Portrait of Dr Silvia Ramos

Dr Silvia Ramos

Senior Lecturer in Materials Physics
Director of Graduate Studies

About

Dr Silvia Ramos studied her degree at the University of Salamanca in Spain. She spent the last year of her degree at the University of Bristol as an Erasmus student and stayed there for her PhD (1997-2001), funded by a Marie Curie Research Fellowship and supervised by Dr G. W. Neilson. She then moved to Grenoble in France to work at the European Synchrotron Radiation Facility as a post-doctoral researcher. 

In 2004 Silvia returned to the UK to work as a post-doctoral research fellow at the University of Birmingham with Professor Ted Forgan. In 2008 she became a beamline scientist at Diamond Light Source, where she was part of the scientific team that built, commissioned and operated one of the X-ray spectroscopy instruments at the synchrotron. Silvia joined the School of Physical Sciences at Kent in 2013, taking the position of Lecturer in Materials.

Research interests

Dr Silvia Ramos's research interests are mainly in the area of strongly correlated electron systems. Within this broad research area, she studies the correlation between structural and electronic properties using X-ray, neutron and muon techniques. The complementarity of the different microscopic probes available at large facilities makes them powerful tools from which much can be learnt. 

Some specific examples of areas in which Sylvia is actively working at the moment are: 

  • the properties of the Ruddlesden-Popper series of rhuthenate materials, iron pnictides and oxychalcogenides
  • the competition between ordered states at interfaces 
  • the local structure of new functional materials by XAS.

Teaching

Sylvia teaches on a number of physics modules including electromagnetism and optics and the undergraduate physics research project.

Publications

Article

  • Somerville, J. et al. (2018). Identification and Characterisation of High Energy Density P2-Type Na2/3[Ni1/3-y/2Mn2/3-y/2Fey]O2 Compounds for Na-ion Batteries. Journal of Materials Chemistry A [Online]. Available at: http://dx.doi.org/10.1039/x0xx00000x.
    The composition space between MnO2, NaFeO2, and Na[Ni1/2Mn1/2]O2 has been explored with the goal of identifying Earth-abundant single-phase P2 cathode materials. This has led to the identification of two compounds, P2 Na2/3[Ni1/3-y/2Mn2/3-y/2Fey]O2 (y = 1/6, 1/3) which exhibit state of the art specific energies. These materials were further evaluated through galvanostatic cycling and x-ray absorption spectroscopy.
  • Maitra, U. et al. (2018). Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2. Nature Chemistry [Online] 10:288-295. Available at: https://doi.org/10.1038/nchem.2923.
    The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li+–O(2p)–Li+ interactions). Na2/3[Mg0.28Mn0.72]O2 exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg2+ resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na2/3[Mg0.28Mn0.72]O2 does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg2+ remains in Na2/3[Mg0.28Mn0.72]O2, which stabilizes oxygen.
  • Vera Stimpson, L. et al. (2017). Investigation of the role of morphology on the magnetic properties of Ca2Mn3O8 materials. Dalton Transactions [Online] 46:14130-14138. Available at: http://dx.doi.org/10.1039/C7DT03053C.
    Ca2Mn3O8 exhibits a complex layered structure comprised of Mn3O84? layers separated by Ca2+ ions. In contrast with the more traditional triangular delafossite layered materials the Mn3O84? layers additionally exhibit an ordered vacancy, which forms a ‘bow-tie’ like arrangement of the Mn4+ ions. We report a comprehensive study of the magnetic properties of a series of Ca2Mn3O8 materials with different morphologies. EXAFS and XANES analysis confirm no differences in either manganese environment or oxidation state between materials. Apparent differences in magnetic order from SQUID magnetometry can be rationalised by uncompensated surface spins arising as a result of changes to the surface to volume ratio between morphologies. Furthermore, these data suggest these materials are potentially frustrated in nature, due to the triangular connectivity of Mn4+ spins, with a simple ‘spin-up/spin-down’ (??) antiferromagnetic model unable to explain the data collected.
  • Düvel, A. et al. (2017). Is Geometric Frustration-Induced Disorder a Recipe for High Ionic Conductivity? Journal of the American Chemical Society [Online] 139:5842-5848. Available at: https://doi.org/10.1021/jacs.7b00502.
    Ionic conductivity is ubiquitous to many industrially important applications such as fuel cells, batteries, sensors, and catalysis. Tunable conductivity in these systems is therefore key to their commercial viability. Here, we show that geometric frustration can be exploited as a vehicle for conductivity tuning. In particular, we imposed geometric frustration upon a prototypical system, CaF2, by ball milling it with BaF2, to create nanostructured Ba1–xCaxF2 solid solutions and increased its ionic conductivity by over 5 orders of magnitude. By mirroring each experiment with MD simulation, including “simulating synthesis”, we reveal that geometric frustration confers, on a system at ambient temperature, structural and dynamical attributes that are typically associated with heating a material above its superionic transition temperature. These include structural disorder, excess volume, pseudovacancy arrays, and collective transport mechanisms; we show that the excess volume correlates with ionic conductivity for the Ba1–xCaxF2 system. We also present evidence that geometric frustration-induced conductivity is a general phenomenon, which may help explain the high ionic conductivity in doped fluorite-structured oxides such as ceria and zirconia, with application for solid oxide fuel cells. A review on geometric frustration [ Nature 2015, 521, 303] remarks that classical crystallography is inadequate to describe systems with correlated disorder, but that correlated disorder has clear crystallographic signatures. Here, we identify two possible crystallographic signatures of geometric frustration: excess volume and correlated “snake-like” ionic transport; the latter infers correlated disorder. In particular, as one ion in the chain moves, all the other (correlated) ions in the chain move simultaneously. Critically, our simulations reveal snake-like chains, over 40 Å in length, which indicates long-range correlation in our disordered systems. Similarly, collective transport in glassy materials is well documented [for example, J. Chem. Phys. 2013, 138, 12A538]. Possible crystallographic nomenclatures, to be used to describe long-range order in disordered systems, may include, for example, the shape, length, and branching of the “snake” arrays. Such characterizations may ultimately provide insight and differences between long-range order in disordered, amorphous, or liquid states and processes such as ionic conductivity, melting, and crystallization.
  • Chadwick, A. et al. (2017). An X-Ray Absorption Spectroscopy Study of Ball-Milled Lithium Tantalate and Lithium Titanate Nanocrystals. IOP Conference Series: Materials Science and Engineering [Online] 169:12015. Available at: https://doi.org/10.1088/1757-899X/169/1/012015.
  • Luo, K. et al. (2016). Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li0.2Ni0.2Mn0.6]O2. Journal of the American Chemical Society [Online] 138:11211-11218. Available at: http://doi.org/10.1021/jacs.6b05111.
    Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g., Mn3+/4+ in LiMn2O4, and this limits the energy storage of Li-ion batteries. Compounds such as Li[Li0.2Ni0.2Mn0.6]O2 exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V versus Li+/Li. The capacity at 4.5 V is dominated by oxidation of the O2– anions accounting for ?0.43 e–/formula unit, with an additional 0.06 e–/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V is mainly O loss, ?0.08 e–/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn4+/Li+). The results have been obtained by combining operando electrochemical mass spec on 18O labeled Li[Li0.2Ni0.2Mn0.6]O2 with XANES, soft X-ray spectroscopy, resonant inelastic X-ray spectroscopy, and Raman spectroscopy. Finally the general features of O redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal–oxygen interaction on anion redox in lithium rich cathode materials.
  • Lester, C. et al. (2015). Field-tunable spin-density-wave phases in Sr3Ru2O7. Nature Materials [Online] 14:373-378. Available at: http://doi.org/10.1038/nmat4181.
    The conduction electrons in a metal experience competing interactions with each other and the atomic nuclei. This competition can lead to many types of magnetic order in metals1. For example, in ?chromium2 the electrons order to form a spin-density-wave (SDW) antiferromagnetic state. A magnetic field may be used to perturb or tune materials with delicately balanced electronic interactions. Here, we show that the application of a magnetic field can induce SDW magnetic order in a quasi-2D metamagnetic metal, where none exists in the absence of the field. We use magnetic neutron scattering to show that the application of a large (B ? 8 T) magnetic field to the perovskite metal Sr3Ru2O7 (refs 3, 4, 5, 6, 7) can be used to tune the material through two magnetically ordered SDW states. The ordered states exist over relatively small ranges in field (?0.4 T), suggesting that their origin is due to a new mechanism related to the electronic fine structure near the Fermi energy, possibly combined with the stabilizing effect of magnetic fluctuations8, 9. The magnetic field direction is shown to control the SDW domain populations, which naturally explains the strong resistivity anisotropy or ‘electronic nematic’ behaviour observed5, 6 in this material.
  • Chadwick, A. et al. (2015). X-Ray Absorption Spectroscopy and Computer Modelling Study of Nanocrystalline Binary Alkaline Earth Fluorides. IOP Conference Series: Materials Science and Engineering [Online] 80:12005. Available at: http://doi.org/10.1088/1757-899X/80/1/012005.
    Nanocrystalline samples of Ba1-xCaxF2 prepared by high-energy milling show an unusually high F? ion conductivity, which exhibit a maximum in the magnitude and a minimum in the activation energy at x = 0.5. Here, we report an X-ray absorption spectroscopy (XAS) at the Ca and Sr K edges and the Ba L3 edge and a molecular dynamics (MD) simulation study of the pure and mixed fluorides. The XAS measurements on the pure binary fluorides, CaF2, SrF2 and BaF2 show that high-energy ball-milling produces very little amorphous material, in contrast to the results for ball milled oxides. XAS measurements of Ba1-xCaxF2 reveal that for 0 < x <1 there is considerable disorder in the local environments of the cations which is highest for x = 0.5. Hence the maximum in the conductivity corresponds to the composition with the maximum level of local disorder. The MD calculations also show a highly disordered structure consistent with the XAS results and similarly showing maximum disorder at x = 0.5.
  • Liu, X. et al. (2015). Structural chemistry and magnetic properties of Y2CoGe4O12. Journal of Solid State Chemistry [Online] 228:183-188. Available at: http://doi.org/10.1016/j.jssc.2015.04.033.
    Polycrystalline Y2CoGe4O12 has been prepared by standard ceramic methods. The crystal structure (space group P4/nbm; a=9.8465(2), c=4.92986(9) Å) consists of metal-rich layers separated from each other by Ge4O12 groups comprised of four corner-sharing GeO4 tetrahedra. Two cation sites lie within the layers; an eight-coordinate site occupied by yttrium and a six-coordinate site occupied by a 1:1 disordered distribution of yttrium and cobalt. Neutron diffraction revealed two-fold disorder on the oxide sublattice; this has been elucidated using Co K-edge EXAFS spectroscopy. The availability of two sites allows each oxide ion to accommodate the coordination preferences of its single Co/Y neighbour; the GeO4 tetrahedra distort to absorb any consequent strain. The octahedron of anions around each Co2+ cation shows a pseudo-tetragonal distortion with a strain (Co–O)eq–(Co–O)ax/(Co–O)eq=?0.173. This results in an unusually large effective magnetic moment of 6.05 µB per Co2+ cation.
  • Sun, H. et al. (2015). Soft Chemical Control of Superconductivity in Lithium Iron Selenide Hydroxides Li1–xFex(OH)Fe1–ySe. Inorganic Chemistry [Online] 54:1958-1964. Available at: http://doi.org/10.1021/ic5028702.
    Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li1–xFex(OH)Fe1–ySe (x ? 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li1–xFex(OH) reservoir layer to fill vacancies in the selenide layer.
  • Blandy, J. et al. (2015). Soft chemical control of the crystal and magnetic structure of a layered mixed valent manganite oxide sulfide. APL Materials [Online] 3:41520. Available at: http://dx.doi.org/10.1063/1.4918973.
    Oxidative deintercalation of copper ions from the sulfide layers of the layered mixed-valent manganite oxide sulfide Sr2MnO2 Cu 1.5S2 results in control of the copper-vacancy modulated superstructure and the ordered arrangement of magnetic moments carried by the manganese ions. This soft chemistry enables control of the structures and properties of these complex materials which complement mixed-valent perovskite and perovskite-related transition metal oxides.
  • Cassidy, S., Ramos, S. and Clarke, S. (2014). Local Structure of Sodium- and Iron-deintercalated NaFeAs. Zeitschrift für anorganische und allgemeine Chemie [Online] 640:2889-2896. Available at: http://doi.org/10.1002/zaac.201400343.
    The local structures around Fe and As were probed using X-ray absorption spectroscopy in metastable derivatives of NaFeAs obtained by deintercalation of the metals with iodine in THF at room temperature. The deintercalated product with a defective ThCr2Si2 structure is unattainable through conventional high temperature routes due to the size-mismatch between the eight-coordinate site and the Na+ radius in the ThCr2Si2 structure type, and is poorly crystalline. X-ray absorption Near Edge Structure (XANES) measurements show a shift of the Fe K-edge consistent with oxidation of iron to the +3 oxidation state. Changes to the features in the XANES at the As K-edge and analysis of the Extended X-ray Absorption Fine Structure (EXAFS) data from both Fe and As K-edges show that the local structures of Fe and As may only be modelled adequately using a highly defective ThCr2Si2 structural model. This shows that deintercalation of both Na and Fe from NaFeAs occurs to produce NaFe1.7As2, which shares local structural features with the related iron selenides K0.8Fe1.6Se2.
  • Corkett, A. et al. (2014). Control of the superconducting properties of Sr2-xCa xVO3FeAs through isovalent substitution. Journal of Solid State Chemistry [Online] 216:91-98. Available at: http://dx.doi.org/10.1016/j.jssc.2014.02.024.
    The effect of the isovalent substitution of Sr2+ by Ca 2+ on the structure and superconducting properties of Sr 2-xCaxVO3FeAs is described in the compositional range 0 â?¤ x â?¤ 0.5. SQUID magnetometry measurements reveal that after an initial increase in Tc, which is maximised at 29.5 K in Sr 1.95Ca0.05VO3FeAs, a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr 2-xCaxVO3FeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in Tc with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with Tc maximised in samples with regular FeAs4-tetrahedra.
  • Dent, A. et al. (2013). Performance of B18, the core EXAFS bending magnet beamline at diamond. Journal of Physics: Conference Series [Online] 430:12023. Available at: http://dx.doi.org/10.1088/1742-6596/430/1/012023.
    B18 has been operational since April 2010 and has hosted a total of 63 EXAFS user experiments up to Jan 2012. B18 is contributing to research programs across a wide range of scientific disciplines, e.g. solid state physics and materials, catalysis, chemistry, soft matter, surfaces and biomaterials. We present a review of its present performance and capabilities.
  • Adroja, D. et al. (2013). Competing4f-electron dynamics in Ce(Ru1?xFex)2Al10(0?x?1.0): Magnetic ordering emerging from the Kondo semiconducting state. Physical Review B [Online] 87:224415. Available at: http://doi.org/10.1103/PhysRevB.87.224415.
    We have carried out muon spin relaxation (?SR), neutron diffraction, and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru1?xFex)2Al10 (x=0, 0.3, 0.5, 0.8, and 1) to investigate the nature of the ground state (magnetic ordered versus paramagnetic) and the origin of the spin-gap formation as evident from the bulk measurements in the end members. Our zero-field ?SR spectra clearly reveal coherent two-frequency oscillations at low temperature in x=0, 0.3, and 0.5 samples, which confirm the long-range magnetic ordering of the Ce moment with Nèel temperature TN=27, 26, and 21 K, respectively. On the other hand, the ?SR spectra of x=0.8 and x=1 down to 1.4 K and 0.045 K, respectively, exhibit a temperature-independent Kubo-Toyabe term, confirming a paramagnetic ground state. The long-range magnetic ordering in x=0.5 below 21 K has been confirmed through the neutron diffraction study. INS measurements of x=0 clearly reveal the presence of a sharp inelastic excitation near 8 meV between 5 K and 26 K, due to an opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. Interestingly, at 4.5 K, the spin-gap excitation broadens in x=0.3 and exhibits two clear peaks at 8.4(3) and 12.0(5) meV in x=0.5. In the x=0.8 sample, which remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of 10–12 meV at 7 K, with a strong wave-vector–dependent intensity. Evidence of a spin gap of 12.5(5) meV has also been found in x=1. The observation of a spin gap in the paramagnetic samples (x=0.8 and 1) is an interesting finding in this study, and it challenges our understanding of the origin of the semiconducting gap in CeT2Al10 (T = Ru and Os) compounds in terms of a hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.
  • Daley, T. et al. (2013). Tracking the formation of nano-sized zinc oxide from zinc peroxide by in situ XAS and XRD. Journal of Physics: Conference Series - Sensors and their Applications XIII [Online] 430:12080. Available at: http://dx.doi.org/10.1088/1742-6596/430/1/012080.
    In this paper we show how the formation of zinc oxide from zinc peroxide takes place during thermal decomposition, through a combination of X-ray Absorption Spectroscopy (XAS) and X-ray Diffraction (XRD). The time-resolved study allowed us to follow the changes in both the local structure of the material, in particular the conversion of octahedral Zn-O coordination to tetrahedral one through EXAFS and XANES analysis and at the same time the particle growth using both EXAFS and diffraction. Multi-cluster analysis provided information on the phase changes which could be cross-correlated with diffraction results which suggests an abrupt change in the long range coordination followed by a slow growth of zinc oxide NPs. This study allowed us to gain a detailed understanding of zinc peroxide decomposition leading to the formation of zinc oxide.
  • Thammajak, N. et al. (2012). Structural chemistry and spin-glass behaviour of Nd18Li8Fe4TiO39. Journal of Solid State Chemistry [Online] 187:75 - 82. Available at: https://doi.org/10.1016/j.jssc.2011.12.029.
  • Shiroka, T. et al. (2012). ?sR studies of superconductivity in eutectically grown mixed ruthenates. Physical Review B - condensed matter and materials physics [Online] 85:134527. Available at: http://dx.doi.org/10.1103/PhysRevB.85.134527.
    The low-temperature magnetic behavior of the double-layered ruthenate Sr 3Ru 2O 7, as grown from a eutectic Sr 2RuO 4-Sr 3Ru 2O 7 system, was investigated via zero- and transverse-field muon-spin rotation. The gradual increase of the muon relaxation rate observed below 2.5 K, even in the absence of applied magnetic fields, indicates the occurrence of a spontaneous breaking of time-reversal symmetry. The onset of the latter at a temperature above 1.5 K, the T c of the single phase Sr 2RuO 4, provides evidence about an unconventional superconducting state in the eutectic phase, which most likely takes place at the interface between the Sr 2RuO 4 and Sr 3Ru 2O 7 domains, or even inside the Sr 3Ru 2O 7 phase. We show that the superconducting state manifests a two-component behavior in the transverse-field response with change-over at about T=2.5 K and T=1.5 K. The comparison with zero-field μSR data in the Ru-Sr 2RuO 4 eutectic system rules out the possibility of spurious effects due to embedded Ru islands.
  • Muñoz-Páez, A. et al. (2011). Reflected extended X-ray absorption fine structure study of the surface region of layered samples. Diamond Light Source Proceedings [Online] 1:e126. Available at: https://doi.org/10.1017/S2044820110000912.
  • Dixon, E. et al. (2011). Mn(I) in an Extended Oxide: The Synthesis and Characterization of La1-xCaxMnO2+d(0.6 = x = 1). Journal of the American Chemical Society [Online] 133:18397-18405. Available at: http://dx.doi.org/10.1021/ja207616c.
    Reduction of La1–xCaxMnO3 (0.6 ? x ? 1) perovskite phases with sodium hydride yields materials of composition La1–xCaxMnO2+?. The calcium-rich phases (x = 0.9, 1) adopt (La0.9Ca0.1)0.5Mn0.5O disordered rocksalt structures. However local structure analysis using reverse Monte Carlo refinement of models against pair distribution functions obtained from neutron total scattering data reveals lanthanum-rich La1–xCaxMnO2+? (x = 0.6, 0.67, 0.7) phases adopt disordered structures consisting of an intergrowth of sheets of MnO6 octahedra and sheets of MnO4 tetrahedra. X-ray absorption data confirm the presence of Mn(I) centers in La1–xCaxMnO2+? phases with x < 1. Low-temperature neutron diffraction data reveal La1–xCaxMnO2+? (x = 0.6, 0.67, 0.7) phases become antiferromagnetically ordered at low temperature.
  • Díaz-Moreno, S., Ramos, S. and Bowron, D. (2011). Solvation Structure and Ion Complexation of La3+ in a 1 Molal Aqueous Solution of Lanthanum Chloride. The Journal of Physical Chemistry A [Online] 115:6575-6581. Available at: http://dx.doi.org/10.1021/jp202961t.
    H/D isotopic substitution neutron scattering and X-ray scattering have been used to investigate the short and intermediate range solution structure in a 1 m aqueous solution of lanthanum chloride. To improve the reliability of the local structural information on the cation environment, information has been incorporated from Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy data into the applied analytical framework. The markedly different structural sensitivities of the experimental probes allow the construction of a detailed three-dimensional atomistic model using the Empirical Potential Structure Refinement (EPSR) technique. The results show that at the investigated concentration La3+ is hydrated by eight water molecules and one chloride ion, forming an inner-sphere ion complex in which the water molecules maintain angular configurations consistent with a tricapped trigonal prism configuration. This local geometry considerably disrupts the bulk solvent structure.
  • Dixon, E. et al. (2011). Mn(I) in an Extended Oxide: The Synthesis and Characterization of La1-xCaxMnO2+? (0.6 ? x ? 1). Journal of the American Chemical Society [Online] 133:18397-18405. Available at: http://dx.doi.org/10.1021/ja207616c.
  • Ramos, S. et al. (2010). B18: What can the new core X-ray absorption spectroscopy beamline at Diamond do? Diamond Light Source Proceedings [Online] 1:e120. Available at: https://doi.org/10.1017/S2044820110000560.
    The new Core-XAS (X-ray absorption spectroscopy) beamline (B18) at Diamond aims to provide a reliable spectrometer for a broad scientific community. With this in mind, B18 has been built as a general-purpose beamline and offers to users a variety of sample environments and detection methods. Here we will present the first commissioning results and some of the capabilities of this versatile instrument.
  • Sardar, K. et al. (2010). Nanocrystalline Cerium?Bismuth Oxides: Synthesis, Structural Characterization, and Redox Properties. Chemistry of Materials [Online] 22:6191-6201. Available at: http://dx.doi.org/10.1021/cm1025848.
  • López-Flores, V. et al. (2009). Development of ReflEXAFS data analysis for deeper surface structure studies. Journal of Physics: Conference Series [Online] 190:12110. Available at: https://doi.org/10.1088/1742-6596/190/1/012110.
    An analytical approach to the analysis of ReflEXAFS data collected from complex multilayer samples, at a range of angles above and below the critical angle is presented. The aim of the technique is to generate a structural model of the investigated system that is consistent with the variable depth sensitivity of the experimental data. The procedure follows three main steps (i) the determination of the free atom reflectivity background for the multilayer system, (ii) the estimation of the depth dependent EXAFS signals and (iii) the calculation of the corresponding ReflEXAFS components. By iterating between steps (ii) and (iii), and varying the estimates of the EXAFS signals, a consistent set of structural parameters is extracted that reflects the bulk structure of the multilayer system through the basic reflectivity signals, and the depth dependent local atomic structure through the estimated EXAFS components. An example of the depth dependent structure of copper in a copper-chromium multilayer stack is presented to illustrate the capabilities of the method.
  • Laver, M. et al. (2009). Structure and degeneracy of vortex lattice domains in pure superconducting niobium: A small-angle neutron scattering study. Physical Review B - Condensed Matter and Materials Physics [Online] 79. Available at: https://doi.org/10.1103/PhysRevB.79.014518.
    High-purity niobium exhibits a surprisingly rich assortment of vortex lattice (VL) structures for fields applied parallel to a fourfold symmetry axis, with all observed VL phases made up of degenerate domains that spontaneously break some crystal symmetry. Yet a single regular hexagonal VL domain is observed at all temperatures and fields parallel to a threefold symmetry axis. We report a detailed investigation of the transition between these lush and barren VL landscapes, discovering new VL structures and phase transitions at high fields. We show that the number and relative population of VL domains is intrinsically tied to the underlying crystal symmetry. We discuss how subtle anisotropies of the crystal may generate the remarkable VLs observed. © 2009 The American Physical Society.
  • Dent, A. et al. (2009). B18: A core XAS spectroscopy beamline for Diamond. [Online] 190. Available at: https://doi.org/10.1088/1742-6596/190/1/012039.
    During the past twenty years, XAS has progressed from being a technique only suitable for specialists to become a widely applicable tool. This situation has resulted from the steady development of reliable spectrometers and new generations of software for data analysis. B18 will be a general purpose XAS beamline on Diamond. It will cover a wide energy range (2 to 35 keV), with a monochromator designed to carry out both conventional and QEXAFS measurements. The main design novelty is that the low and high energy optic branches will run in parallel and the appropriate branch for a given experiment will be selected by changing the position of the slits, instead of by moving the optical elements. This will allow us to develop a very high level of automation in the operation of the instrument. Detection systems will include transmission, fluorescence and electron yield. Experience shows that considerable value is added by combining techniques. Therefore provision has been made for wide angle X-ray diffraction studies to be incorporated into the beamline architecture. The instrument will offer a variety of sample environments and the flexibility to integrate set-ups designed by the users. Hence, B18 will be able to contribute to research programs across a wide range of scientific disciplines, e.g. solid state physics and materials, catalysis, chemistry, soft matter, surfaces and biomaterials. The instrument will open to first users in April 2010. © 2009 IOP Publishing Ltd.
  • Zeidler, A. et al. (2009). Establishing the structure of GeS 2 at high pressures and temperatures: a combined approach using x-ray and neutron diffraction. Journal of Physics: Condensed Matter [Online] 21:474217. Available at: https://doi.org/10.1088/0953-8984/21/47/474217.
  • López-Flores, V. et al. (2007). Optimized end station and operating protocols for reflection extended x-ray absorption fine structure (ReflEXAFS) investigations of surface structure at the European Synchrotron Radiation Facility beamline BM29. Review of Scientific Instruments [Online] 78. Available at: https://doi.org/10.1063/1.2409763.
    The development of the capability to engineer the surface properties of materials to match specific requirements demands high quality surface characterization techniques. The ideal tool should provide chemically specific structural characterization as well as surface sensitivity and depth profiling. Ideally the characterization method should also be applicable to systems both with and without long range order. X-ray absorption spectroscopy fine structure, when using the standard transmission detection system, provides all this information with the significant exception of surface sensitivity. In contrast, by detecting the reflected instead of the transmitted beam, it encompasses all these requirements because when the incident beam impinges onto a sample surface at glancing angles, in conditions close to the total reflection, only the outermost regions of the system under study are sampled. Such a technique provides information about the local structure as a function of depth as well as thin layer structure in the case of layered samples. Although it is potentially the ideal tool to study surface modified materials, experimental difficulties have hampered its widespread use in the fields of surface and materials sciences. As a solution to the experimental challenges, we provide a detailed description of an appropriate experimental station, the sample requirements, the measuring protocols, and software routines needed to optimize the collection of the data. To illustrate the capabilities of the technique the results obtained for a model multilayer sample are presented and analyzed under the total external reflection approximation. © 2007 American Institute of Physics.
  • Ramos, S. et al. (2007). Spin dynamics in Sr3 Ru2 O7 near the metamagnetic transition by inelastic neutron scattering. Physica B: Condensed Matter [Online] 403:1270-1272. Available at: https://doi.org/10.1016/j.physb.2007.10.123.
    The bilayered ruthenate Sr3 Ru2 O7 has a metamagnetic quantum critical point. This is the first example of a material with a quantum-critical end-point. This work addresses the problem of the metamagnetic criticality in this system through the measurement of spin fluctuations by inelastic neutron scattering. Our results show that incommensurate antiferromagnetic fluctuations are present in a wide range of applied fields. We have also observed how ferromagnetic fluctuations develop at the metamagnetic field. Finally, the intensity of the two types of fluctuations is followed as a function of the energy transfer between the neutron and the spins. © 2007 Elsevier B.V. All rights reserved.
  • Laver, M. et al. (2006). Spontaneous symmetry-breaking vortex lattice transitions in pure niobium. Physical Review Letters [Online] 96. Available at: https://doi.org/10.1103/PhysRevLett.96.167002.
    We report an extensive investigation of magnetic vortex lattice (VL) structures in single crystals of pure niobium with the magnetic field applied parallel to a fourfold symmetry axis, so as to induce frustration between the cubic crystal symmetry and hexagonal VL coordination expected in an isotropic situation. We observe new VL structures and phase transitions; all the VL phases observed (including those with an exactly square unit cell) spontaneously break some crystal symmetry. One phase even has the lowest possible symmetry of a two-dimensional Bravais lattice. This is quite unlike the situation in high-Tc or borocarbide superconductors, where VL structures orient along particular directions of high crystal symmetry. The causes of this behavior are discussed. © 2006 The American Physical Society.
  • Ansell, S. et al. (2006). X-ray and neutron scattering studies of the hydration structure of alkali ions in concentrated aqueous solutions. Biophysical Chemistry [Online] 124:171-179. Available at: https://doi.org/10.1016/j.bpc.2006.04.018.
    The presence of ions in water provides a rich and varied environment in which many natural processes occur with important consequences in biology, geology and chemistry. This article will focus on the structural properties of ions in water and it will be shown how the 'difference' methods of neutron diffraction with isotopic substitution (NDIS) and anomalous X-ray diffraction (AXD) can be used to obtain direct information regarding the radial pair distribution functions of many cations and anions in solution. This information can subsequently be used to calculate coordination numbers and to determine ion-water conformation in great detail. As well as enabling comparisons to be made amongst ions in particular groups in the periodic table, such information can also be contrasted with results provided by molecular dynamics (MD) simulation techniques. To illustrate the power of these 'difference' methods, reference will be made to the alkali group of ions, all of which have been successfully investigated by the above methods, with the exception of the radioactive element francium. Additional comments will be made on how NDIS measurements are currently being combined with MD simulations to determine the structure around complex ions and molecules, many of which are common in biological systems. © 2006 Elsevier B.V. All rights reserved.
  • Charalambous, D. et al. (2006). Driven vortices in type-II superconductors: A muon spin rotation study. Physical Review B - Condensed Matter and Materials Physics [Online] 73. Available at: https://doi.org/10.1103/PhysRevB.73.104514.
    We present a detailed study of vortex motion in a type-II superconductor using the muon spin rotation (μSR) technique. The vortices were set in motion by an alternating transport current. By adjusting the frequency and amplitude of the driving force so that vortices "visible" to the implanted muons did not cross the sample boundaries, a μSR line shape was obtained corresponding to almost perfectly ordered vortex motion. We also observed sidebands to the μSR line shape which correspond to the frequency at which vortices pass the implanted muon, and allow a direct measurement of the vortex velocity. Both these features of the line shape confirm numerical predictions reported in a previous study [see Charalambous, Phys. Rev. B 66, 054506 (2002)]. © 2006 The American Physical Society.
  • Martínez-Criado, G. et al. (2005). Mn-rich clusters in GaN: Hexagonal or cubic symmetry? Applied Physics Letters [Online] 86:1-3. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-20244378230&partnerID=40&md5=b44d4d934fcf826234dcd8851d594b20.
    In this study, we report the application of synchrotron radiation microprobe to the analysis of Mn-rich clusters in GaN. From the Mn and Ga fluorescence line intensity ratio, an average Mn concentration of 11% was deduced. A combination of fluorescence mapping with spectroscopic techniques enabled us to examine not only the cluster elemental nature but also their crystallographic orientation on the submicron scale. The strong polarization-dependent x-ray absorption near-edge structure features showed the preservation of the hexagonal symmetry in both cluster-free and Mn-rich regions. However, from the x-ray absorption data taken inside the clusters, a preferential disorder was observed in the direction parallel to the crystal growth rather than perpendicular to it. © 2005 American Institute of Physics.
  • Ramos, S. et al. (2005). An anomalous x-ray diffraction study of the hydration structures of Cs+ and I- in concentrated solutions. The Journal of chemical physics [Online] 123. Available at: https://doi.org/10.1063/1.2128706.
    Anomalous x-ray diffraction experiments were carried out on concentrated aqueous solutions of sodium iodide (6 molal) and cesium iodide (3 molal). Data were gathered at two energies below the absorption edges of the Cs+ and I- ions in order to avoid contributions from fluorescence. The statistics and quality of the raw data were improved by the use of a focusing analyzer crystal. Differences were taken between the data sets and used to calculate the hydration structures of Cs+ and I-. The structures found are more complex than anticipated for such large ions with relatively low charge densities and show evidence of ion-pair formation in both solutions. A two-Gaussian fit to the Cs+ data gives information about the Cs+-O and Cs+-I- correlations. The central position of the Gaussian representing the Cs+-O was fixed at 3.00 A, that is, the maximum of this contribution. The other parameters were allowed to vary freely, giving a Cs+-I- distance of 3.84+/-0.05 A and coordination numbers of 7.9 and 2.7, respectively, for the Cs+-O and Cs+-I- correlations. The results on the structure of I- in the 6 molal NaI aqueous solution were also fitted to a model based on Gaussians; this gives correlations for I- -O and I- -Na+ at 3.17+/-0.06 and 3.76+/-0.06 A with respective coordination numbers of 8.8 and 1.6. The structure of I- in the 3 molal CsI solution shows overlapping contributions due to I- -H, I- -O, and I- -Cs+. The best Gaussian fit gives two peaks centered at 3.00+/-0.08 and 3.82+/-0.04 A and shows that the latter two correlations are unresolved. The hydration structures are compared with those of other alkali and halide ions. The results are also found to be in good agreement with those obtained from standard x-ray diffraction and computer simulation.
  • Sánchez-Benítez, J. et al. (2005). Manganese sites and valence states in RCu3-xMn 4+xO12(R = Ca or La) magnetoresistive perovskites. 1st International Meeting on Applied Physics, APHYS-2003 [Online] T118:251-254. Available at: https://doi.org/10.1238/Physica.Topical.118a00251.
    The manganese environment has been investigated by X-ray Absorption Spectroscopy (XAS) in RCu3-xMn4+xO12(R = Ca or La) magnetoresistive perovskites. The analysis of the data evidence the presence of two different Mn edges, each one related with the two different sites in which Mn can be located. Mn-O distances at both sublattices were obtained by Extended X-ray Absorption Fine Structure (EXAFS) analysis. The variation of the threshold energy corresponding to the Mn at A sites is driven by the changes in the Mn-O distances. On the contrary, the edge of Mn at Mn-O6 sites, which is equivalent to the one observed in manganites, is dominated by the change in its valence. The results suggest that electrical conduction in this series of compounds is due to the Mn eg electrons at B site (mixed valence Mn) and not to the localized electrons from Mn at A site of the perovskite. © Physica Scripta 2005.
  • Ramos, S. et al. (2003). Anomalous x-ray diffraction studies of Sr2+ hydration in aqueous solution. Journal of Chemical Physics 118:5542-5546.
    An overview is given of anomalous x-ray diffraction (AXD) results on the hydration of Sr2+ in a concentrated solution of strontium chloride. Results from AXD on the hydration of the Sr2+ ion are compared with those obtained from previous techniques. These results are also compared in conjunction with results for Mg2+ and Ca2+ in order to discern any trends in the hydration properties of the alkaline earth series.
  • Ramos, S. et al. (2001). An anomalous X-ray diffraction study of yttrium(III) hydration. Journal of Physical Chemistry B [Online] 105:2694-2698. Available at: http://dx.doi.org/10.1021/jp003869c.
  • Neilson, G. et al. (2001). Neutron and X-ray scattering studies of hydration in aqueous solutions. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences [Online] 359:1575-1591. Available at: http://dx.doi.org/10.1098/rsta.2001.0866.
    The presence of ions and/or apolar species in water provides a rich and varied environment in which many natural processes occur. This review provides results of recent structural studies of aqueous solutions derived from state-of-the-art neutron and X-ray scattering methods. The enhanced resolution provided by methods such as neutron diffraction and isotopic substitution, and anomalous X-ray diffraction, have given scientists new insights into the contrasting hydration structures of a variety of ions and small molecules, and crucially into how these structures might affect the general properties of solutions. The discussion points out common features of ionic hydration within particular series, such as the alkalis, halides and transition metals, and also indicates where significant differences in hydration structure appear.
  • Ramos, S. et al. (2000). Anomalous X-ray diffraction studies of hydration effects in concentrated aqueous electrolyte solutions. Chemical Physics [Online] 258:171-180. Available at: https://doi.org/10.1016/S0301-0104(00)00132-4.
    The method of anomalous X-ray diffraction (AXD) was applied to two aqueous electrolyte solutions: 6 m rubidium bromide and 4 m rubidium chloride. Data from the former solution were used to determine Br- hydration and those from the latter to determine Rb+ hydration. The results are compared with those obtained from other techniques and contrasted with the hydration of other ions in their respective series. A critical appraisal of the AXD method and its potential for future studies of ionic solutions are given. (C) 2000 Published by Elsevier Science B.V.
  • Ramos, S. et al. (2000). The hydration structure of Br- from anomalous x-ray diffraction. Journal of Physics Condensed Matter [Online] 12:A195-A201. Available at: https://doi.org/10.1088/0953-8984/12/8A/324.
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