Professor Mark Green

Professor of Materials Chemistry

About

Professor Mark Green is a Professor of Materials Chemistry for the School of Physical Sciences.

Publications

Showing 50 of 136 total publications in the Kent Academic Repository. View all publications.

Article

  • Orlandi, F. et al. (2018). Incommensurate atomic and magnetic modulations in the spin-frustrated ?-NaMnO2 triangular lattice. Physical Review Materials [Online] 2:74407. Available at: https://doi.org/10.1103/PhysRevMaterials.2.074407.
    The layered ?-NaMnO2, a promising Na-ion energy-storage material, has been investigated for its triangular
    lattice capability to promote complex magnetic configurations that may release symmetry restrictions for the
    coexistence of ferroelectric and magnetic orders. The complexity of the neutron powder diffraction patterns
    underlines that the routinely adopted commensurate structural models are inadequate. Instead, a single-phase
    superspace symmetry description is necessary, demonstrating that the material crystallizes in a compositionally
    modulated q = (0.077(1), 0, 0) structure. Here Mn3+ Jahn-Teller distorted MnO6 octahedra form corrugated
    layer stacking sequences of the ?-NaMnO2 type, which are interrupted by flat sheets of the ?-like oxygen
    topology. Spontaneous long-range collinear antiferromagnetic order, defined by the propagation vector k =
    (1/2, 1/2, 1/2), appears below TN1 = 200 K. Moreover, a second transition into a spatially modulated properscrew
    magnetic state (k ± q) is established at TN2 = 95 K, with an antiferromagnetic order parameter resembling
    that of a two-dimensional (2D) system. The evolution of 23Na NMR spin-lattice relaxation identifies a magnetically
    inhomogeneous state in the intermediate T region (TN2 <T <TN1), while its strong suppression below TN2
    indicates that a spin gap opens in the excitation spectrum. High-resolution neutron inelastic scattering confirms that
    the magnetic dynamics are indeed gapped (? ? 5 meV) in the low-temperature magnetic phase, while simulations
    on the basis of the single-mode approximation suggest that Mn spins residing on adjacent antiferromagnetic
    chains, establish sizable 2D correlations. Our analysis points out that novel structural degrees of freedom promote
    cooperative magnetism and emerging dielectric properties in this nonperovskite type of manganite.
  • Songvilay, M. et al. (2018). Anharmonic Magnon Excitations in Noncollinear and Charge-Ordered RbFe2+Fe3+F6. Physical Review Letters [Online] 121. Available at: https://doi.org/10.1103/PhysRevLett.121.087201.
    RbFe2+Fe3+F6 is an example of a charge ordered antiferromagnet where iron sites, with differ-
    ing valences, are structurally separated into two interpenetrating sublattices. The low temperature
    magnetically ordered Fe2+ (S=2) and Fe3+ (S=5/2) moments form a noncollinear orthogonal struc-
    ture with the Fe3+ site displaying a reduced static ordered moment. Neutron spectroscopy on single
    crystals finds two distinct spin wave branches with a dominant coupling along b-axis Fe3+ chain
    axis. High resolution spectroscopic measurements find an intense energy and momentum broadened
    magnetic band of scattering bracketing a momentum-energy region where two magnon processes
    are kinematically allowed. These anharmonic excitations are enhanced in this non collinear magnet
    owing to the orthogonal spin arrangement.
  • Green, M. et al. (2017). Competing spin density wave, collinear, and helical magnetism in Fe1+xTe. Physical Review B: Condensed Matter and Materials Physics [Online] 95. Available at: http://dx.doi.org/10.1103/PhysRevB.95.144407.
    The Fe1+xTe phase diagram consists of two distinct magnetic structures with collinear order present at low interstitial iron concentrations and a helical phase at large values of x with these phases separated by a Lifshitz point. We use unpolarized single crystal diffraction to confirm the helical phase for large interstitial iron concentrations and polarized single crystal diffraction to demonstrate the collinear order for the iron deficient side of the Fe1+xTe phase diagram. Polarized neutron inelastic scattering show that the fluctuations associated with this collinear order are predominately transverse at low energy transfers, consistent with a localized magnetic moment picture. We then apply neutron inelastic scattering and polarization analysis to investigate the dynamics and structure near the boundary between collinear and helical order in the Fe1+xTe phase diagram. We first show that the phase separating collinear and helical order is characterized by a spin-density wave with a single propagation wave vector of (\sim 0.45, 0, 0.5). We do not observe harmonics or the presence of a charge density wave. The magnetic fluctuations associated with this wavevector are different from the collinear phase being strongly longitudinal in nature and correlated anisotropically in the (H,K) plane. The excitations preserve the C4 symmetry of the lattice, but display different widths in momentum along the two tetragonal directions at low energy transfers. While the low energy excitations and minimal magnetic phase diagram can be understood in terms of localized
    interactions, we suggest that the presence of density wave phase implies the importance of electronic and orbital properties.
  • Zajdel, P. et al. (2017). Structure and magnetism in the bond-frustrated spinel ZnCr2Se4. Physical Review B [Online] 95. Available at: https://doi.org/10.1103/PhysRevB.95.134401.
    The crystal and magnetic structures of stoichiometric ZnCr2Se4 have been investigated using synchrotron x-ray and neutron powder diffraction, muon spin relaxation (?SR), and inelastic neutron scattering. Synchrotron x-ray diffraction shows a spin-lattice distortion from the cubic Fd¯3m spinel to a tetragonal I41/amd lattice below TN = 21 K, where powder neutron diffraction confirms the formation of a helical magnetic structure with magnetic moment of 3.04(3)?B at 1.5 K, close to that expected for high-spin Cr3+. ?SR measurements show prominent local spin correlations that are established at temperatures considerably higher (<100 K) than the onset of long-range magnetic order. The stretched exponential nature of the relaxation in the local spin-correlation regime suggests a wide distribution of depolarizing fields. Below TN, unusually fast (>100?s?1) muon relaxation rates are suggestive of rapid site hopping of the muons in static field. Inelastic neutron scattering measurements show a gapless mode at an incommensurate propagation vector of k = [000.4648(2)] in the low-temperature
    magnetic ordered phase that extends to 0.8 meV. The dispersion is modeled by a two-parameter Hamiltonian, containing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions with a Jnnn/Jnn = ?0.337.
  • Goto, Y. et al. (2017). Pressure-Stabilized Cubic Perovskite Oxyhydride BaScO2H. Inorganic Chemistry [Online] 56:4840-4845. Available at: https://doi.org/10.1021/acs.inorgchem.6b02834.
    We report a scandium oxyhydride BaScO2H prepared by solid state reaction under high pressure. Rietveld refinements against powder synchrotron X-ray and neutron diffraction data revealed that BaScO2H adopts the ideal cubic perovskite structure (Pm3?m), where oxide (O2–) and hydride (H–) anions are disordered. 1H nuclear magnetic resonance (NMR) spectroscopy provides a positive chemical shift of about +4.4 ppm, which can be understood by the distance to the nearest (and possibly the next nearest) cation from the H nucleus. A further analysis of the NMR data and calculations based on ab initio random structure searches suggest a partial cis preference in ScO4H2 octahedra. The present oxyhydride, if compositionally or structurally tuned, may become a candidate for H– conductors.
  • Minns, J. et al. (2017). Structure and interstitial iodide migration in hybrid perovskite methylammonium lead iodide. Nature Communications [Online] 8:15152. Available at: https://doi.org/10.1038/ncomms15152.
    Hybrid perovskites form an emerging family of exceptional light harvesting compounds. However, the mechanism underpinning their photovoltaic effect is still far from understood, which is impeded by a lack of clarity on their structures. Here we show that iodide ions in the methylammonium lead iodide migrate via interstitial sites at temperatures above 280?K. This coincides with temperature dependent static distortions resulting in pseudocubic local symmetry. Based on bond distance analysis, the migrating and distorted iodines are at lengths consistent with the formation of I2 molecules, suggesting a 2I??I2+2e? redox couple. The actual formula of this compound is thus (CH3NH3)PbI3?2x(I2)x where x?0.007 at room temperature. A crucial feature of the tetragonal structure is that the methylammonium ions do not sit centrally in the A-site cavity, but disordered around two off-centre orientations that facilitate the interstitial ion migration via a gate opening mechanism.
  • Stock, C. et al. (2017). Orphan spins in the S = 5/2 antiferromagnet CaFe2O4. Physical Review Letters [Online] 119. Available at: https://doi.org/10.1103/PhysRevLett.119.257204.
    CaFe2O4 is an anisotropic S=5/2 antiferromagnet with two competing A (????) and B (????) magnetic order parameters separated by static antiphase boundaries at low temperatures. Neutron diffraction and bulk susceptibility measurements, show that the spins near these boundaries are weakly correlated and a carry an uncompensated ferromagnetic moment that can be tuned with a magnetic field. Spectroscopic measurements find these spins are bound with excitation energies less than the bulk magnetic spin waves and resemble the spectra from isolated spin clusters. Localized bound orphaned spins separate the two competing magnetic order parameters in CaFe2O4.
  • Yajima, T. et al. (2016). Selective and low temperature transition metal intercalation in layered tellurides. Nature Communications [Online] 7:13809. Available at: http://doi.org/10.1038/ncomms13809.
    Layered materials embrace rich intercalation reactions to accommodate high concentrations of foreign species within their structures, and find many applications spanning from energy storage, ion exchange to secondary batteries. Light alkali metals are generally most easily intercalated due to their light mass, high charge/volume ratio and in many cases strong reducing properties. An evolving area of materials chemistry, however, is to capture metals selectively, which is of technological and environmental significance but rather unexplored. Here we show that the layered telluride T2PTe2 (T=Ti, Zr) displays exclusive insertion of transition metals (for example, Cd, Zn) as opposed to alkali cations, with tetrahedral coordination preference to tellurium. Interestingly, the intercalation reactions proceed in solid state and at surprisingly low temperatures (for example, 80?°C for cadmium in Ti2PTe2). The current method of controlling selectivity provides opportunities in the search for new materials for various applications that used to be possible only in a liquid.
  • Stock, C. et al. (2016). Solitary Magnons in the S=5/2 Antiferromagnet CaFe2O4. Physical Review Letters [Online] 117. Available at: http://doi.org/10.1103/PhysRevLett.117.017201.
    CaFe2O4 is a S=52 anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A (????) and B (????) phases, which differ by the c-axis stacking of ferromagnetic stripes. We apply neutron scattering to demonstrate that the competing A and B phase order parameters result in magnetic antiphase boundaries along c which freeze on the time scale of ?1??ns at the onset of magnetic order at 200 K. Using high resolution neutron spectroscopy, we find quantized spin wave levels and measure 9 such excitations localized in regions ?1–2 c-axis lattice constants in size. We discuss these in the context of solitary magnons predicted to exist in anisotropic systems. The magnetic anisotropy affords both competing A+B orders as well as localization of spin excitations in a classical magnet.
  • Bakaimi, I. et al. (2016). Hydration-induced spin-glass state in a frustrated Na-Mn-O triangular lattice. Physical Review B [Online] 93. Available at: http://doi.org/10.1103/PhysRevB.93.184422.
    Birnessite compounds are stable across a wide range of compositions that produces a remarkable diversity in their physical, electrochemical, and functional properties. These are hydrated analogs of the magnetically frustrated, mixed-valent manganese oxide structures, with general formula, NaxMnO2. Here we demonstrate that the direct hydration of layered rock-salt type ?-NaMnO2, with the geometrically frustrated triangular lattice topology, yields the birnessite type oxide, Na0.36MnO2?0.2H2O, transforming its magnetic properties. This compound has a much-expanded interlayer spacing compared to its parent ?-NaMnO2 compound. We show that while the parent ?-NaMnO2 possesses a Néel temperature of 45 K as a result of broken symmetry in the Mn3+ sublattice, the hydrated derivative undergoes collective spin freezing at 29 K within the Mn3+/Mn4+ sublattice. Scaling-law analysis of the frequency dispersion of the ac susceptibility, as well as the temperature-dependent, low-field dc magnetization confirm a cooperative spin-glass state of strongly interacting spins. This is supported by complementary spectroscopic analysis [high-angle annular dark-field scanning transmission electron miscroscopy (TEM), energy-dispersive x-ray spectroscopy, and electron energy-loss spectroscopy] as well as by a structural investigation (high-resolution TEM, x-ray, and neutron powder diffraction) that yield insights into the chemical and atomic structure modifications. We conclude that the spin-glass state in birnessite is driven by the spin frustration imposed by the underlying triangular lattice topology that is further enhanced by the in-plane bond-disorder generated by the mixed-valent character of manganese in the layers.
  • Yajima, T. et al. (2015). A labile hydride strategy for the synthesis of heavily nitridized BaTiO3. Nature Chemistry [Online] 7:1017-1023. Available at: http://dx.doi.org/10.1038/nchem.2370.
    Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800-1,300 °C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N3- species in the lattice place significant constraints on the composition and structure-and hence the properties-of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (&lt;500 °C), using a perovskite oxyhydride as a host. The lability of H-in BaTiO3-xHx (x � 0.6) allows H-/N3- exchange to occur, and yields a room-temperature ferroelectric BaTiO3-xN2x/3. This anion exchange is accompanied by a metal-to-insulator crossover via mixed O-H-N intermediates. These findings suggest that this 'labile hydride' strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds. © 2015 Macmillan Publishers Limited. All rights reserved.
  • Yajima, T. et al. (2015). Superconductivity in the hypervalent compound Ba2Bi(Sb1-xBix)2 with a square-honeycomb lattice with a square-honeycomb lattice. Journal of the Physical Society of Japan [Online] 83:73705-73705. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84921340196&partnerID=40&md5=5177d9ccadc0901fd985cecfd958c63d.
    We studied the structural and physical properties of the hypervalent system Ba2Bi(Sb1-xBix)2 (0 ¯ x ¯ 1) with an anisotropic "square-honeycomb" layer. We found that the orthorhombic Ba2BiSb2 (x = 0) shows a charge density wave (CDW) transition at approximately 230K accompanied by a significant elongation of the b-axis, indicating the quasi-one- dimensional nature along the b-axis in its electronic state, as supported by first-principles calculations. This CDW transition is rapidly suppressed with increasing x, leading to the appearance of superconductivity for 0.375 ¯ x ¯ 1. The superconducting transition temperature Tc increases slightly with x and the maximum Tc was 4.4K for Ba2Bi3 (x = 1). © 2014 The Physical Society of Japan.
  • Zhang, Y. et al. (2015). Interlayer Communication in Aurivillius Vanadate to Enable Defect Structures and Charge Ordering. Inorganic Chemistry [Online] 54:10925-10933. Available at: http://dx.doi.org/10.1021/acs.inorgchem.5b01964.
    The fluorite-like [Bi2O2]2+ layer is a fundamental building unit in a great variety of layered compounds. Here in this contribution, we presented a comprehensive study on an unusual Aurivillius phase Bi3.6V2O10 with respect to its defect chemistry and polymorphism control as well as implications for fast oxide ion transport at lower temperatures. The bismuth oxide layer in Bi4V2O11 is found to tolerate a large number of Bi vacancies without breaking the high temperature prototype I4/mmm structure (γ-phase). On cooling, an orthorhombic distortion occurs to the γ-phase, giving rise to a different type of phase (B-phase) in the intermediate temperature region. Cooling to room temperature causes a further transition to an oxygen-vacancy ordered A-phase, which is accompanied by the charge ordering of V4+ and V5+ cations, providing magnetic (d1) and nonmagnetic (d0) chains along the a axis. This is a novel charge ordering transition in terms of the concomitant change of oxygen coordination. Interestingly, upon quenching, both the γ- and B-phase can be kinetically trapped, enabling the structural probing of the two phases at ambient temperature. Driven by the thermodynamic forces, the oxide anion in the γ-phase undergoes an interlayer diffusion process to reshuffle the compositions of both Bi-O and V-O layers. © 2015 American Chemical Society.
  • Stock, C. et al. (2014). Soft striped magnetic fluctuations competing with superconductivity in Fe1+xTe. Physical Review B - Condensed Matter and Materials Physics [Online] 90:0-0. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84907452407&partnerID=40&md5=6943972eb3702a5714e889d63a3a121c.
    Neutron spectroscopy is used to investigate the magnetic fluctuations in Fe1+xTe - a parent compound of chalcogenide superconductors. Incommensurate "stripelike" excitations soften with increased interstitial iron concentration. The energy crossover from incommensurate to stripy fluctuations defines an apparent hourglass dispersion. Application of sum rules of neutron scattering find that the integrated intensity is inconsistent with an S=1Fe2+ ground state and significantly less than S=2 predicted from weak crystal field arguments pointing towards the Fe2+ being in a superposition of orbital states. The results suggest that a highly anisotropic order competes with superconductivity in chalcogenide systems. © 2014 American Physical Society.
  • Takatsu, H. et al. (2014). Magnetic structure of the conductive triangular-lattice antiferromagnet PdCrO 2. Physical Review B: Condensed Matter and Materials Physics [Online] 89:104408-104408. Available at: http://dx.doi.org/10.1103/PhysRevB.89.104408.
  • Tassel, C. et al. (2014). Direct Synthesis of Chromium Perovskite Oxyhydride with a High Magnetic-Transition Temperature. Angewandte Chemie International Edition [Online] 53:10377-10380. Available at: http://doi.org/10.1002/anie.201405453.
    We report a novel oxyhydride SrCrO2H directly synthesized by a high-pressure high-temperature method. Powder neutron and synchrotron X-ray diffraction revealed that this compound adopts the ideal cubic perovskite structure with O2?/H? disorder. Surprisingly, despite the non-bonding nature between Cr?3d?t2g orbitals and the H?1s orbital, it exhibits G-type spin ordering at TN?380?K, which is higher than that of RCrO3 (R=rare earth) and any chromium oxides. The enhanced TN in SrCrO2H with four Cr-O-Cr bonds in comparison with RCr3+O3 with six Cr-O-Cr bonds is reasonably explained by the tolerance factor. The present result offers an effective strategy to tune octahedral tilting in perovskites and to improve physical and chemical properties through mixed anion chemistry.
  • Rodriguez, E. et al. (2013). Magnetic and structural properties near the Lifshitz point in Fe 1+xTe. Physical Review B - Condensed Matter and Materials Physics [Online] 88:0-0. Available at: http://dx.doi.org/10.1103/PhysRevB.88.165110.
    We construct a phase diagram of the parent compound Fe1+xTe as a function of interstitial iron x in terms of the electronic, structural, and magnetic properties. For a concentration of x<10%, Fe1+xTe undergoes a "semimetal" to metal transition at approximately 70 K that is also first-order and coincident with a structural transition from a tetragonal to a monoclinic unit cell. For x?14%, Fe1+xTe undergoes a second-order phase transition at approximately 58 K corresponding to a semimetal to semimetal transition along with a structural orthorhombic distortion. At a critical concentration of x?11%, Fe1+xTe undergoes two transitions: the higher-temperature one is a second-order transition to an orthorhombic phase with incommensurate magnetic ordering and temperature-dependent propagation vector, while the lower-temperature one corresponds to nucleation of a monoclinic phase with a nearly commensurate magnetic wave vector. While both structural and magnetic transitions display similar critical behavior for x<10% and near the critical concentration of x?11%, samples with large interstitial iron concentrations show a marked deviation between the critical response indicating a decoupling of the order parameters. Analysis of temperature dependent inelastic neutron data reveals incommensurate magnetic fluctuations throughout the Fe1+xTe phase diagram are directly connected to the "semiconductor"-like resistivity above TN and implicates scattering from spin fluctuations as the primary reason for the semiconducting or poor metallic properties. The results suggest that doping driven Fermi surface nesting maybe the origin of the gapless and incommensurate spin response at large interstitial concentrations.
  • Nakano, K. et al. (2013). Tc enhancement by aliovalent anionic substitution in superconducting BaTi2(Sb1-xSnx)2O. Journal of the Physical Society of Japan [Online] 82:0-0. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84879815803&partnerID=40&md5=2b9de235e6b0b10a07406693f2c690f5.
    BaTi2Sb2O is a Tc = 1.2K superconductor with a d1 square lattice, and isovalent Bi substitution for Sb can increase its Tc to 4.6K (BaTi2Bi2O), accompanied by the complete suppression of charge density wave (CDW) or spin density wave (SDW) transition. In the present study, we demonstrate that aliovalent Sn substitution (hole doping) also increases Tc up to 2.5K for BaTi2(Sb 0.7Sn0.3)2O, while suppressing CDW/SDW transition only slightly. The overall electronic phase diagram of BaTi2(Sb,Sn) 2O is qualitatively similar to that of cation-substituted (hole-doped) (Ba,Na)Ti2Sb2O, but quantitative differences such as in Tc are observed, which is discussed in terms of Ti-Pn hybridization and chemical disorder. © 2013 The Physical Society of Japan.
  • Bao, W. et al. (2013). Superconductivity tuned by the iron vacancy order in KxFe 2-ySe2. Chinese Physics Letters [Online] 30:0-0. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84874323326&partnerID=40&md5=c70e14a1a3578fd2c311b697f6def2a2.
    Combining in-depth neutron diffraction and systematic bulk studies, we discover that the 5 � 5 Fe vacancy order, with its associated block antiferromagnetic order, is the ground state with varying occupancy ratios of the iron 16i and vacancy 4d sites across the phase-diagram of K xFe2-ySe2. The orthorhombic order, with one of the four Fe sites vacant, appears only at intermediate temperatures as a competing phase. The material experiences an insulator to metal crossover when the 5 � 5 order is highly developed. Superconductivity occurs in such a metallic phase. © 2013 Chinese Physical Society and IOP Publishing Ltd.
  • Starowicz, P. et al. (2013). A flat band at the chemical potential of a Fe1.03Te 0.94S0.06 superconductor observed by angle-resolved photoemission spectroscopy. Journal of Physics Condensed Matter [Online] 25:0-0. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84876845662&partnerID=40&md5=86129b5f3bcb8f0d9a64effbee2b0b71.
    The electronic structure of superconducting Fe1.03Te 0.94S0.06 has been studied by angle-resolved photoemission spectroscopy (ARPES). Experimental band topography is compared to the calculations using the methods of Korringa-Kohn-Rostoker (KKR) with the coherent potential approximation (CPA) and the linearized augmented plane wave with local orbitals (LAPW+LO) method. The region of the Î? point exhibits two hole pockets and a quasiparticle peak close to the chemical potential (μ) with undetectable dispersion. This flat band with mainly dz2 orbital character is most likely formed by the top of the outer hole pocket or is evidence of a third hole band. It may cover up to 3% of the Brillouin zone volume and should give rise to a Van Hove singularity. Studies performed for various photon energies indicate that at least one of the hole pockets has a two-dimensional character. The apparently nondispersing peak at μ is clearly visible for 40 eV and higher photon energies, due to an effect of the photoionization cross-section rather than band dimensionality. Orbital characters calculated by LAPW+LO for stoichiometric FeTe do not reveal the flat dz2 band but are in agreement with the experiment for the other dispersions around Î? in Fe1.03Te0.94S 0.06. © 2013 IOP Publishing Ltd.
  • Nakatsuji, S. et al. (2012). Spin-orbital short-range order on a honeycomb-based lattice. Science [Online] 336:559-563. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84860487946&partnerID=40&md5=a346116b81a011dd013e81abcd623df8.
    Frustrated magnetic materials, in which local conditions for energy minimization are incompatible because of the lattice structure, can remain disordered to the lowest temperatures. Such is the case for Ba 3CuSb2O9, which is magnetically anisotropic at the atomic scale but curiously isotropic on mesoscopic length and time scales. We find that the frustration of Wannier's Ising model on the triangular lattice is imprinted in a nanostructured honeycomb lattice of Cu2+ ions that resists a coherent static Jahn-Teller distortion. The resulting two-dimensional random-bond spin-1/2 system on the honeycomb lattice has a broad spectrum of spin-dimer-like excitations and low-energy spin degrees of freedom that retain overall hexagonal symmetry.
  • Katayama, N. et al. (2012). Conductivity and incommensurate antiferromagnetism of Fe 1.02Se 0.10Te 0.90 under pressure. EPL [Online] 98:0-0. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84862088081&partnerID=40&md5=502e6a963b2cee3731eef4e0f1fcfd8b.
    By performing resistivity, bulk susceptibility and neutron diffraction on single crystals of Fe 1.02Se 0.10Te 0.90 under pressure, we show that an external pressure can enhance both conductivity and magnetism simultaneously. Surprisingly, the pressure-induced magnetic order is nearly three-dimensional, and incommensurate along both the c-axis and the a-axis, which is accompanied by a structural phase separation. Our powder neutron diffraction data show that the Fe(Se, Te) 4 tetrahedron becomes more regular under pressure, which is consistent with the enhancement of the metallicity but is counterintuitive to the three-dimensional magnetic order. © 2012 Europhysics Letters Association.
  • Dissanayake, S. et al. (2012). Magnetic-field-induced instability of the cooperative paramagnetic state in Zn xCo 4-x(OD) 6Cl 2. Physical Review B - Condensed Matter and Materials Physics [Online] 85:0-0. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84861700391&partnerID=40&md5=0bb40da9b8f79f8102ffa51eed7c6e54.
    Using elastic and inelastic neutron scattering techniques with and without application of an external magnetic field H, the magnetic ground states of Zn xCo 4-x(OD) 6Cl 2 (x=0,1) were studied. Our results show that for x=0, the ground state is a magnetic long-range ordered (LRO) state where each tetrahedron forms an "umbrella"-type structure. On the other hand, for x=1, no static ordering was observed down to 1.5 K, which resembles the behavior found in the isostructural quantum system Zn xCu 4-x(OD) 6Cl 2. When H field is applied, however, the x=1 system develops the same LRO state as x=0. This indicates that the x=1 disordered state is in the vicinity of the x=0 ordered state. © 2012 American Physical Society.
  • Deka, U. et al. (2012). Confirmation of isolated Cu 2+ ions in SSZ-13 zeolite as active sites in NH 3-selective catalytic reduction. Journal of Physical Chemistry C [Online] 116:4809-4818. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84857437443&partnerID=40&md5=b8d8bbd407f1167aa5f7339e5f0c68ea.
    NH 3-Selective Catalytic Reduction (NH 3-SCR) is a widely used technology for NO x reduction in the emission control systems of heavy duty diesel vehicles. Copper-based ion exchanged zeolites and in particular Cu-SSZ-13 (CHA framework) catalysts show both exceptional activity and hydrothermal stability for this reaction. In this work, we have studied the origin of the SCR activity of Cu-SSZ-13 as evidenced from a combination of synchrotron-based and laboratory techniques. Synchrotron-based in situ XAFS/XRD measurements were used to provide complementary information on the local copper environment under realistic NH 3-SCR conditions. Crucial then to the catalytic activity of Cu-SSZ-13 is the local environment of the copper species, particularly in the zeolite. Cu-SSZ-13 contains mononuclear Cu 2+ species, located in the face of the double-6-ring subunit of the zeolite after calcination where it remains under reaction conditions. At lower temperatures (with low activity), XAFS and XRD data revealed a conformational change in the local geometry of the copper from a planar form toward a distorted tetrahedron as a result of a preferential interaction with NH 3. This process appears necessary for activity, but results in a stymieing of activity at low temperatures. At higher temperatures, the Cu 2+ possess a local coordination state akin to that seen after calcination. © 2012 American Chemical Society.
  • Yeon, J. et al. (2012). Syntheses, crystal structures, and characterization of two new Tl +-Cu 2+-Te 6+ oxides: Tl 4CuTeO 6 and Tl 6CuTe 2O 10. Journal of Solid State Chemistry [Online] 196:607-613. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84867653705&partnerID=40&md5=c1b355afe8263b4e4b2252cd7d447676.
    Crystals and polycrystalline powders of two new oxide materials, Tl 4CuTeO6 and Tl 6CuTe 2O 10, have been synthesized by hydrothermal and solid-state methods. The materials were structurally characterized by single-crystal X-ray diffraction. Tl 4CuTeO6 and Tl 6CuTe 2O 10 exhibit one dimensional anionic slabs of [CuTeO 6] 4� and [CuTe 2O 10] 6�, respectively. Common to both slabs is the occurrence of Cu 2+O 4 distorted squares and Te 6+O 6 octahedra. The slabs are separated by Tl + cations. For Tl 4CuTeO 6, magnetic measurements indicate a maximum at �8 K in the temperature dependence of the susceptibility. Low temperature neutron diffraction data confirm no long-range magnetic ordering occurs and the susceptibility was adequately accounted for by fits to a Heisenberg alternating chain model. For Tl 6CuTe 2O 10 on the other hand, magnetic measurements revealed paramagnetism with no evidence of long-range magnetic ordering. Infrared, UV-vis spectra, thermogravimetric, and differential thermal analyses are also reported. Crystal data: Tl 4CuTeO 6, Triclinic, space group P-1 (No. 2), a=5.8629(8)A, b=8.7848(11) A, c=9.2572(12)A, α = 66.0460(10), β = 74.2010(10), γ = 79.254(2), V=417.70(9) A 3, andZ=2; Tl 6CuTe 2O 10, orthorhombic, space group Pnma (No. 62), a=10.8628(6) �, b=11.4962(7)�, c=10.7238(6) �, V= 1339.20(13) � 3, and Z=4. © 2012 Elsevier Inc. All rights reserved.
  • BÅ?achowski, A. et al. (2012). Mössbauer study of the 11 iron-based superconductors parent compound Fe 1+xTe. Journal of Physics Condensed Matter [Online] 24:0-0. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84866130428&partnerID=40&md5=da6e4ba32bfbb778b8765192d21154e6.
    57Fe Mössbauer spectroscopy was applied to investigate the superconductor parent compound Fe 1+xTe for x=0.06, 0.10, 0.14, 0.18 within the temperature range 4.2-300K. A spin density wave (SDW) within the iron atoms occupying regular tetrahedral sites was observed, with the square root of the mean square amplitude at 4.2K varying between 9.7 and 15.7T with increasing x. Three additional magnetic spectral components appeared due to the interstitial iron distributed over available sites between the Fe-Te layers. The excess iron showed hyperfine fields at approximately 16, 21 and 49T for three respective components at 4.2K. The component with a large field of 49T indicated the presence of isolated iron atoms with large localized magnetic moments in interstitial positions. Magnetic ordering of the interstitial iron disappeared in accordance with the fallout of the SDW with increasing temperature. © 2012 IOP Publishing Ltd.
  • Kim, S. et al. (2012). RbFe 2+Fe 3+F 6: Synthesis, structure, and characterization of a new charge-ordered magnetically frustrated pyrochlore-related mixed-metal fluoride. Chemical Science [Online] 3:741-751. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84863149985&partnerID=40&md5=1d9a4df73801d3a3c3eef024ff6ca898.
    A new charge-ordered magnetically frustrated mixed-metal fluoride with a pyrochlore-related structure has been synthesized and characterized. The material, RbFe 2F 6 (RbFe 2+Fe 3+F 6) was synthesized through mild hydrothermal conditions. The material exhibits a three-dimensional pyrochlore-related structure consisting of corner-shared Fe 2+F 6 and Fe 3+F 6 octahedra. In addition to single-crystal diffraction data, neutron powder diffraction and magnetometry measurements were carried out. Magnetic data clearly reveal strong antiferromagnetic interactions (a Curie-Weiss temperature of -270 K) but sufficient frustration to prevent ordering until 16 K. No structural phase transformation is detected from the variable-temperature neutron diffraction data. Infrared, UV-vis, thermogravimetric, and differential thermal analysis measurements were also performed. First-principles density functional theory (DFT) electronic structure calculations were also done. Crystal data: RbFe 2F 6, orthorhombic, space group Pnma (no. 62), a = 7.0177(6), b = 7.4499(6), c = 10.1765(8) �, V = 532.04(8) � 3, Z = 4. © 2012 The Royal Society of Chemistry.
  • Stock, C., Rodriguez, E. and Green, M. (2012). Spin fluctuations and superconductivity in powders of Fe 1+xTe 0.7Se 0.3 as a function of interstitial iron concentration. Physical Review B - Condensed Matter and Materials Physics [Online] 85:94507-94507. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84858425218&partnerID=40&md5=a3af08c595dd570fbefafa56eb6489a9.
    Using neutron inelastic scattering, we investigate the role of interstitial iron on the low-energy spin fluctuations in powder samples of Fe 1+xTe 0.7Se 0.3. We demonstrate how combining the principle of detailed balance along with measurements at several temperatures allows us to subtract both temperature-independent and phonon backgrounds from S(Q,Ï?) to obtain purely magnetic scattering. For small values of interstitial iron [x=0.009(3)], the sample is superconducting (T c=14 K) and displays a spin gap of 7 meV peaked in momentum at wave vector q 0=(Ï?,Ï?) consistent with single-crystal results. On populating the interstitial iron sites, the superconducting volume fraction decreases and we observe a filling in of the low-energy magnetic fluctuations and a decrease of the characteristic wave vector of the magnetic fluctuations. For large concentrations of interstitial iron [x=0.048(2)] where the superconducting volume fraction is minimal, we observe the presence of gapless spin fluctuations at a wave vector of q 0=(Ï?,0). We estimate the absolute total moment for the various samples and find that the amount of interstitial iron does not change the total magnetic spectral weight significantly, but rather has the effect of shifting the spectral weight in Q and energy. These results show that the superconducting and magnetic properties can be tuned by doping small amounts of iron and are suggestive that interstitial iron concentration is also a controlling dopant in the Fe 1+xTe 1-ySe y phase diagram in addition to the Te/Se ratio. © 2012 American Physical Society.
  • Natali Sora, I. et al. (2012). Crystal structures and magnetic properties of strontium and copper doped lanthanum ferrites. Journal of Solid State Chemistry [Online] 191:33-39. Available at: http://doi.org/10.1016/j.jssc.2012.02.020.
    The crystal and magnetic structures of La0.8Sr0.2Fe1?xCuxO3?w compounds, which exhibit coercive fields larger than any others reported for iron-based perovskites, have been analyzed at room temperature with the neutron powder diffraction technique and the Rietveld method of profile fitting. For x in the range 0.05–0.10 the material is monophasic with orthorhombic symmetry (space group Pnma), and crystallizes in the perovskite-like cell of LaFeO3, Fe/Cu cations occupy octahedral sites, La/Sr cations are twelve-fold coordinated. For x=0.20 the material is biphasic, with a main orthorhombic phase (space group Pnma) and a secondary rhombohedral phase with space group R-3c (hexagonal setting). The structural transition from the orthorhombic to the rhombohedral phase reduces the structural distortion of the (Fe/Cu)O6 octahedron. The average bond distance (Fe/Cu)–O and the pseudo-cubic unit cell volume decrease with increasing Cu content in accordance with the presence of higher valence states of the transition metals. The magnetic structure was modeled for the monophasic samples (x=0.05 and 0.10) assuming an antiferromagnetic interaction between Fe/Cu neighboring cations (G-type): the magnetic moments order antiferromagnetically along the b-axis, with the spin direction along a-axis. The magnetic moments of the Fe/Cu atoms are ?x=2.66(3)?B and 2.43(3)?B for the compositions x=0.05 and 0.10, respectively. By measuring the first magnetization curve and the hysteresis loops, coexisting antiferromagnetic and weak ferromagnetic interactions were observed for all samples.
  • Qian, B. et al. (2012). Ferromagnetism in CuFeSb: Evidence of competing magnetic interactions in iron-based superconductors. Physical Review B - Condensed Matter and Materials Physics [Online] 85:0-0. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84860485709&partnerID=40&md5=6d8197fc67689b82156493042e7efdef.
    We have synthesized a new layered iron-pnictide CuFeSb. This material shares a similar layered tetragonal structure with iron-based superconductors, with Fe square planar sheets forming from the edge-sharing iron antimony tetrahedral network. CuFeSb differs remarkably from Fe-based superconductors in the height of anion Z anion from the Fe plane; Z Sb for CuFeSb is �1.84 �, much larger than Z As (1.31-1.51 �) in FeAs compounds and Z Te (�1.77 �) in Fe 1+yTe. In contrast with the metallic antiferromagnetic (AFM) or superconducting state of iron pnictides and chalcogenides under current studies, CuFeSb exhibits a metallic, ferromagnetic (FM) state with T c=375 K. This finding suggests that the competition between AFM and FM coupling may exist in Fe-based superconductors and that the nature of magnetic coupling within the Fe plane is indeed dependent on the height of anion as predicted in theories. © 2012 American Physical Society.
  • Rodriguez, E. et al. (2011). Noncollinear spin-density-wave antiferromagnetism in FeAs. Physical Review B - Condensed Matter and Materials Physics [Online] 83:0-0. Available at: http://dx.doi.org/10.1103/PhysRevB.83.134438.
    The nature of the magnetism in the simplest iron arsenide is of fundamental importance in understanding the interplay between localized and itinerant magnetism and superconductivity. We present the magnetic structure of the itinerant monoarsenide FeAs with the B31 structure. Powder neutron diffraction confirms incommensurate modulated magnetism with wave vector q=(0.395±0.001)c* at 4 K, but can not distinguish between a simple spiral and a collinear spin-density-wave structure. Polarized single-crystal diffraction confirms that the structure is best described as a noncollinear spin-density wave arising from a combination of itinerant and localized behavior with spin amplitude along the b-axis direction being (15±5)% larger than in the a direction. Furthermore, the propagation vector is temperature dependent, and the magnetization near the critical point indicates a two-dimensional Heisenberg system. The magnetic structures of closely related systems are discussed and compared to that of FeAs. © 2011 American Physical Society.
  • Kruk, I. et al. (2011). Coupled Commensurate Cation and Charge Modulation in the Tunneled Structure, Na0.40(2)MnO2. Journal of the American Chemical Society [Online] 133:13950-13956. Available at: http://dx.doi.org/10.1021/ja109707q.
    Na0.40(2)MnO2 belongs to a family of mixed Mn3+ and Mn4+ porous oxides that contains both octahedral and square pyramidal Mn–O units. Neutron and synchrotron radiation studies identify the presence of both sodium ordering (TNa ? 310 K) and Mn charge and orbital ordering. Below TNa, the centrosymmetric Pbam structure adopts an (ab 4c) supercell of Pnnm symmetry that accommodates a coupled commensurate modulation down the c-axis channels of both Na position and occupancy with Mn valence.
  • Bao, W. et al. (2011). A Novel Large Moment Antiferromagnetic Order in K0.8Fe1.6Se2Superconductor. Chinese Physics Letters [Online] 28:86104. Available at: http://doi.org/10.1088/0256-307X/28/8/086104.
    The discovery of cuprate high TC superconductors has inspired the search for unconventional superconductors in magnetic materials. A successful recipe has been to suppress long-range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point. We report an exception to this rule in the recently discovered potassium iron selenide. The superconducting composition is identified as the iron vacancy ordered K0.83(2)Fe1.64(1)Se2 with TC above 30 K. A novel large moment 3.31 ?B/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature TN ? 559 K for a bulk superconductor. Staggeringly polarized electronic density of states is thus suspected, which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances.
  • Nambu, Y. et al. (2011). Erratum: Incommensurate magnetism in FeAs strips: Neutron scattering from CaFe4As3. Physical Review Letters 107:59903-59903.
  • Rodriguez, E. et al. (2011). Chemical control of interstitial iron leading to superconductivity in Fe 1+xTe 0.7Se 0.3. Chemical Science [Online] 2:1782-1787. Available at: http://dx.doi.org/10.1039/c1sc00114k.
    Although it possesses the simple layered topology of the tetragonal anti-PO structure, the Fe(Te,Se) series has a complex structural and magnetic phase diagram that is dependent on composition and occupancy of a secondary interstitial Fe site. Here we show that superconductivity in Fe 1+xTe 0.7Se 0.3 is enhanced by topotactic deintercalation of the interstitial iron with iodine, demonstrating the competing roles of the two iron positions. We follow the evolution of the structure and magnetic properties as a function of interstitial iron. Powder neutron diffraction reveals a flattening of the Fe(Te,Se) 4 tetrahedron on Fe removal and an unusual temperature dependence of the lattice parameters that increases strongly below 150 K along with lattice strain. Inelastic neutron scattering shows gapless paramagnetic scattering evolves into a gapped excitation at 6 meV on removal of interstitial iron. This work highlights the robustness of the superconductivity across different Fe(Te,Se) compositions and geometries. © The Royal Society of Chemistry 2011.
  • Bhatia, V. et al. (2011). Phase separation and superconductivity in Fe 1+xTe 0.5Se 0.5. Chemical Communications [Online] 47:11297-11299. Available at: http://dx.doi.org/10.1039/c1cc13878b.
    Fe 1+xTe 0.5Se 0.5 is the archetypical iron-based superconductor. Here we show that the superconducting state is controlled by the stacking of its anti-PbO layers, such that homogeneous ordering hinders superconductivity and the highest volume fractions are observed in phase separated structures as evidenced by either a distribution of lattice parameters or microstrain. © 2011 The Royal Society of Chemistry.
  • Zavalij, P. et al. (2011). Structure of vacancy-ordered single-crystalline superconducting potassium iron selenide. Physical Review B - Condensed Matter and Materials Physics [Online] 83:132509-132509. Available at: http://dx.doi.org/10.1103/PhysRevB.83.132509.
    With single-crystal x-ray diffraction studies, we compare the structures of three samples showing optimal superconductivity: K0.775(4)Fe 1.613(1)Se2, K0.737(6)Fe 1.631(3)Se2, and Cs0.748(2)Fe 1.626(1)Se2. All have an almost identical ordered vacancy structure with a (?5 × ?5 × 1) supercell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250 K of a = b = 8.729(2) Å and c = 14.120(3) Å, a = b = 8.7186(12) Å and c = 14.0853(19) Å, and at 295 K, a = b = 8.8617(16) Å and c = 15.304(3) Å for the three crystals, respectively. The structure contains two iron sites; one is almost completely empty while the other is fully occupied. There are similarly two alkali metal sites that are occupied in the range of 72.2(2)%-85.3(3)%. The inclusion of alkali metals and the presence of vacancies within the structure allows for considerable relaxation of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series, and the resulting shift of the Se-Fe-Se bond angles to less distorted geometry could be important in understanding the associated increase in the superconducting transition temperature. The structure of these superconductors are distinguished from the structure of the nonsuperconducting phases by an almost complete absence of Fe on the (0 0.5 0.25) site, as well as lower alkali metal occupancy that ensures an exact Fe2+ oxidation state, which are clearly critical parameters in the promotion of superconductivity. © 2011 American Physical Society.
  • Nambu, Y. et al. (2011). Incommensurate mgnetism in FeAs srips: Neutron sattering from CaFe 4As3. Physical Review Letters: Moving Physics Forward [Online] 106:37201-37201. Available at: http://dx.doi.org/10.1103/PhysRevLett.106.037201.
    Magnetism in the orthorhombic metal CaFe4As3 was examined through neutron diffraction for powder and single crystalline samples. Incommensurate [qm?(0.37-0.39)×b*] and predominantly longitudinally (b) modulated order develops through a 2nd order phase transition at TN=89.63(6)K with a 3D Heisenberg-like critical exponent ?=0.365(6). A 1st order transition at T2=25.6(9)K is associated with the development of a transverse component, locking qm to 0.375(2)b*, and increasing the moments from 2.1(1) to 2.2(3) ?B for Fe2+ and from 1.3(3) to 2.4(4)?B for Fe+. The ab initio Fermi surface is consistent with a nesting instability in cross-linked FeAs strips. © 2011 American Physical Society.
  • Rodriguez, E. et al. (2011). Magnetic-crystallographic phase diagram of the superconducting parent compound Fe1+xTe. Physical Review B - Condensed Matter and Materials Physics [Online] 84:0-0. Available at: http://dx.doi.org/10.1103/PhysRevB.84.064403.
    Through neutron diffraction experiments, including spin-polarized measurements, we find a collinear incommensurate spin-density wave with propagation vector k= [0.4481(4)012] at base temperature in the superconducting parent compound Fe1+xTe. This critical concentration of interstitial iron corresponds to x?12% and leads to crystallographic phase separation at base temperature. The spin-density wave is short-range ordered with a correlation length of 22(3) Å, and as the ordering temperature is approached its propagation vector decreases linearly in the H direction and becomes long-range ordered. Upon further populating the interstitial iron site, the spin-density wave gives way to an incommensurate helical ordering with propagation vector k= [0.3855(2)012] at base temperature. For a sample with x?9(1)%, we also find an incommensurate spin-density wave that competes with the bicollinear commensurate ordering close to the Néel point. The shifting of spectral weight between competing magnetic orderings observed in several samples is supporting evidence for the phase separation being electronic in nature, and hence leads to crystallographic phase separation around the critical interstitial iron concentration of 12%. With results from both powder and single crystal samples, we construct a magnetic-crystallographic phase diagram of Fe1+xTe for 5%<x<17%
  • Stock, C. et al. (2011). Interstitial iron tuning of the spin fluctuations in the nonsuperconducting parent phase Fe1+xTe. Physical Review B - Condensed Matter and Materials Physics [Online] 84:45124-45124. Available at: http://dx.doi.org/10.1103/PhysRevB.84.045124.
    Using neutron inelastic scattering, we investigate the low-energy spin fluctuations in Fe1+xTe as a function of both temperature and interstitial iron concentration. For Fe1.057(7)Te, the magnetic structure is defined by a commensurate wave vector of (12,0,12). The spin fluctuations are gapped with a sharp onset at 7 meV and are three dimensional in momentum transfer, becoming two dimensional at higher-energy transfers. On doping with interstitial iron, we find, in Fe1.141(5)Te, the ordering wave vector is located at the (0.38,0,12) position and the fluctuations are gapless with the intensity peaked at an energy transfer of 4 meV. These results show that the spin fluctuations in the Fe1+xTe system can be tuned not only through selenium doping, but also with interstitial iron. We also compare these results with superconducting concentrations and, in particular, the resonance mode in the Fe1+xTe1-ySey system. © 2011 American Physical Society.
  • Zajdel, P. et al. (2010). Single crystal growth and structural properties of iron telluride doped with nickel. Crystal Research and Technology [Online] 45:1316-1320. Available at: http://dx.doi.org/10.1002/crat.201000375.
    We report the single crystal growth of Fe1.1-xNix Te with nominal compositions x = 0.01, 0.025, 0.05 and 0.1 using a slow cooled melt technique. Cylindrical crystals 0.5 cm in diameter and 2 cm long were obtained. The initial characterization, done using single-crystal diffractometer, revealed that the lattice parameter a increases and c decreases with Ni content. The crystal structure refinement was performed in the P4/nmm space group. Chemical analysis confirmed the presence of nickel in the system close to the nominal content. © 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
  • Xiang, S. et al. (2010). Open metal sites within isostructural metal-organic frameworks for differential recognition of acetylene and extraordinarily high acetylene storage capacity at room temperature. Angewandte Chemie - International Edition [Online] 49:4615-4618. Available at: http://dx.doi.org/10.1002/anie.201000094.
    Chemical equcation Presented On the metal: Open metal sites within isostructural [M2(DHTP)] metal-organic frameworks (M = Co 2-, Mn2+, Mg2+, and Zn2+; DHTP = 2,5-dihydroxyterephthalate) exhibit differential molecular recognition with acetylene. The extremely strong interaction of Co2+with acetylene (see structure) makes [Co2(DHTP)] the highest volumetric acetylene storage material with a capacity of 230 cm3cm-3 at 295 K and 1 atm. © 2010 Wiley-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
  • Zajdel, P. et al. (2010). Phase separation and suppression of the structural and magnetic transitions in superconducting doped iron tellurides, Fe1+ xTe 1-ySy. Journal of the American Chemical Society [Online] 132:13000-13007. Available at: http://dx.doi.org/10.1021/ja105279p.
    Single crystal and powder samples of the series of iron chalcogenide superconductors with nominal composition, Fe1.15Te 1-ySy, are found to form for 0 â?¤ y â?¤ 0.15. They crystallize in the tetragonal anti-PbO structure, which is composed of layers of edge-shared Fe(Te, S)4 tetrahedra. For y = 0, Fe1+xTe (x â?? 0.12(1)) is nonsuperconducting and undergoes a tetragonal (P4/nmm) to monoclinic (P21/m) structural transition at â?¼65 K, associated with the onset of commensurate antiferromagnetic order at q = (0.5 0 0.5). We show that on sulfur substitution, Fe1+xTe1-ySy becomes orthorhombic (Pmmn) at low temperature for 0 â?¤ y â?¤ 0.015, where the greatly suppressed magnetic scattering is now incommensurate at q = (0.5-δ 0 0.5) and possesses short ranged magnetic correlations that are well fitted with a two-dimensional Warren peak shape. At much higher concentrations of S (y â?¤ 0.075), there is suppression of both the structural and magnetic transitions and a superconducting transition at 9 K is observed. Between these two composition regimes, there exists a region of phase separation (0.025 â?¤ y â?¤ 0.05), where the low temperature neutron diffraction data is best refined with a model containing both the tetragonal and orthorhombic phases. The increase in the amount of sulfur is found to be associated with a reduction in interstitial iron, x. Microprobe analysis of a single crystal of composition Fe1.123(5)Te0.948(4)S0.052(4) confirms the presence of compositional variation within the crystals, rationalizing the observed phase separation. © 2010 American Chemical Society.
  • Kim, S. et al. (2010). Experimental and computational investigation of the polar ferrimagnet VOSe2O5. Chemistry of Materials [Online] 22:5074-5083. Available at: http://dx.doi.org/10.1021/cm1011839.
    We have re-examined the crystal structure and the physical properties of VOSe2O5 by performing single crystal X-ray and powder neutron diffraction, alternating current (AC) and direct current (DC) magnetization measurements, heat capacity, dielectric properties, and second-harmonic generation (SHG) measurements. From these studies, we observed that the compound undergoes three magnetic transitions near 4, 5.5, and 8 K. In addition, we observed ferrimagnetic behavior as the magnetic ground state, confirmed by the isothermal magnetization measured below 8 K that reveals a saturated magnetic moment of 0.5 μB per formula unit, consistent with density functional calculations of the magnetically ordered ground state. We propose a ferrimagnetic spin arrangement that is consistent with neutron diffraction measurements as well. Frequency dependence in the AC magnetic susceptibility, observed at 5.5 K, is considered as short-range magnetic ordering and may be associated with the competition between nearest neighbor and next nearest neighbor interactions of the V4+ cations. A dielectric anomaly near 240 K and non-centrosymmetric functional properties, notably, second harmonic generation and electric polarization, are also discussed. © 2010 American Chemical Society.
  • Rodriguez, E. et al. (2010). Iodine as an Oxidant in the Topotactic Deintercalation of Interstitial Iron in Fe1+xTe. Journal of the American Chemical Society [Online] 132:10006-10008. Available at: http://dx.doi.org/10.1021/ja104004t.
    The layered telluride, Fe1+xTe, is a parent compound of the isostructural and superconducting phases, Fe1+x(Te, Se, S). Here we show that, through a simple reaction of I2 vapor with both powder and single crystal samples, the interstitial iron can be removed from the FeTe framework topotactically. Neutron powder diffraction and X-ray single crystal diffraction confirm that the iron being extracted is the partially occupied site that lies between the 2-D blocks of edge-sharing FeTe4 tetrahedra. The deintercalation process has consequences for both magnetic and crystallographic phase transitions in the compound at low temperatures. This technique could be of use for the tuning of stoichiometry of the superconducting phases and therefore enable more careful studies on how chemical composition affects magnetic and superconducting properties.
  • Green, M. (2010). Crystal engineering in two dimensions: Surface attraction. Nature Materials [Online] 9:539-540. Available at: http://dx.doi.org/10.1038/nmat2786.
    A new route to layer-by-layer assembly of metal-organic framework thin films affords highly ordered and controllable surfaces with potential in chemical sensing and catalyst applications. The fabrication of nanoscale MOF films is a particularly valuable objective towards their commercial utilization. The Langmuir-Blodgett method of producing homogeneous layers is an ingenious extension of this experiment, whereby, after a compacting process, the monolayers on the surface of a liquid are swept onto a substrate by a simple immersion procedure. (Metallo)porphyrins have been shown to constitute a versatile pallet for chemists to explore new structures and bonding. The fundamental importance of the work, on the other hand, is arguably a combination of the facile synthesis and the simply staggering possibilities available in the future in adapting the combination of metals and ligands to produce distinct multiply stacked nanofilms.
  • Ji, S. et al. (2010). Orbital order and partial electronic delocalization in a triangular magnetic metal Ag2MnO2. Physical Review B - Condensed Matter and Materials Physics [Online] 81:94421-94421. Available at: http://dx.doi.org/10.1103/PhysRevB.81.094421.
    Magnetic and electrical properties of Ag2MnO2 were examined by elastic and inelastic neutron-scattering measurements and by density-functional calculations. The spins of the triangular antiferromagnet metal Ag2MnO2 are found to freeze into a gapless short-range collinear state below 50 K because of a ferro-orbital ordering and spin-orbit coupling of the high-spin Mn3+ ions. The decrease in the spin-spin correlation lengths of Ag2 MnO2 in the order, ξb â?« ξa â?« ξc, is explained by the spin-exchange interactions calculated for the ferro-orbital ordered state. The electronic states around the Fermi level have significant contributions from the spin-polarized Mn3d and O2p states, which makes electron-electron scattering dominate over electron-phonon scattering at low temperatures leading to the Ï?â?? T2 behavior below 50 K. © 2010 The American Physical Society.
  • Kim, M. et al. (2010). Antiferromagnetic ordering in the absence of structural distortion in Ba (Fe1-x Mnx )2 As2. Physical Review B - Condensed Matter and Materials Physics [Online] 82:0-0. Available at: http://dx.doi.org/10.1103/PhysRevB.82.220503.
    Neutron and x-ray diffraction studies of Ba (Fe1-x Mn x)2 As2 for low doping concentrations (x�0.176) reveal that at a critical concentration, 0.102&lt;x&lt;0.118, the tetragonal-to- orthorhombic transition abruptly disappears whereas magnetic ordering with a propagation vector of (1 2 1 2 1) persists. Among all of the iron arsenides this observation is unique to Mn doping, and unexpected because all models for "stripelike" antiferromagnetic order anticipate an attendant orthorhombic distortion due to magnetoelastic effects. We discuss these observations and their consequences in terms of previous studies of Ba (Fe 1-x T Mx) 2 As2 compounds (TM=transitionmetal), and models for magnetic ordering in the iron arsenide compounds. © 2010 The American Physical Society.

Conference or workshop item

  • Bakaimi, I. et al. (2014). Crystal, magnetic and dielectric studies of the 2D antiferromagnet: β-NaMnO2. in: 5th Annual Oxide Based Materials and Devices Conference. SPIE. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84901763630&partnerID=40&md5=f84bb59411d9a591f17d4ab1df3bc8d2.
    In this paper we present our recent studies on the crystal, magnetic and dielectric properties of the β-NaMnO2. Experimental results of neutron powder and electron diffraction combined with measurements of the dielectric permittivity suggest that the β-NaMnO2 is an excellent candidate for studying the coupling between the magnetic and electric degrees of freedom. Neutron powder diffraction data reveal the existence of a commensurate and an incommensurate magnetic structure at 200 K and below 100 K, respectively. Dielectric anomalies which appear at the temperature regions where the two magnetic structures emerge, indicate the appearance of magnetodielectric coupling. © 2014 SPIE.
  • Natali Sora, I. et al. (2013). Charge compensation and magnetic properties in Sr and Cu doped La-Fe perovskites. in: EPJ Web of Conferences.. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84879749580&partnerID=40&md5=0f1b0ffbbda3f17665ed69f7ae3a9cc9.
    Orthorhombic lanthanum orthoferrites La0.8Sr 0.2Fe1-yCuyO3-w (y = 0 and 0.10) have been studied using X-rays and neutron powder diffraction (XRPD and NPD), magnetization measurements and 57Fe Mössbauer spectroscopy. Rietveld refinements on XRPD and NPD data show that they adopt an orthorhombic ABO3 perovskite symmetry with La/Sr and Fe/Cu atoms randomly distributed on crystal A and B sites, respectively. The magnetic structure at room temperature is antiferromagnetic, with the Fe/Cu magnetic moments aligned along the a axis. Magnetization curves versus temperature show that the compounds exhibit an overall antiferromagnetic and a weak ferromagnetic behaviour in the range 5-298 K. 57Fe Mössbauer spectroscopy measurements indicate that Fe3+ and Fe5+ ions coexist in both compounds, and the relative percentage of Fe5+ is almost the same at 77 and 170 K, rejecting a charge disproportion mechanism. © 2013 Owned by the authors published by EDP Sciences.
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