Dr Maria Alfredsson
Reader in Theoretical Materials
Director of Undergraduate Studies for Chemistry Programmes
About
Dr Maria Alfredsson is a Senior Lecturer in Theoretical Materials.
Publications
Showing 50 of 60 total publications in the Kent Academic Repository. View all publications.
Article
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Blidberg, A., Valvo, M., Alfredsson, M., Tengstedt, C., Gustafsson, T. and Björefors, F. (2019). Electronic changes in poly(3,4-ethylenedioxythiophene)-coated LiFeSO4F during electrochemical lithium extraction. Journal of Power Sources [Online] 418:84-89. Available at: https://doi.org/10.1016/j.jpowsour.2019.02.039.The redox activity of tavorite LiFeSO4F coated with poly (3,4-ethylenedioxythiophene), i.e. PEDOT, is investigated by means of several spectroscopic techniques. The electronic changes and iron-ligand redox features of this LiFeSO4F-PEDOT composite are probed upon delithiation through X-ray absorption spectroscopy. The PEDOT coating, which is necessary here to obtain enough electrical conductivity for the electrochemical reactions of LiFeSO4F to occur, is electrochemically stable within the voltage window employed for cell cycling. Although the electronic configuration of PEDOT shows also some changes in correspondence of its reduced and oxidized forms after electrochemical conditioning in Li half-cells, its p-type doping is fully retained between 2.7 and 4.1 V with respect to Li+/Li during the first few cycles. An increased iron-ligand interaction is observed in LixFeSO4F during electrochemical lithium extraction, which appears to be a general trend for polyanionic insertion compounds. This finding is crucial for a deeper understanding of a series of oxidation phenomena in Li-ion battery cathode materials and helps paving the way to the exploration of new energy storage materials with improved electrochemical performances.
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Arayamparambil, J., Mann, M., Liu, X., Alfredsson, M., Dronskowski, R., Stievano, L. and Sougrati, M. (2019). Electrochemical Evaluation of Pb, Ag, and Zn Cyanamides/Carbodiimides. ACS Omega [Online] 4:4339-4347. Available at: https://doi.org/10.1021/acsomega.8b02748.PbNCN, Ag2NCN, and ZnNCN were tested as negative
electrode materials for Li-ion batteries. A thorough analysis of the
electrochemical mechanism by X-ray diffraction and X-ray absorption
spectroscopy showed that, unlike transition metal carbodiimides, these
compounds react with lithium via a two-step reaction, starting with
conversion followed by alloying. The conversion reaction is highly
irreversible for the three compounds, whereas the reversibility of the alloying
reaction depends on the metal, that is, highly irreversible for PbNCN and
Ag2NCN which contain the cyanamide group (NC−N2−) and more
reversible for ZnNCN containing carbodiimide (−NCN−). In the case
of the more covalent, cyanamide-type PbNCN and Ag2NCN, the conversion
reaction occurs at a higher voltage compared to the more ionic,
carbodiimide-type ZnNCN, correlated with the nature of bonding in the
NCN group and in the phases themselves. Compared to transition metal carbodiimides, these materials show rather low
performance, with no improvement in capacity as it would have been expected from the combination of conversion and
alloying. -
Fehse, M., Darwiche, A., Sougrati, M., Kelder, E., Chadwick, A., Alfredsson, M., Monconduit, L. and Stievano, L. (2017). In-Depth Analysis of the Conversion Mechanism of TiSnSb vs Li by Operando Triple-Edge X-ray Absorption Spectroscopy: a Chemometric Approach. Chemistry of Materials [Online] 29:10446-10454. Available at: https://doi.org/10.1021/acs.chemmater.7b04088.The electrochemical cycling mechanism of the ternary intermetallic TiSnSb, a promising conversion-type negative electrode material for lithium batteries, was thoroughly studied by operando X-ray absorption spectroscopy (XAS) at three different absorption edges, i.e., Ti, Sn, and Sb K-edge. Chemometric tools such as principal component analysis and multivariate curve resolution-alternating least squares were applied on the extensive data set to extract the maximum contained information in the whole set of operando data. The evolution of the near-edge (XANES) fingerprint and of the extended fine-structure (EXAFS) of the XAS spectra confirms the reversibility of the conversion mechanism, revealing that Ti forms metallic nanoparticles upon lithiation and binds back to both Sn and Sb upon the following delithiation. The formation of both Li7Sn2 and Li3Sb upon lithiation was also clearly confirmed. The application of chemometric tools allowed the identification of a time shift between the reaction processes of Sn and Sb lithiation, indicating that the two metals do not react at the same time, in spite of a certain overlap between their respective reaction. Furthermore, XANES and EXAFS fingerprint show that the Ti–Sn–Sb species formed after one complete lithiation/delithiation cycle is distinct from the starting material TiSnSb.
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Caccami, M., Hogan, M., Alfredsson, M., Marrocco, G. and Batchelor, J. (2017). A Tightly Integrated Multilayer Battery Antenna for RFID Epidermal Applications. IEEE Transactions on Antennas and Propagation [Online] 66:609-617. Available at: http://dx.doi.org/10.1109/TAP.2017.2780899.For the acceptance 1 of biointegrated devices in daily life, radio systems must be developed, which are minimally invasive to the skin, and they must have ultralow-profile local power sources to support data-logging functionality without compromising shape conformability. This contribution proposes a tightly integrated multilayer battery-antenna system (65 × 23 mm2), that is, ultrathin (just 200 ?m), flexible, and lighter than 1 g, making it suitable for epidermal applications. The negative electrode (anode) current collector of the battery is a radio frequency identification tag antenna coated by a conductive polymer (Pedot:PSS) working as anode material. Since the battery is a dynamic device, subjected to discharging, the antenna design must include the variable dielectric properties of the conductive polymer which are here first characterized in the UHF band for real charge/discharge battery conditions. The communication performance of the prototype composite device is hence evaluated through the measurement of the realized gain of the tag antenna (?19.6 dBi at 870 MHz) when it is placed directly onto a volunteer’s forearm. The read range of 1.3–3 m is suitable for occasional data download from the epidermal data logger when the user comes close to a reader-equipped gate.
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Brownrigg, A., Mountjoy, G., Chadwick, A., Alfredsson, M., Bras, W., Billaud, J., Armstrong, A., Bruce, P., Dominko, R. and Kelder, E. (2015). In situ Fe K-edge X-ray absorption spectroscopy study during cycling of Li2FeSiO4 and Li2.2Fe0.9SiO4 Li ion battery materials. Journal of Materials Chemistry A [Online] 3:7314-7322. Available at: http://www.dx.doi.org/10.1039/c4ta06305h.In situ X-ray Absorption Spectroscopy (XAS) results are presented for Li2FeSiO4 and Li2.2Fe0.9SiO4, promising cathode materials for lithium-ion batteries. The aims are to establish the valence and local structure of Fe during charge and discharge to understand if the Fe3+/Fe4+ redox pair can be reached in the current battery design. It is found that the valence state changes between Fe2+ and Fe3+, with no evidence of Fe4+ before the onset of electrolyte degradation. There is a reversible contraction and extension of the Fe–O bond lengths during cycling while the Fe–Si distance remains constant, which underlines the stability of the Li2FeSiO4 material. The same observations apply to Li2.2Fe0.9SiO4 cathode material indicating that changing the stoichiometry does not provide any additional structural stability.
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Kossoff, D., Dubbin, W., Alfredsson, M., Edwards, S., Macklin, M. and Hudson-Edwards, K. (2014). Mine tailings dams: Characteristics, failure, environmental impacts, and remediation. Applied Geochemistry [Online] 51:229-245. Available at: http://dx.doi.org/10.1016/j.apgeochem.2014.09.010.On a global scale demand for the products of the extractive industries is ever increasing. Extraction of the targeted resource results in the concurrent production of a significant volume of waste material, including tailings, which are mixtures of crushed rock and processing fluids from mills, washeries or concentrators that remain after the extraction of economic metals, minerals, mineral fuels or coal. The volume of tailings is normally far in excess of the liberated resource, and the tailings often contain potentially hazardous contaminants. A priority for a reasonable and responsible mining organization must be to proactively isolate the tailings so as to forestall them from entering groundwaters, rivers, lakes and the wind. There is ample evidence that, should such tailings enter these environments they may contaminate food chains and drinking water. Furthermore, the tailings undergo physical and chemical change after they have been deposited. The chemical changes are most often a function of exposure to atmospheric oxidation and tends to make previously, perhaps safely held contaminants mobile and available. If the tailings are stored under water, contact with the atmosphere is substantially reduced, thereby forestalling oxygen-mediated chemical change. It is therefore accepted practice for tailings to be stored in isolated impoundments under water and behind dams. However, these dams frequently fail, releasing enormous quantities of tailings into river catchments. These accidents pose a serious threat to animal and human health and are of concern for extractive industries and the wider community. It is therefore of importance to understand the nature of the material held within these dams, what best safety practice is for these structures and, should the worst happen, what adverse effects such accidents might have on the wider environment and how these might be mitigated. This paper reviews these factors, covering the characteristics, types and magnitudes, environmental impacts, and remediation of mine tailings dam failures. © 2014 Elsevier Ltd.
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Moulki, H., Faure, C., Mihelcic, M., Surca Vuk, A., Svegl, F., Orel, B., Campet, G., Alfredsson, M., Chadwick, A., Gianolio, D. and Rougier, A. (2014). Electrochromic performances of nonstoichiometric NiO thin films. Thin Solid Films [Online] 553:63-66. Available at: http://www.dx.doi.org/10.1016/j.tsf.2013.10.154.Electrochromic (EC) performances of Ni3 + containing NiO thin films, called modified NiO thin films, prepared either by pulsed laser deposition or by chemical route are reported. When cycled in lithium based electrolyte, the comparison of the EC behavior of nonstoichiometric NiO thin films points out a larger optical contrast for the films synthesized by chemical route with the absence of an activation period on early electrochemical cycling due in particular to a larger porosity. Herein we demonstrate faster kinetics for modified NiO thin films cycled in lithium ion free electrolyte. Finally, X-ray absorption spectroscopy is used for a preliminary understanding of the mechanism involved in this original EC behavior linked to the film characteristics including their disorder character, the presence of Ni3 + and their porous morphology.
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Kossoff, D., Hudson-Edwards, K., Dubbin, W., Alfredsson, M. and Geraki, T. (2012). Cycling of As, P, Pb and Sb during weathering of mine tailings: Implications for fluvial environments. Mineralogical Magazine [Online] 76:1209-1228. Available at: http://dx.doi.org/10.1180/minmag.2012.076.5.14.The weathering and oxidation of mine tailings has the potential to contaminate water and soil with toxic elements. To understand the mechanisms, extent and products of the long-term weathering of complex Bolivian tailings from the Cerro Rico de PotosÃ, and their effects on As, Pb, P and Sb cycling, three-year long laboratory column experiments were carried out to model 20 years of dry-and wet-season conditions in the Pilcomayo basin. Chemical analysis of the leachate and column solids, optical mineralogy, X-ray diffraction, scanning electron microscopy, electron probe microanalysis, microscale X-ray absorption near edge structure spectroscopy, Bureau Commun de Référence sequential extraction and water-soluble chemical extractions, and speciation modelling have shown that the weathering of As-bearing pyrite and arsenopyrite, resulted in a loss of 13-29% of the original mass of As. By contrast, Pb and Sb showed much lower mass losses (0.1-1.1% and 0.6-1.9%, respectively) due to the formation of insoluble Pb-and Sb(V)-rich phases, which were stable at the low pH (~2) conditions that prevailed by the end of the experiment. The experiment also demonstrated a link between the cycling of As, Sb, and the oxidation of Fe(II)-bearing sphalerite, which acted as a nucleation point for an Fe-As-Sb-O phase. Phosphorus was relatively immobile in the tailings columns (up to 0.3% mass loss) but was more mobile in the soil-bearing columns (up to 10% mass loss), due to the formation of soluble P-bearing minerals or mobilization by organic matter. These results demonstrate the influence of mine tailings on the mobility of P from soils and on the potential contamination of ecosystems with As, and strongly suggest that these materials should be isolated from fluvial environments. © 2012 Mineralogical Society.
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Canepa, P., Ugliengo, P. and Alfredsson, M. (2012). Elastic and Vibrational Properties of a- and ß-PbO. Journal of Physical Chemistry C [Online] 116:21514-21522. Available at: http://dx.doi.org/10.1021/jp3036988.The structure, electronic, and dynamic properties of the two-layered ? (litharge) and ? (massicot) phases of PbO have been studied by density functional methods. The role of London dispersion interactions as leading component of the total interaction energy between layers has been addressed by using the Grimme’s approach in which new parameters for Pb and O atoms have been developed. Both gradient-corrected and hybrid functionals have been adopted using Gaussian-type basis sets of polarized triple-? quality for O atoms and small-core pseudopotential for the Pb atoms. Basis set superposition error (BSSE) has been accounted for by the Boys–Bernardi correction to compute the interlayer separation. Cross-checks with calculations adopting plane waves that are BSSE free have also been performed for both structures and vibrational frequencies. With the new set of proposed Grimme’s type parameters, structures and dynamical parameters for both PbO phases are in good agreement with experimental data.
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Vidal-Abarca, C., Lavela, P., Tirado, J., Chadwick, A., Alfredsson, M. and Kelder, E. (2012). Improving the cyclability of sodium-ion cathodes by selection of electrolyte solvent. Journal of Power Sources [Online] 197:314-318. Available at: http://www.dx.doi.org/10.1016/j.jpowsour.2011.09.008.A composite material containing orthorhombic Na1.8FePO4F and carbon is prepared by mechanical activation and ceramic procedures. The material is studied in sodium test cells as a potential candidate for sodium-ion battery cathodes. The effect of the solvents in the electrolyte on the electrochemical performance is analysed by X-ray absorption spectroscopy. The structural changes on cycling are small, while the changes in the oxidation state of iron agree with the sodium insertion–extraction processes. The oxidation state is especially affected by the upper limit of the voltage window, and the discharge capacity is strongly affected when using propylene carbonate solvent. Capacity and capacity retention are higher for sodium cells using mixtures of ethylene carbonate and diethyl carbonate as the solvent of NaPF6 electrolytes.
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Kossoff, D., Hudson-Edwards, K., Dubbin, W. and Alfredsson, M. (2012). Major and trace metal mobility during weathering of mine tailings: Implications for floodplain soils. Applied Geochemistry [Online] 27:562-576. Available at: http://dx.doi.org/10.1016/j.apgeochem.2011.11.012.Mine tailings discharged to river systems have the potential to release significant quantities of major and trace metals to waters and soils when weathered. To provide data on the mechanisms and magnitudes of short- and long-term tailings weathering and its influence on floodplain environments, three calendar year-long column leaching experiments that incorporated tailings from PotosÃ, Bolivia, and soil from unaffected downstream floodplains, were carried out. These experiments were designed to model 20 cycles of wet and dry season conditions. Two duplicate columns modeled sub-aerial tailings weathering alone, a third modeled the effects of long-term floodplain tailings contamination and a fourth modeled that of a tailings dam spill on a previously contaminated floodplain. As far as was practical local climatic conditions were modeled. Chemical analysis of the leachate and column solids, optical mineralogy, XRD, SEM, EPMA, BCR and water-soluble chemical extractions and speciation modeling were carried out to determine the processes responsible for the leaching of Al, Ca, Cu, K, Na, Mg, Mn, Sn, Sr and Ti. Over the 20 cycles, the pH declined to a floor of ca. 2 in all columns. Calcium, Cu, Mg, Mn and Na showed significant cumulative losses of up to 100%, 60%, 30%, 95% and 40%, respectively, compared to those of Al, K, Sr, Sn and Ti, which were up to 3%, 1.5%, 5%, 1% and 0.05%, respectively. The high losses are attributed to the dissolution of relatively soluble minerals such as biotite, and oxidation of chalcopyrite and Cu-sulfosalts, while low losses are attributed to the presence of sparingly soluble minerals such as svanbergite, cassiterite and rutile. These results strongly suggest that the release of tailings to floodplains should be limited or prohibited, and that all tailings should be removed from floodplains following dam spills. © 2011 Elsevier Ltd.
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Nwokeke, U., Chadwick, A., Alcantara, R., Alfredsson, M. and Tirado, J. (2011). Nanocrystalline Fe1-xCoxSn2 solid solutions prepared by reduction of salts in tetraethylene glycol. Journal of Alloys and Compounds [Online] 509:3074-3079. Available at: http://www.dx.doi.org/10.1016/j.jallcom.2010.11.202.In an effort to improve the electrochemical performance of tin intermetallic phases as electrode active material for lithium-ion batteries, Fe1?xCoxSn2 solid solutions with x = 0.0, 0.25, 0.3, 0.5, 0.6 and 0.8 were prepared by chemical reduction in tetraethylene glycol. Precise control of the synthesis conditions allowed single-phase nanocrystalline materials to be prepared, with particle diameters of about 20 nm and cubic, nanorods, and U-shaped morphologies. The substitution of iron by cobalt induced a contraction of the unit cell volume. The hyperfine parameters of the 57Fe Mössbauer spectra were sensitive to the Co/Fe substitution and revealed a superparamagnetic behaviour. In lithium cells nanocrystalline Fe1?xCoxSn2 active materials delivered reversible capacities above 500 mAh g?1 that depended on the composition and cycling conditions. The intermediate compositions exhibit better electrochemical performance than the end compositions CoSn2 and FeSn2.
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Kossoff, D., Hudson-Edwards, K., Dubbin, W. and Alfredsson, M. (2011). Incongruent weathering of Cd and Zn from mine tailings: A column leaching study. Chemical Geology [Online] 281:52-71. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-78751580444&partnerID=40&md5=287d70279e088899f943d436efb5dbf2.The weathering of discharged mine tailings can contaminate groundwaters, rivers and floodplains with potentially toxic Cd and Zn, depending on tailings mineralogy, storage, dispersal and climatic conditions. The mechanisms of long-term tailings weathering and its influence on waste piles and floodplain environments were assessed by a column leaching experiment that incorporated tailings and soil from PotosÃ, Bolivia, and modelled 20 cycles of wet and dry season conditions over three calendar years. Chemical analysis of the leachate and column solids, optical mineralogy, XRD, SEM, EPMA, BCR and water-soluble chemical extractions and speciation modelling were carried out to determine the processes responsible for the leaching of Cd, Fe, S and Zn. Over this period, approximately 50 to 95% of the original Cd and 50 to 60% of the Zn were leached from the columns. Large amounts of leached Cd and Zn at the beginning of the experiment are attributed to the dissolution of soluble sulphate minerals present in the original tailings and formed after the first wetting of the columns. The Zn/Cd mass ratios of the tailings and soil, initially 429 and 400, respectively, vary considerably over the course of the experiment. Low values (between 220 and 300) in the early cycles are attributed to preferential weathering of Cd-rich wurtzite [Zn,Fe)S] and sequestration of Zn in preference to Cd in secondary Fe phases forming in the columns. In the middle cycles, dissolution of secondary Fe(OH) 3 under low pH (<3) conditions, and of ferroan (Cd-poor) sphalerite [Zn,Fe)S], releases Zn and raises the Zn/Cd ratio to 550-600 in the tailings-only columns and up to 1500 in the mixed tailings-soil columns. The very high ratios in the latter are also ascribed to the formation of low molecular weight organic ligands that have high affinity for Zn over Cd. In the later column-cycles, Zn/Cd ratios return to near-initial values, due to the weathering of Fe-poor sphalerite and secondary Fe phases, and the declining preference of Zn over Cd in the soil organic acids under the strongly acidic conditions prevailing in the columns. The formation and dissolution of secondary soluble sulphate minerals also play a role in Cd and Zn cycling, especially at the beginning of the experiment. © 2010 Elsevier B.V.
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Canepa, P., Schofield, E., Chadwick, A. and Alfredsson, M. (2011). Comparison of a calculated and measured XANES spectrum of ?-Fe2O3. Physical Chemistry Chemical Physics [Online] 13:12826-12834. Available at: http://dx.doi.org/10.1039/C1CP00034A.Comparison and prediction of the experimental XANES spectrum is a good measurement of the quality of the electronic structure calculations employed, and their ability to predict electronic transitions in solids. Here we present a comparison between BLYP + U and hybrid-BLYP calculations regarding the geometric, magnetic and electronic structures of ?-Fe2O3 (hematite). Several values of U and different percentages of Fock-exchange have been screened to see how their contributions affect different properties of hematite, paying particular attention to the electronic structure. To estimate the quality of the various methods the calculated density-of-states were compared to the experimentally collected XANES spectrum of the iron K-edge, providing information about the orbitals describing the conduction band. We find that in agreement with previous studies DFT + U and hybrid-functional simulations can correctly predict the character of the valence band, but only Fock-exchange higher than 30% or U-values equal or larger than 6 eV properly reproduce the order between the tg and e orbitals in the conduction band, and can, therefore, be used to study and predict XANES spectra and electronic transitions in hematite.
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Canepa, P., Hanson, R., Ugliengo, P. and Alfredsson, M. (2011). J-ICE: a new Jmol interface for handling and visualizing crystallographic and electronic properties. Journal of Applied Crystallography [Online] 44:225-229. Available at: http://dx.doi.org/10.1107/S0021889810049411.The growth in complexity of quantum mechanical software packages for modelling the physicochemical properties of crystalline materials may hinder their usability by the vast majority of non-specialized users. Consequently, a free operating-system-independent graphical user interface (GUI) has been developed to drive the most common simulation packages for treating both molecules and solids. In order to maintain maximum portability and graphical efficiency, the popular molecular graphics engine Jmol, written in the portable Java language, has been combined with a specialized GUI encoded in HTML and JavaScript. This framework, called J-ICE, allows users to visualize, build and manipulate complex input or output results (derived from modelling) entirely via a web server, i.e. without the burden of installing complex packages. This solution also dramatically speeds up both the development procedure and bug fixing. Among the range of software appropriate for modelling condensed matter, the focus of J-ICE is currently only on CRYSTAL09 and VASP.
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Canepa, P., Kossoff, D., Hudson-Edwards, K., Dubbin, W. and Alfredsson, M. (2009). Hematite contaminated by heavy metals. Geochimica Et Cosmochimica Acta [Online] 73:A189. Available at: http://dx.doi.org/10.1016/j.gca.2009.05.026.
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Ono, S., Brodholt, J., Alfe, D., Alfredsson, M. and Price, G. (2008). Ab initio molecular dynamics simulations for thermal equation of state of B2-type NaCl. Journal of Applied Sciences [Online] 103:23510. Available at: http://dx.doi.org/10.1063/1.2832632.The pressure as a function of volume and temperature has been investigated for B2-type NaCl over the pressure range of 20-360 GPa and at temperatures between 300 and 3000 K. The simulations were performed using ab initio molecular dynamics method within the density-functional theory framework. A Vinet equation of state fitted to the 300 K data yielded a bulk modulus of B(Ta)=128.66 GPa and a pressure derivative of B(Ta)'=4.374 at standard state pressure of 30 GPa. The thermal pressure contribution was determined to be of the form Delta P(th)=[alpha B(T)(V(a))+(partial derivative B(T)/partial derivative T)(V) ln(V(a)/V)]Delta T. When alpha B(T)(V(a)) is assumed to be constant, the fit to the data yielded alpha B(T)(V(a))=0.0033 GPa/K at standard volume, corresponding to the pressure of 30 GPa. In contrast, the volume dependence of the thermal pressure was very small, and fitting yielded (partial derivative B(T)/partial derivative T)(V)=0.000 87.
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Kossoff, D., Hudson-Edwards, K., Dubbin, W. and Alfredsson, M. (2008). Incongruent weathering of Cd and Zn from mine tailings. Mineralogical Magazine [Online] 72:81-84. Available at: http://dx.doi.org/10.1180/minmag.2008.072.1.81.Weathering of discharged mine tailings contaminates streams, rivers and floodplains with toxic metals on a vast scale. The magnitude of the problem depends on input tailings mineralogy, storage and dispersal, and climatic conditions. To better understand the mechanisms of long-term tailings weathering, a leaching column study was established, incorporating tailings and soil from Potosi, Bolivia, with the aim of modelling a 25 year field period. The Zn/Cd molar ratio of the tailings leachate water, initially 738 for the unaltered tailings, is highly variable over 15 model years of leaching, particularly in the mixed tailings-soil columns. Columns with soil have ratios as high as 2563, while pure tailings columns reach ratios of <376. We employ complementary techniques, involving atomistic computational modelling, leachate analysis and mineralogical characterization, to elucidate the mechanisms governing these incongruent Cd and Zn weathering dynamics.
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Blanchard, M., Alfredsson, M., Brodholt, J., Wright, K. and Catlow, C. (2007). Arsenic incorporation into FeS2 pyrite and its influence on dissolution: A DFT study. Geochimica Et Cosmochimica Acta [Online] 71:624-630. Available at: http://dx.doi.org/10.1016/j.gca.2006.09.021.FeS2 pyrite can incorporate large amounts of arsenic (up to ca. 10 wt%) and hence has a strong impact on the mobility of this toxic metalloid. Focussing on the lowest arsenic concentrations for which the incorporation occurs in solid solution, the substitution mechanisms involved have been investigated by assuming simple incorporation reactions in both oxidising and reducing conditions. The solution energies were calculated by Density Functional Theory (DFT) calculations and we predict that the formation of AsS dianion groups is the most energetically favourable mechanism. The results also suggest that the presence of arsenic will accelerate the dissolution and thus the generation of acid drainage, when the crystal dissolves in oxidising conditions.
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Fortes, A., Wood, I., Alfredsson, M., Vocadlo, L., Knight, K., Marshall, W., Tucker, M. and Fernandez-Alonso, F. (2007). The high-pressure phase diagram of ammonia dihydrate. High Pressure Research [Online] 27:201-212. Available at: http://dx.doi.org/10.1080/08957950701265029.We have investigated the P-T phase diagram of ammonia dihydrate (ADH), ND3 center dot 2D(2)O, using powder neutron diffraction methods over the range 0-9 GPa, 170-300 K. In addition to the ambient pressure phase, ADH I, we have identified three high-pressure phases, ADH II, III, and IV, each of which has been reproduced in at least three separate experiments. Another, apparently body-centred-cubic, phase of ADH has been observed on a single occasion above 6 GPa at 170 K. The existence of a dehydration boundary has been confirmed where, upon compression or warming, ADH IV decomposes to a high-pressure ice phase (ice VII or VIII) and a high-pressure phase of ammonia monohydrate (AMH V or VI).
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Alfredsson, M., Cora, F., Dobson, D., Davy, J., Brodholt, J., Parker, S. and Price, G. (2007). Dopant control over the crystal morphology of ceramic materials. Surface Science [Online] 601:4793-4800. Available at: http://dx.doi.org/10.1016/j.susc.2007.07.025.Doping is a common way to activate the behavior of ceramics. Its effect is not limited to the bulk: segregation of dopants to the surfaces also yields a way to modify, and ultimately control the crystal morphology. We propose a model that allows us to calculate the surface energy beyond the Langmuir isotherm for doped and defective surfaces from atomic-level simulations. The model also allows us to account for different compositions between the bulk and surface. Computational materials design can thus be applied to optimize simultaneously the crystal behavior at the atomic (surface structure and composition) and mesoscopic (crystal size and shape) length scales. We exemplify the model with orthorhombic CaTiO3 perovskite doped with Mg2+, Fe2+, Ni2+, Sr2+, Ba2+ and Cd2+ ions, by predicting the effect that different dopants and dopant concentrations have on the crystal morphology. We find that a higher proportion of reactive {021} and {111} surfaces are exposed with the presence of divalent Mg2+, Fe2+ and Ni2+ ions than in the undoped material and in perovskite doped with Ba2+ and Sr2+center dot Cd2+ has only minor effects on crystal morphologies. These findings have important implications for predicting the reactivity of crystals doped with different ions and we show how this can be related to a simple parameter such as the ionic radius. We have tested our newly derived model by comparison with laboratory flux grown single crystals of CaTiO3, (Ni,Ca)TiO3 and (Ba,Ca)TiO3 and find excellent agreement between theory and experiment. (c) 2007 Elsevier B.V. All rights reserved.
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Dobson, D., Alfredsson, M., Holzapfel, C. and Brodholt, J. (2007). Grain boundary enrichment of iron on magnesium silicate perovskite. European Journal of Mineralogy [Online] 19:617-622. Available at: http://dx.doi.org/10.1127/0935-1221/2007/0019-1756.We present a combined experimental and numerical simulations study of grain boundaries in (Fe, Mg)SiO3 perovskite. TEM observations of grain boundaries in a well equilibrated perovskite sample consistently show small but significant enrichments of Fe. This result is in good agreement with classical potentials simulations of grain boundary structures in MgSiO3 perovskite which predict substitution of iron onto grain boundaries to be energetically favourable over iron substitution in the bulk perovskite structure.
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Fortes, A., Wood, I., Alfredsson, M., Vocadlo, L. and Knight, K. (2006). The thermoelastic properties Of MgSO4 center dot 7D(2)O (epsomite) from powder neutron diffraction and ab initio calculation. European Journal of Mineralogy [Online] 18:449-462. Available at: http://dx.doi.org/10.1127/0935-1221/2006/0018-0449.time-of-flight powder neutron diffraction has been used to measure the molar volume of MgSO4.7D(2)O (i) from 1.8 - 300 K at ambient pressure, (ii) from 50 - 290 K at 1.4, 3.0, and 4.5 kbar, (iii) from 0 - 5.5 kbar at 290 K, and (iv) from 0 - 4.5 kbar at 50 K. The data have allowed us to determine the temperature dependence of the incompressibility, (partial derivative K/partial derivative T)(p), (thermodynamically equivalent to the pressure dependence of the thermal expansion, (partial derivative alpha/partial derivative P)(T)) of epsomite throughout its stability field. We observed that the a-axis exhibits negative thermal expansion, alpha(a), from 30 - 250 K at room pressure, turning positive above 250 K and being zero below 30 K. However, each of the crystallographic axes exhibits a sharp change in (partial derivative alpha/partial derivative T) at similar to 125 K, and this appears to correspond to significant changes in the axial incompressibilities with the a- and c-axes softening, and the b-axis stiffening considerably below similar to 125 K. Our thermoelastic results are in agreement with ab initio calculations at zero Kelvin; however the calculations offer no obvious insight into the mechanism responsible for the change in behaviour at low temperature.
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Alfe, D., Alfredsson, M., Brodholt, J., Gillan, M., Towler, M. and Needs, R. (2005). Quantum Monte Carlo calculations of the structural properties and the B1-B2 phase transition of MgO. Physical Review B: Condensed Matter and Materials Physics [Online] 72:7. Available at: http://dx.doi.org/10.1103/PhysRevB.72.014114.We report diffusion Monte Carlo (DMC) calculations on MgO in the rock-salt and CsCl structures. The calculations are based on Hartree-Fock pseudopotentials, with the single-particle orbitals entering the correlated wave function being represented by a systematically convergeable cubic-spline basis. Systematic tests are presented on system-size errors using periodically repeating cells of up to over 600 atoms. The equilibrium lattice parameter of the rocksalt structure obtained within DMC is almost identical to the Hartree-Fock result, which is close to the experimental value. The DMC result for the bulk modulus is also in good agreement with the experimental value. The B1-B2 transition pressure (between the rocksalt and CsCl structures) is predicted to be just below 600 GPa, which is beyond the experimentally accessible range, in accord with other predictions based on Hartree-Fock and density functional theories.
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Alfredsson, M., Brodholt, J., Dobson, D., Oganov, A., Catlow, C., Parker, S. and Price, G. (2005). Crystal morphology and surface structures of orthorhombic MgSiO3 perovskite. Physics and Chemistry of Minerals [Online] 31:671-682. Available at: http://dx.doi.org/10.1007/s00269-004-0429-4.Orthorhombic MgSiO3 perovskite is thought to be the most abundant mineral in the mantle of the Earth. Its bulk properties have been widely studied, but many geophysical and rheological processes are also likely to depend upon its surface and grain boundary properties. As a first step towards modelling these geophysical properties, we present here an investigation of the structures and energetics of the surfaces of MgSiO3-perovskite, employing both shell-model atomistic effective-potential simulations, and density-functional-theory (DFT) calculations. Our shell-model calculations predict the {001} surfaces to be the energetically most stable surfaces: the calculated value of the surface energy being 2.2 J/m(2) for the MgO-terminated surface, which is favoured over the SiO2-terminated surface (2.7 J/m(2)). Also for the polar surfaces {111}, {101} and {011} the MgO-terminated surfaces are energetically more stable than the Si-terminated surfaces. In addition we report the predicted morphology of the MgSiO3 perovskite structure, which is dominated by the energetically most stable {001} and {110} surfaces, and which appears to agree well with the shape of grown single crystals.
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Li, L., Brodholt, J., Stackhouse, S., Weidner, D., Alfredsson, M. and Price, G. (2005). Elasticity of (Mg, Fe)(Si, Al)O-3-perovskite at high pressure. Earth and Planetary Science Letters [Online] 240:529-536. Available at: http://dx.doi.org/10.1016/j.epsl.2005.09.030.The most abundant mineral on Earth has a perovskite crystal structure and a chemistry that is dominated by MgSiO3 with the next most abundant cations probably being aluminum and ferric iron. The dearth of experimental elasticity data for this chemically complex mineral limits our ability to calculate model seismic velocities for the lower mantle. We have calculated the single crystal elastic moduli (c(ij)) for (Mg, Fe3+)(Si, Al)O-3 perovskite using density functional theory in order to investigate the effect of chemical variations and spin state transitions of the Fe3+ ions. Considering the favored coupled substitution of Mg2+-Si4+ by Fe3+-Al3+, we find that the effect of ferric iron on seismic properties is comparable with the same amount of ferrous iron. Ferric iron lowers the elastic moduli relative to the Al charge-coupled substitution. Substitution of Fe3+ for Al3+, giving rise to an Fe/Mg ratio of 6%, causes 1.8% lower longitudinal velocity and 2.5% lower shear velocity at ambient pressure and 1.1% lower longitudinal velocity and 1.8% lower shear velocity at 142 GPa. The spin state of the iron for this composition has a relatively small effect (<0.5% variation) on both bulk modulus and shear modulus. (C) 2005 Elsevier B.V. All rights reserved.
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Alfredsson, M., Brodholt, J., Wilson, P., Price, G., Cora, F., Calleja, A., Bruin, R., Blanshard, L. and Tyer, R. (2005). Structural and magnetic phase transitions in simple oxides using hybrid functionals. Molecular Simulation [Online] 31:367-377. Available at: http://dx.doi.org/10.1080/08927020500066684.We present the structural as well as elastic properties of the alkaline earth oxides and FeO, calculated using hybrid exchange functionals within DFT. We show that by empirically fitting the amount of Fock-exchange in the hybrid functionals, we can accurately reproduce the pressure-induced phase transitions for MgO, CaO, SrO and BaO. For FeO the hybrid functionals predict an insulator <-> metal transition at ca. 150 GPa, associated with an i-B8 <-> B8 structural phase transition. The structural phase transition is accompanied by a spin transition from a high- to low-spin electron configuration on the Fe2+ ions. Hence, FeO undergoes a magnetic phase transition from an anti-ferromagnetic to non-magnetic structure. We also find that as the ionicity of the polymorphs increases a higher fraction of Fock-exchange is required to reproduce the structural volumes reported from experiments.
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Alfredsson, M., Cora, F., Brodholt, J., Parker, S. and Price, G. (2005). Crystal morphology and surface structures of orthorhombic MgSiO3 in the presence of divalent impurity ions. Physics and Chemistry of Minerals [Online] 32:379-387. Available at: http://dx.doi.org/10.1007/s00269-005-0009-2.Many rheological and transport properties of rocks are determined by the grain boundary structures of their constituent minerals. These grain boundaries often also hold a high concentration of dopant ions. Here, as a first step towards modelling the transport and rheological behaviour of the lower mantle, we report the results of lattice static simulations on the surface structures of Fe2+ and Ca2+- doped orthorhombic MgSiO3-perovskite. For all the surfaces we studied, the energies of the doped structures are lowered, sometimes by more than 1 J/m(2), with respect to the pure surfaces. From our calculated crystal morphologies, we predict that the grains become more tabular as the concentration of Fe2+ ions increases, while under equilibrium conditions the grains are cubic. By calculating the replacement energies of Mg2+ by Fe2+ and Ca2+ ions in the six outermost surface layers, we conclude that these divalent ions would tend to segregate onto the crystal surfaces. We suggest, therefore, that the grain boundary structure and rheology of MgSiO3- perovskite dominated rocks will be strongly affected by the presence of minor elements in the lower mantle.
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Blanchard, M., Alfredsson, M., Brodholt, J., Price, G., Wright, K. and Catlow, C. (2005). Electronic structure study of the high-pressure vibrational spectrum of FeS2 pyrite. Journal of Physical Chemistry B [Online] 109:22067-22073. Available at: http://dx.doi.org/10.1021/jp053540x.Plane-wave density functional calculations are used to investigate the pressure dependence of the geometry and Gamma-point phonons of FeS2 pyrite up to 150 GPa. The linear response method is employed to calculate the vibrational properties. Raman-active modes are in excellent agreement with the experimental data available up to 50 GPa,(1) and we predict the evolution with pressure of the IR-active modes for which no high-pressure spectroscopic data have been reported so far. Over the wide pressure range investigated here, all vibrational frequencies depend nonlinearly on pressure; their pressure dependence is quantified by determining the full set of mode Gruneisen parameters and their pressure derivatives.
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Dove, M., Calleja, M., Bruin, R., Wakelin, J., Tucker, M., Lewis, G., Hasan, S., Alexandrov, V., Keegan, M., Ballard, S., Tyer, R., Todorov, I., Wilson, P., Alfredsson, M., Price, G., Chapman, C., Emmerich, W., Wells, S., Marmier, A., Parker, S. and Du, Z. (2005). Erratum: The eMinerals collaboratory: tools and experience. Molecular Simulation [Online] 31:1107-1107. Available at: http://dx.doi.org/10.1080/10503300500520106.Erratum for article featured in vol 31, Issue 14-15, pg 329 of Molecular Simulation, 2005.
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Fortes, A., Wood, I., Alfredsson, M., Vocadlo, L. and Knight, K. (2005). The incompressibility and thermal expansivity of D2O ice II determined by powder neutron diffraction. Journal of Applied Crystallography [Online] 38:612-618. Available at: http://dx.doi.org/10.1107/S0021889805014226.Using high-resolution neutron powder diffraction, the molar volume of a pure sample of D2O ice II has been measured, within its stability field, at 225 K, over the pressure range 0.25 < P < 0.45 GPa. Ar gas was used as the pressure medium, to avoid the formation of 'stuffed ice' gas hydrates encountered when using He. The third-order Birch - Murnaghan equation of state parameters of helium-free D2O ice II, referenced to 225 K, are: V-0,V-225 = 306.95 +/- 0.04 angstrom(3) ( 1299.7 +/- 0.2 kg m(-3)), K-0,K-225 = 12.13 +/- 0.07 GPa, with K'(0.225) fixed at 6.0. The thermal expansivity of metastable D2O ice II samples recovered to ambient pressure has also been measured, over the range 4.2 < T < 160 K; above 160 K an irreversible transition to ice Ic was observed. The volumetric expansion coefficient, alpha(V), at P = 0 and T = 225 K, is predicted to be 2.48 x 10(-4) K-1.
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Dove, M., Calleja, M., Bruin, R., Wakelin, J., Tucker, M., Lewais, G., Hasan, S., Alexandrov, V., Keegan, M., Ballard, S., Tyer, R., Todorov, I., Wilson, P., Alfredsson, M., Price, G., Chapman, C., Emmerich, W., Wells, S., Marmier, A., Parker, S. and Du, Z. (2005). The eMinerals collaboratory: tools and experience. Molecular Simulation [Online] 31:329-337. Available at: http://dx.doi.org/10.1080/08927020500066163.Collaboratories provide an environment where researchers at distant locations work together at tackling important scientific and industrial problems. In this paper we outline the tools and principles used to form the eMinerals collaboratory, and discuss the experience, from within, of working towards establishing the eMinerals project team as a functioning virtual organisation. Much of the emphasis of this paper is on experience with the IT tools. We introduce a new application sharing tool.
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Li, L., Brodholt, J., Stackhouse, S., Weidner, D., Alfredsson, M. and Price, G. (2005). Electronic spin state of ferric iron in Al-bearing perovskite in the lower mantle. Geophysical Research Letters [Online] 32:4. Available at: http://dx.doi.org/10.1029/2005GL023045.We investigate the effect of pressure on the electronic spin state of ferric iron on Al-bearing MgSiO3-perovskite using first-principle computations. Ferric iron (6.25 mol%) and Al ( 6.25 mol%) substitute for Mg and Si respectively. Five substitution models on different atomic position pairs are examined. Our results show that spin state transition from high spin (HS) to low spin (LS) occurs on the Fe3+ ions at high pressure, while there is no stability field for the intermediate spin state. Fe3+ alone can be responsible for the spin state transition. The five models witness a transition pressure ranging from 97-126 GPa. Differential stress can change the pressure for the spin collapse. The lowest pressure spin state transition occurs where Al3+ and Fe3+ are in adjacent sites. These results are one explanation to the reported experimental observations that the spin transition occurs over a wide pressure range. This finding may have important implications for the dynamics and seismic signature of the lower mantle.
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Fortes, A., Wood, I., Grigoriev, D., Alfredsson, M., Kipfstuhl, S., Knight, K. and Smith, R. (2004). No evidence for large-scale proton ordering in Antarctic ice from powder neutron diffraction. Journal of Chemical Physics [Online] 120:11376-11379. Available at: http://dx.doi.org/10.1063/1.1765099.We have examined a sample of 3000 year old Antarctic ice, collected at the Kohnen Station, by time-of-flight powder neutron diffraction to test the hypothesis of Fukazawa et al. [e.g., Ann. Glaciol. 31, 247 (2000)] that such ice may be partially proton ordered. Great care was taken to keep our sample below the proposed ordering temperature (237 K) at all times, but we did not observe any evidence of proton ordering.
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Alfredsson, M., Richard, C. and Catlow, A. (2004). Predicting the metal growth mode and wetting of noble metals supported on c-ZrO2. Surface Science [Online] 561:43-56. Available at: https://doi.org/10.1016/j.susc.2004.03.073.Employing periodic density-functional calculations, we predict the metal growth mode for ideal and defective interfaces of noble metals (Pd and Pt) deposited on top of cubic-ZrO2. We discuss various energetic contributions and surface textures. We propose that isolated noble metal atoms are supported on both steps and terraces on the surfaces. Pt, on the other hand, shows a higher probability to decorate steps and other low oxygen coordinated sites, while Pd forms larger metal aggregates on both high oxygen coordinated sites (terraces) as well as low oxygen coordinated sites (steps). Oxygen defects on the support act as nucleation sites for the metal cluster growth. We also investigate metal wetting of Pd and Pt on the {111} surface of cubic-zirconia. Our calculations predict that the wetting angles are ca. 110-130degrees for 1 and 2 mono-layers of Pd and Pt, respectively, suggesting that metal wetting decreases as the metal loading increases.
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Alfredsson, M., Price, G., Catlow, C., Parker, S., Orlando, R. and Brodholt, J. (2004). Electronic structure of the antiferromagnetic B1-structured FeO. Physical Review B: Condensed Matter and Materials Physics [Online] 70. Available at: http://link.aps.org/abstract/PRB/v70/e165111.Density functional theory (DFT) famously fails to correctly describe the electronic band structure of Mott insulators, such as FeO. Here, we present calculations on the structural, elastic, and electronic properties of antiferromagnetic, NaCl-structured (B1) FeO, employing hybrid functionals that combine fractions of the Becke and Fock exchange in combination with the Lee-Yang-Parr correlation functional. We find that in detail different properties of FeO require the inclusion of different amounts of Fock exchange in order for the simulation to reproduce the experimental values, correctly, e.g., geometry, band gap, and elastic constant; but in general between 30%-60% Fock-exchange gives a good description of lattice parameters, bulk modulus as well as the electronic structure, and this approach is therefore an alternative to the LDA+U method that has traditionally been used in the past.
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Herschend, B., Hermansson, K., Alfredsson, M., Zhukovskii, Y., Kotomin, E. and Jacobs, P. (2003). Characterization of the metal-ceramic bonding in the Ag/MgO(001) interface from ab initio calculations. Journal of Physical Chemistry B [Online] 107:11893-11899. Available at: https://doi.org/10.1021/jp030305a.The nature of the metal-ceramic interaction in the Ag/MgO(001) interface is studied using periodic Hartree-Fock calculations with density functional theory a posteriori correlation corrections. Different aspects of the Ag-MgO interaction have been studied by analysis of the electronic properties: total and projected density of states, multipole moments, bond population, and the difference electron density. By linking these properties to the adsorption energy and making a comparative analysis for interfaces with different degrees of coverage and different adsorption models (one- and two-sided adsorption), a detailed description of the Ag-MgO has been acquired.
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Fortes, A., Wood, I., Brodholt, J., Alfredsson, M., Vocadlo, L., McGrady, G. and Knight, K. (2003). A high-resolution neutron powder diffraction study of ammonia dihydrate (ND3 center dot 2D(2)O) phase I. Journal of Chemical Physics [Online] 119:10806-10813. Available at: http://dx.doi.org/10.1063/1.1619371.We have measured the thermal expansivity of ammonia dihydrate (ND3.O2D(2)O) phase I from 4.2 to 174 K at ambient pressure, and the incompressibility at 174 K from 0 to 0.45 GPa, using time-of-flight neutron powder diffraction. The unit cell volume as a function of temperature, V(T), was fitted with a Gruneisen approximation to the zero-pressure equation of state (with the lattice vibrational energy calculated from a double-Debye model fitted to heat capacity data) having the following parameters at zero pressure and temperature: V-0,V-0=356.464+/-0.005 Angstrom(3), (K-0,K-0/gamma)=7.163+/-0.024 GPa, and K'(0,0)=5.41+/-0.33 (where V-P,V-T is the unit cell volume at pressure P and temperature T, K-P,K-T is the isothermal bulk modulus, K'(P,T) is its first pressure derivative, and gamma is the Gruneisen ratio). The two Debye temperatures are theta(A)(D)=165+/-3 K and theta(D)(B)=729+/-4 K. The unit cell volume at 174 K as a function of pressure, V(P), was fitted with a third-order Birch-Murnaghan equation of state having the following parameters: V-0,V-174=365.69+/-0.16 Angstrom(3), K-0,K-174=7.02+/-0.25 GPa, and K'(0,174)=9.56+/-1.28. The volume thermal expansion coefficient, alpha(V), at 174 K and atmospheric pressure is 281.3x10(-6) K-1. The proton disorder manifested at high homologous temperatures is seen to be frozen in, on the time scale of these experiments, down to 4.2 K. A high-pressure polymorph of ammonia dihydrate was observed following melting of the sample at 179 K and 0.46 GPa. (C) 2003 American Institute of Physics.
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Cora, F., Alfredsson, M., Barker, C., Bell, R., Foster, M., Saadoune, I., Simperler, A. and Catlow, C. (2003). Modeling the framework stability and catalytic activity of pure and transition metal-doped zeotypes. Journal of Solid State Chemistry [Online] 176:496-529. Available at: http://dx.doi.org/10.1016/S0022-4596(03)00275-5.We present a thorough computational study of transition metal-doped zeolite and aluminophosphate (AIPO) frameworks. The structural and electronic chemistry of the dopants is examined with ab initio quantum mechanical calculations, and the results correlated with the Bronsted and Lewis acid strength, and with the redox potential of the dopant ions in the framework. The energetics of doping is provided, and is employed to analyze the mode of dopant incorporation, and its site ordering in the microporous framework. In total, 23 dopant ions are examined in the isostructural framework of chabasite and AlPO-34. These cover most of the isomorphous framework replacements known to occur experimentally, but also framework replacements that have not yet been achieved. In this case, ab initio modeling techniques are employed in a predictive way. Finally, we present a computational study of the alkene epoxidation on titanosilicates, that covers the whole catalytic cycle. (C) 2003 Elsevier Inc. All rights reserved.
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Catlow, C., French, S., Sokol, A., Alfredsson, M. and Bromley, S. (2003). Understanding the interface between oxides and metals. Faraday Discussions [Online] 124:185-203. Available at: https://doi.org/10.1039/b211661h.Computational methods based on DFT are applied to modelling structure and bonding in oxide supported metals. We examined three systems of contemporary interest: Cu/ZnO, Pd,Pt/ZrO2 and bimetallic transition metal clusters on silica supports.
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Saadoune, I., Cora, F., Alfredsson, M. and Catlow, C. (2003). Computational study of the structural and electronic properties of dopant ions in microporous AlPOs. 2. Redox catalytic activity of trivalent transition metal ions. Journal of Physical Chemistry B [Online] 107:3012-3018. Available at: https://doi.org/10.1021/jp027286.Periodic ab initio QM calculations are employed in order to study the structure and redox properties of Cr, Mn, Fe. and Co trivalent transition metal dopants in AlPO-34. Our results show that the local environment of Mn and Co dopants is a distorted tetrahedron due to Jahn-Teller distortions. The bonding between the Me-III dopants and the neighboring oxygens is ionic in nature, and explains the Lewis acidity of the Me-III ions. The replacement energy DeltaE(III) of the 3+ dopant ions in AlPO-34 increases linearly as a function of the Me-O bond distance. Finally, the calculated redox energies of the Me-II/Me-III couples indicate that, among the transition metal ions investigated, Fe is the most stable in the 3+ oxidation state, whereas Mn is the most stable as 2+ ion. Cr and Co, instead, have intermediate behavior and can switch more easily between the two oxidation states. These results contribute to elucidate the mechanistic details of catalytic processes occurring in MeAlPOs.
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Alfredsson, M. and Hermansson, K. (2002). OH frequency calculations for the hydroxylated MgO(001) surface. Molecular Simulation [Online] 28:663-681. Available at: http://dx.doi.org/10.1080/08927020290030198.We have performed periodic Hartree-Fock calculations for OH groups adsorbed on the MgO(001) surface considering different surface coverages. Six types of OH groups are discussed: OH-, OH, H+, H and hydrogen-bonded OH and H. It is found that when both OH and H are present on the surface, the two groups are best described as OH-. We suggest that the highest-frequency fundamental band (similar to3750 cm(-1) in the experimental OH spectrum) is assigned to OH- groups adsorbed on top of Mg2+, while H+ adsorbed on top of O2- give rise to the broader band at similar to3550 cm(-1).
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Alfredsson, M. and Catlow, C. (2002). A comparison between metal supported c-ZrO2 and CeO2. Physical Chemistry Chemical Physics [Online] 4:6100-6108. Available at: http://dx.doi.org/10.1039/b204680f.We present periodic density-functional theory calculations within the GGA Hamiltonian for monolayers of Pd and Pt supported on the {111} surfaces of cubic-ZrO2 and CeO2. We find that the adsorption energies for the Pd-layer on both the zirconia and ceria supports are ca. 200 kJ mol(-1), while the corresponding adsorption energies for the Pt-layers are ca. 400 kJ mol(-1). Our calculations also predict that the Pt Pt interactions are three times stronger (similar to300 kJ mol(-1)) than the corresponding Pd Pd interactions, explaining the different cluster shapes and metal growth modes observed experimentally. Our calculations indicate that the electronic structure of the support determines the geometrical structure of the interface: a tilting of ca. 20 compared to the underlaying ZrO2 substrate is identified, which is not observed for the CeO2 substrate. Furthermore, we propose that the bonding character between the metal ad-layers and the oxide supports is dominated by electrostatic forces, owing to the polarisation of the metal ad-layers; yet a weak hybridisation between the outermost oxygen ions and the metal ad-layers is also present. The CeO2 interfaces investigated show metallic properties as a result of metal induced gap states originating from the metal ad-layers, while the ZrO2 interfaces show a small band gap of ca. 0.1 eV.
Book section
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Cora, F., Alfredsson, M., Mallia, G., Middlemiss, D., Mackrodt, W., Dovesi, R. and Orlando, R. (2004). The performance of hybrid density functionals in solid state chemistry. In: Principles and Applications of Density Functional Theory in Inorganic Chemistry II. Berlin: Springer Berlin / Heidelberg, pp. 171-232. Available at: https://doi.org/10.1007/b97944.We examine the performance of hybrid (HF-DFT) exchange functionals within Density Functional Theory (DFT) in describing the properties of crystalline solids. Recent applications are reviewed, and an extensive set of new results presented on transition metal compounds. The features of the electronic density and of several calculated properties are examined as the weight of the HF (exact) exchange in the hybrid functional is increased. Clear trends emerge in the structural and electronic properties; in particular, HF exchange increases the electronic localisation. This features causes an increase in the ionicity of the materials, a systematic decrease of the lattice parameter and increase of the elastic constants and bulk moduli. When HF and standard (LDA or GGA) DFT solutions yield systematically errors with opposite sign with respect to experiment, the formulation of hybrid functionals improves the accuracy of the calculations. This is the case for band gaps, phonon spectra, magnetic coupling constants, and all properties that depend on the extent of electronic localisation at either perfect or defective lattice sites. A different weight of HF exchange is required to reproduce the experimental value of different observables; as a general rule, however, a higher fraction of HF exchange than the 20% optimised in the molecular B3LYP formulation is required in the solid state.
Conference or workshop item
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Caccami, M., Horgan, M., Marrocco, G., Alfredsson, M. and Batchelor, J. (2018). Development of a new class of on-skin radio-sensors boosted by thin polymer-based batteries. In: IMWS-AMP 2017 : IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes. Italy: IEEE. Available at: http://dx.doi.org/10.1109/IMWS-AMP.2017.8247436.Conductive polymers are currently collecting interest for the development of low-profile eco-friendly and biocompatible non-metallic batteries capable of providing a local power source for the next-generation flexible body-integrated electronics. In this contribution, we demonstrate the feasibility of an organic ultrathin and multilayered polymer-based battery integrating a radiofrequency identification (RFID) tag antenna by means of an electromagnetic characterization in UHF band of the polymeric films. The optimally modeling of the battery plus the antenna aimed to develop a new class of shape-conformable radio-sensors suitable to adhere to the skin as a tattoo as well as a plaster. A prototype of the device was manufactured and its communication performances were characterized through the measurement of the realized gain of the tag attached directly onto a volunteer's skin.
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Ow, J., Musso, F., Bertuzzo, M., Alfredsson, M. and Corà , F. (2015). Doped apatites for novel solid oxide fuel cell applications. In: ECS Conference on Electrochemical Energy Conversion & Storage With SOFC-XIV - 14th International Symposium on Solid Oxide Fuel Cells, SOFC 2015. Electrochemical Society Inc., pp. 529-537. Available at: http://dx.doi.org/10.1149/06801.0529ecst.The aim of this study is to gain insight into the local coordination of doped apatites (e.g. La8Y2Ge6O27) using the GULP (General Utility Lattice Program) code. Apatite ceramics are believed to facilitate oxygen conduction by rearranging its local structure in the presence of oxygen excess. By understanding the oxygen conduction mechanism we can design materials with improved functionality. The objectives of this study are twofold: firstly, to investigate the energetics of doping in the Ge apatites and secondly, to further investigate the interstitial-oxide migration pathway and energy barriers involved in the migration. It has been found that isovalent doping with ions of smaller ionic radii than Ge4+, e.g. Si4+ and Ti4+, are favourable, as is hypervalent doping on the Sr site, leading to an oxide ion excess in the structure. The excess oxide ion migration pathway takes place via a wheel mechanism with an energy barrier of 0.66 eV. This mechanism is different from the one proposed in Si-based apatites, which is done by a sinusoidal trajectory. © The Electrochemical Society.
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Woods, M., Rakibet, O., Young, P., Alfredsson, M. and Batchelor, J. (2014). Integrated Antenna-Battery for Low-profile Short Range Communications. In: EuCAP 2014. Available at: http://www.eucap2014.org/.An integrated antenna-battery was designed in order to function at 2.45GHz, with short range Bluetooth applications considered. The model was derived from a previous study in which an integrated system was first explored. The tan ? was investigated for the substrate and this new structure was found to give favourable gains and efficiencies compared to the previous study. Experimental parameters were also applied to the model in the presence of a dielectric material. These experimental
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Dove, M., White, T., Walker, A., Bruin, R., Austen, K., Artacho, E., Sullivan, L., Calleja, M., Tucker, M., Tyer, R., Couch, P., Van Dam, K., Allan, R., Todorov, I., Chapman, C., Emmerich, W., Marmier, A., Parker, S., Blanchard, M., Catlow, C., Du, Z., De Leeuw, N., Lewis, G., Alexandrov, V., Alfredsson, M., Brodholt, J. and Murray-Rust, P. (2006). Computational grids for mid-sized collaborative projects: The eMinerals experience. In: ARRAY(0x7ffa4013ea88). Available at: http://dx.doi.org/10.1109/E-SCIENCE.2006.261179.Grid computing has the potential to revolutionise how small groups of simulation scientists work together to tackle new science problems. In this paper we report how the eMinerals project has developed a small scale integrated compute and data grid infrastructure - the eMinerals minigrid - and developed generic job submission tools that exploit this infrastructure and which enable the science users to also access other grid systems. © 2006 IEEE.
Thesis
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Woods, M. (2016). A Novel Approach to Design an Integrated Antenna-Battery System.In this study, an integrated antenna-battery was explored. Studying the systems separately allowed information to be obtained relating to the materials' performance and feasibility of an integrated system.
Conducting polymers are promising in modern day lithium ion batteries. With high electrical conductivity as well as good ionic conductivity, they are now becoming more widely used. Here, we present a study of a co-block polymer (PEDOT-PEG) in which a polymer with high electrical conductivity is linked to a polymer with lithium ion conductivity, using a combination of atomistic simulations and experiments.
Simulations showed that the diffusion and ionic conductivity for PEDOT-PEG agreed well with experiments. A trend was identified as a function of lithium salt concentration, in which the ionic conductivity decreased with increasing concentration. This was identified to be down to the significant ion pairing occurring in the system between lithium and the counterion.
Requirements for the antenna were the ability to be mounted easily onto a battery substrate without a significant loss in efficiency and bandwidth. Studies were undertaken in which a slot dipole antenna was modified so as to incorporate properties more closely associated with battery materials i.e. permittivity and dielectric loss. An ultra-thin Mylar prototype was also synthesised and mounted onto a variety of surfaces, to assess how the antenna performed in different environments.
Results for the antenna showed usable bandwidths and efficiencies when the antenna structure was modified to closely resemble a solid state battery. Despite a reduction seen in certain cases, these losses were not significant, and showed promise with regards to designing an integrated system. The Mylar prototype showed a good match between simulation and experiment in free space and when mounted on surfaces such as polymers, indicating that an ultra-thin antenna-battery is feasible.
Forthcoming
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Blidberg, A., Valvo, M., Alfredsson, M., Tengstedt, C., Gustafsson, T. and Björefors, F. (2019). Electronic changes in poly(3,4-ethylenedioxythiophene)-coated LiFeSO4F during electrochemical lithium extraction. Journal of Power Sources [Online] 418:84-89. Available at: https://doi.org/10.1016/j.jpowsour.2019.02.039.The redox activity of tavorite LiFeSO4F coated with poly (3,4-ethylenedioxythiophene), i.e. PEDOT, is investigated by means of several spectroscopic techniques. The electronic changes and iron-ligand redox features of this LiFeSO4F-PEDOT composite are probed upon delithiation through X-ray absorption spectroscopy. The
PEDOT coating, which is necessary here to obtain enough electrical conductivity for the electrochemical reactions of LiFeSO4F to occur, is electrochemically stable within the voltage window employed for cell cycling.
Although the electronic configuration of PEDOT shows also some changes in correspondence of its reduced and
oxidized forms after electrochemical conditioning in Li half-cells, its p-type doping is fully retained between 2.7
and 4.1 V with respect to Li+/Li during the first few cycles. An increased iron-ligand interaction is observed in
LixFeSO4F during electrochemical lithium extraction, which appears to be a general trend for polyanionic insertion compounds. This finding is crucial for a deeper understanding of a series of oxidation phenomena in Liion battery cathode materials and helps paving the way to the exploration of new energy storage materials with
improved electrochemical performances
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