Browsing by Author "Miiller, W"
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- ItemAb initio parametrized polarizable force field for rutile-type SnO2(Springer, 2012-05-01) Miiller, W; Kearley, GJ; Ling, CDWe report a new, polarizable classical force field for the rutile-type phase of SnO2, casserite. This force field has been parametrized using results from ab initio (density functional theory) calculations as a basis for fitting. The force field was found to provide structural, dynamical and thermodynamic properties of tin oxide that compare well with both ab initio and experimental results at ambient and high pressures. © 2012, Springer.
- ItemCoexistence of spin glass and antiferromagnetic orders in Ba3Fe2.15W0.85O8.72(IOP Publishing, 2012-03-23) Miiller, W; Auckett, JE; Avdeev, M; Ling, CDBa 3 Fe 2.15 W 0.85 O 8.72 has been grown as large single crystals using the floating-zone method, permitting very precise characterization of the nuclear and magnetic structures by neutron and synchrotron diffraction methods. The results of our structural investigation are combined with dc and ac magnetization and heat capacity measurements to give an unusually complete and detailed picture of a complex magnetic system. The compound crystallizes in the hexagonal perovskite structure (space group P 6 3 / m m c ) and reveals antiferromagnetic order below T N = 290 K. Frequency-dependent ac susceptibility and the presence of magnetic viscosity suggest the onset of a spin glass component in this material below T f = 60 K. These findings are discussed on the basis of detailed analysis of the crystalo-chemical properties, supported by ab initio (density functional theory) calculations.(c) 2011 IOP Publishing Ltd.
- ItemCombined experimental and computational study of oxide ion conduction dynamics in Sr2Fe2O5 brownmillerite(American Chemical Society, 2013-08-13) Auckett, JE; Studer, AJ; Pellegrini, E; Ollivier, J; Johnson, MR; Schober, H; Miiller, W; Ling, CDWe report a detailed study of the dynamics of oxide ionic conduction in brownmillerite-type Sr2Fe2O5, including lattice anisotropy, based on neutron scattering studies of a large (partially twinned) single crystal in combination with ab initio molecular dynamics simulations. Single-crystal diffraction reveals supercell peaks due to long-range ordering among chains of corner-sharing FeO4 tetrahedra, which disappears on heating above 540 °C due to confined local rotations of tetrahedra. Our simulations show that these rotations are essentially isotropic, but are a precondition for the anisotropic motion that moves oxide ions into the tetrahedral layers from the octahedral layers, which we observe experimentally as a Lorentzian broadening of the quasielastic neutron scattering spectrum. This continual but incoherent movement of oxide ions in turn creates conduction pathways and activates long-range diffusion at the interface between layers, which appears to be largely isotropic in two dimensions, in contrast with previously proposed mechanisms that suggest diffusion occurs preferentially along the c axis.© 2013, American Chemical Society.
- ItemComplex 5d magnetism in a novel S= 1/2 trimer system, the 12L hexagonal perovskite ba4biir3o12(American Chemical Society, 2013-10-21) Miiller, W; Dunstan, MT; Huang, Z; Mohamed, Z; Kennedy, BJ; Avdeev, M; Ling, CDThe 12L hexagonal perovskite Ba4BiIr3O12 has been synthesized for the first time and characterized using high-resolution neutron and synchrotron X-ray diffraction as well as physical properties measurements. The structure contains Ir3O12 linear face-sharing octahedral trimer units, bridged by corner-sharing BiO6 octahedra. The average electronic configurations of Ir and Bi are shown to be +4(d5) and +4(s1), respectively, the same as for the S = 1/2 dimer system Ba3BiIr2O9, which undergoes a spin-gap opening with a strong magnetoelastic effect at T* = 74 K. Anomalies in magnetic susceptibility, heat capacity, electrical resistivity, and unit cell parameters indeed reveal an analogous effect at T* ≈ 215 K in Ba4BiIr3O12. However, the transition is not accompanied by the opening of a gap in spin excitation spectrum, because antiferromagnetic coupling among S = 1/2 Ir4+ (d5) cations leads to the formation of a S = 1/2 doublet within the trimers, vs S = 0 singlets within dimers. The change in magnetic state of the trimers at T* leads to a structural distortion, the energy of which is overcompensated for by the formation of S = 1/2 doublets. Extending this insight to the dimer system Ba3BiIr2O9 sheds new light on the more pronounced low-temperature anomalies observed for that compound. © 2013, American Chemical Society.
- ItemFrustrated magnetism and local structural disorder in pyrochlore-type Bi1.89Fe1.16Nb0.95O6.95(Institute of Physics, 2010-12-08) Miiller, W; Causeret, L; Ling, CDThe magnetic properties of pyrochlore-type Bi1.89Fe1.16Nb0.95O6.95 have been investigated for the first time using AC and DC susceptibility. The compound is shown to behave as a classical spin glass due to strong competition/coexistence of ferro- and antiferromagnetic exchange. The study was accompanied by the first-ever growth of single crystals of this compound using the floating-zone method, allowing us to carry out a single-crystal neutron diffraction experiment that confirmed and extended our understanding of local structural disorder, driven by the stereochemically active 6s2 electron lone pair on Bi3 + ions. The magnetic properties are discussed in terms of both the topologically frustrated Fe3 + lattice and the role of this local structural disorder. © 2010, Institute of Physics
- ItemGiant magnetoelastic effect at the opening of a spin-gap in Ba3BiIr2O9(American Chemical Society, 2012-01-26) Miiller, W; Avdeev, M; Zhou, Q; Kennedy, BJ; Sharma, N; Kutteh, R; Kearley, GJ; Schmid, S; Knight, KS; Blanchard, PER; Ling, CDAs compared to 3d (first-row) transition metals, the 4d and 5d transition metals have much more diffuse valence orbitals. Quantum cooperative phenomena that arise due to changes in the way these orbitals overlap and interact, such as magnetoelasticity, are correspondingly rare in 4d and 5d compounds. Here, we show that the 6H-perovskite Ba3BiIr2O9, which contains 5d Ir4+ (S = 1/2) dimerized into isolated face-sharing Ir2O9 bioctahedra, exhibits a giant magnetoelastic effect, the largest of any known 5d compound, associated with the opening of a spin-gap at T* = 74 K. The resulting first-order transition is characterized by a remarkable 4% increase in Ir–Ir distance and 1% negative thermal volume expansion. The transition is driven by a dramatic change in the interactions among Ir 5d orbitals, and represents a crossover between two very different, competing, ground states: one that optimizes direct Ir–Ir bonding (at high temperature), and one that optimizes Ir–O–Ir magnetic superexchange (at low temperature). © 2012 American Chemical Society
- ItemGiant magnetoelasticity at a spin gap transition in the 5d oxide Ba3Bilr2O9(Australian Institute of Physics, 2013-02-06) Ling, CD; Miiller, W; Kennedy, BJ; Avdeev, MNot available
- ItemGiant magnetoelasticity at a spin gap transition in the 5d oxide Ba3Bilr2O9(Australian Institute of Nuclear Science and Engineering, 2013-12-03) Ling, CD; Miiller, W; Kennedy, BJ; Avdeev, MThe 4d and 5d transition metals have much more diffuse valence orbitals than their 3rd (first-row) transition metals. Quantum cooperative phenomena that arise due to changes in the way these orbitals overlap and interact, such as magnetoelasticity, are therefore relatively rare in 4d and 5d compounds. However, we recently discovered a 6H-perovskite Ba_3Bilr_2O_9, containing 5d lr"4"+ (S = 1/2) dimerised into isolated face-sharing lr_2O_9 bi-octahedra, which exhibits a giant magnetoelastic effect - the largest of any known 5d compound - upon the opening a spin-gap at T* = 74 K. The first-order transition observed by neutron powder diffraction is characterised by a remarkable 4% increase in Ir-Ir distance and 1% negative thermal volume expansion. The transition is driven by a dramatic change in the interactions among Ir 5d orbitals, and represents a crossover between two very different, competing, ground states: one that optimises direct lr-lr bonding (at high temperature); and one that optimises lr-O-lr magnetic superexchange (at low temperature).
- ItemImpact of Jahn-Teller active Mn3+ on strain effects and phase transitions in Sr0.65Pr0.35MnO3(American Physical Society, 2012-03-19) Tan, TY; Kennedy, BJ; Zhou, QD; Ling, CD; Miiller, W; Howard, CJ; Carpenter, MA; Knight, KSThe mixed-valence manganite Sr(0.65)Pr(0.35)MnO(3) has been prepared and its crystal and magnetic structure investigated between 7 and 1200 K using high-resolution powder neutron diffraction. The structural and lattice parameter data have been used to determine the octahedral tilting and spontaneous strains associated with the structural, electronic, and magnetic phase transitions. At room temperature, the structure is tetragonal and is characterized by cooperative out-of-phase tilts of the MnO(6) octahedra about the c axis and a large Jahn-Teller-type distortion due to the presence of Mn(3+). The sample exhibits a reversible phase transition from the cubic Pm3m perovskite to a tetragonal I4/mcm structure at 750 K. The Pm (3) over barm <-> I4/mcm phase transition is continuous, and the tetragonal strain, which is dominated by the Jahn-Teller-type distortion of the MnO(6) octahedra, exhibits an unusual e(0.5)(t)alpha(T(c)-T) temperature dependence. At low temperatures, a C-type antiferromagnetic structure develops with a Neel temperature T(N) of 250 K. TheMnmagnetic moment at 7 K is 2.99(2) mu(B)/Mn. The magnetic ordering introduces additional tetragonal strain, and this strain shows the expected quadratic dependence on the magnetic moment at low temperatures. An increase in the octahedral tilt angle at TN demonstrates an effective coupling between the magnetic ordering process and octahedral tilting. © 2012, American Physical Society
- ItemKey role of Bismuth in the magnetoelastic transitions of Ba3BiIr2O9 and Ba3BiRu2O9 as revealed by chemical doping(America Chemical Society, 2013-12-24) Blanchard, PER; Huang, ZX; Kennedy, BJ; Liu, S; Miiller, W; Reynolds, EM; Zhou, QD; Avdeev, M; Zhang, ZM; Aitken, JB; Cowie, BCC; Jang, LY; Tan, TT; Li, S; Ling, CDThe key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba3BiRu2O9 and Ba3BiIr2O9 has been confirmed by systematically replacing bismuth with La3+ and Ce4+. Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba3BiRu2O9 and Ba3BiIr2O9 have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L3-edge spectrum of Ba3BiRu2O9). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba3BiRu2O9 and Ba3BiIr2O9, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials. © 2013 American Chemical Society.
- ItemLocal structure, dynamics, and the mechanisms of oxide ionic conduction in Bi26Mo10O69(American Chemical Society, 2012-12-11) Ling, CD; Miiller, W; Johnson, MR; Richard, D; Rols, S; Madge, J; Evans, IRWe report the results of a computational and experimental study into the stabilized fluorite-type delta-Bi(2)O(3)-related phase Bi(26)Mo(10)O(69) aimed at clarifying the local and average structure, for which two distinct models have previously been proposed, and the oxide ionic diffusion mechanism, for which three distinct models have previously been proposed. Concerning the structure, we propose a new model in which some molybdenum atoms have higher coordination numbers than 4; that is, some MoO(5) trigonal bipyramids coexist with MoO(4) tetrahedra. This accounts for the additional oxygen required to achieve the nominal composition (a tetrahedrononly model gives Bi(26)Mo(10)O(68)) without invoking a previously proposed unbonded interstitial site, which we found to be energetically unfavorable. All these MoO(x) units are rotationally disordered above a first-order transition at 310 degrees C, corresponding to a first-order increase in conductivity. Concerning oxide ionic diffusion above that transition temperature, we found excellent agreement between the results of ab initio molecular dynamics simulations and quasielastic neutron scattering experiments. Our results indicate a mechanism related to that proposed by Holmes et al. (Chem. Mater. 2008, 20, 3638), with the role previously assigned to partially occupied interstitial oxygen sites played instead by transient but stable MoO(5) trigonal bipyramids and with more relaxed requirements in terms of the orientation and timing of the diffusive jumps. © 2012, American Chemical Society.
- ItemPerovskites in low dimensional multi-layer structure types(International Union of Crystallography, 2014-08) Liu, S; Miiller, W; Liu, Y; Blanchard, PER; Avdeev, M; Kennedy, BJ; Ling, CDThis study introduces examples of structure property relationships within the multi-layered Sillen-Aurivillius family (shown in Figure) and aims to investigate the effect of chemical doping and lattice matching effects. The first example involves doping 1/3 of the n = 3 ferroelectric perovskite layers with magnetic transition metal cations in Bi 5 PbTi 3 O 1 4 Cl [1] with charge balancing by removing Pb 2 + for Bi 3 + . A statistical 1:2 distribution of M 3 + and Ti 4 + across all three perovskite layers was found in Bi 6 Ti 2 MO 1 4 Cl, M = Cr 3 + , Mn 3 + , Fe 3 + , resulting in highly strained structures (enhancing the ferroelectricity compared to Bi 5 PbTi 3 O 1 4 Cl) and pronounced spin-glass behavior below T i r r (0) = 4.46 K. Ferroelectric transitions were observed at high temperature for each of the new compounds. Ferroelectric properties were also measured on Bi 6 Ti 2 FeO 1 4 Cl using piezoresponse force microscopy showing hysteretic phase behavior. A new n = 2 Sillen-Aurivillius compound Bi 3 Sr 2 Nb 2 O 1 1 Br, based on Bi 3 Pb 2 Nb 2 O 1 1 Cl [2], was synthesized by simultaneously replacing Pb 2 + with Sr 2 + and Cl - with Br - . Inter-layer mismatch prevented the formation of Bi 3 Sr 2 Nb 2 O 1 1 Cl and Bi 3 Pb 2 Nb 2 O 1 1 Br. Sr 2 + doping reduces the impact of the stereochemically active 6s 2 lone pair found on Pb 2 + and Bi 3 + , resulting in a stacking contraction in the lattice parameters by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br - . X-ray Absorption Near Edge Structure spectra analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi 3 Sr 2 Nb 2 O 1 1 Br compared to Bi 3 Pb 2 Nb 2 O 1 1 Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion. © International Union of Crystallography
- ItemSillen-aurivillius intergrowth phases as templates for naturally layered multiferroics(American Chemical society, 2012-9-21) Liu, S; Miiller, W; Liu, Y; Avdeev, M; Ling, CDThe ferroelectric Sillen-Aurivillius phase Bi5PbTi3O14Cl, a layered structure containing three-layer perovskite-type blocks, has been modified by substituting magnetic transition metal cations M3+ = Cr3+, Mn3+, or Fe3+ for 1/3 of the Ti4+ cations, accompanied by co-doping of Bi3+ for Pb2+. The aim of the modification was to produce naturally layered ferroelectromagnetic compounds. Rietveld refinements against high-temperature synchrotron X-ray powder diffraction data show that the resulting new compounds Bi6Ti2MO14Cl undergo non-centrosymmetric (P2an) to centrosymmetric (P4/mmm) ferroelectric phase transitions for Bi6Ti2CrO14Cl at 974.6(2) K, Bi6Ti2MnO14Cl at 913.5(6) K, and Bi6Ti2FeO14Cl at 1044.8(1) K. Ferroelectric properties were measured on Bi6Ti2FeO14Cl using piezoresponse force microscopy which showed typical ferroelectric hysteresis behavior in the polarization with varying field strength as well as a piezoelectric strain. Combined Rietveld refinements against X-ray and neutron powder diffraction data indicate a statistical 1:2 distribution of M3+ and Ti4+ across all three perovskite layers, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi5PbTi3O14Cl) and pronounced spin-glass (cluster glass-type) behavior below T-irr(0) = 4.46 K that we have characterized by detailed magnetic susceptibility and heat capacity measurements. © 2012, American Chemical Society.
- ItemSpin-gap opening accompanied by a strong magnetoelastic response in the S=1 magnetic dimer system Ba3BiRu2O9(American Physical Society, 2011-12-15) Miiller, W; Avdeev, M; Zhou, Q; Studer, AJ; Kennedy, BJ; Kearley, GJ; Ling, CDNeutron diffraction, magnetization, resistivity, and heat-capacity measurements on the 6H-perovskite Ba3BiRu2O9 reveal simultaneous magnetic and structural dimerization driven by strong magnetoelastic coupling. An isostructural but strongly displacive first-order transition on cooling through T∗=176 K is associated with a change in the nature of direct Ru–Ru bonds within Ru2O9 face-sharing octahedra. Above T∗, Ba3BiRu2O9 is an S=1 magnetic dimer system with intradimer exchange interactions J0/kB=320 K and interdimer exchange interactions J′/kB=−160 K. Below T∗, a spin-gapped state emerges with Δ≈220 K. Ab initio calculations confirm antiferromagnetic exchange within dimers, but the transition is not accompanied by long-range magnetic order. ©2011 American Physical Society
- ItemStructural disorder and classical spin-glass behaviour in Ba3Fe2SbO9(CSIRO Publishing, 2014-09-08) Auckett, JE; Miiller, W; Avdeev, M; Kimpton, JA; Ling, CDA new 6H-type perovskite Ba3Fe2SbO9 has been synthesised for the first time. Synchrotron and neutron powder diffraction data reveal complete structural disorder between Sb and Fe in the octahedral perovskite B sites. This results in classical spin-glass behaviour, which we characterise using magnetic susceptibility, magnetisation, and heat capacity measurements, although some evidence is seen for a transition to a partially ordered spin-glass like state below 24 K. The behaviour of Ba3Fe2SbO9 is compared with that of the 6H-type perovskite Ba3Fe2WO9, which displays antiferromagnetic character below TN = 290K before entering a glassy state below Tf = 60 K. Differences between the magnetism in these two phases are discussed in terms of the complete structural disorder between the Fe and Sb ions in the former case, versus partial disorder (limited to the distribution and local orientation of Fe-W and Fe-Fe dimer units) in the latter. © CSIRO 1996-2021
- ItemSynthesis and characterization of the crystal structure and magnetic properties of the new fluorophosphate LiNaCo[PO4]F(American Chemical Society, 2012-08-02) Ben Yahia, H; Shikano, M; Koike, S; Tatsumi, K; Kobayashi, H; Kawaji, H; Avdeev, M; Miiller, W; Ling, CD; Liu, J; Whangbo, MHThe new compound LiNaCo[PO4]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo[PO4]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo[PO4]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9334(6), b = 6.2934(11), c = 11.3556(10) angstrom, and Z = 8. The structure consists of edge-sharing CoO4F2 octahedra forming CoFO3 chains running along the b axis. These chains are interlinked by PO4 tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie Weiss behavior above 60 K with theta = -21 K. The specific heat and magnetization measurements show that LiNaCo[PO4]F undergoes a three-dimensional magnetic ordering at T-mag = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO3 chain along the b-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the a-direction but have a noncollinear arrangement along the c-direction. The noncollinear spin arrangement implies the presence of spin conflict along the c-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations. © 2012, American Chemical Society.
- ItemSynthesis and characterization of the crystal structure, the magnetic and the electrochemical properties of the new fluorophosphate LiNaFe[PO4]F(Royal Society of Chemistry, 2012-8-16) Ben Yahia, H; Shikano, M; Sakaebe, H; Koike, S; Tabuchi, M; Kobayashi, H; Kawaji, H; Avdeev, M; Miiller, W; Ling, CDThe new compound LiNaFe[PO4]F was synthesized by a solid state reaction route, and its crystal structure was determined using neutron powder diffraction data. LiNaFe[PO4]F was characterized by 57Fe Mossbauer spectroscopy, magnetic susceptibility, specific heat capacity, and electrochemical measurements. LiNaFe[PO4]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9568(6) A, b = 6.3959(3) A, c = 11.4400(7) A, V = 801.7(1) A3 and Z = 8. The structure consists of edge-sharing FeO4F2 octahedra forming FeFO3 chains running along the b axis. These chains are interlinked by PO4 tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The specific heat and magnetization measurements show that LiNaFe[PO4]F undergoes a three-dimensional antiferromagnetic ordering at TN = 20 K. The neutron powder diffraction measurements at 3 K show that each FeFO3 chain along the b-direction is ferromagnetic (FM), while these FM chains are antiferromagnetically coupled along the a and c-directions with a non-collinear spin arrangement. The galvanometric cycling showed that without any optimization, one mole of alkali metal is extractable between 1.0 V and 5.0 V vs. Li+/Li with a discharge capacity between 135 and 145 mAh g-1. © 2012, Royal Society of Chemistry.
- ItemSynthesis, structural and magnetic studies of the double perovskites Ba2CeMO6 (M = Ta, Nb)(American Chemical Society, 2012-07-16) Zhou, QD; Blanchard, PER; Kennedy, BJ; Reynolds, EM; Zhang, Z; Miiller, W; Aitken, JB; Avdeev, M; Jang, LY; Kimpton, JATwo Ce(3+) containing double perovskites Ba(2)CeMO(6) (M = Nb and Ta) have been prepared through the use of mildly reducing conditions, and the Ce valence state has been shown to be +3 through Ce L-edge X-ray absorption measurements. Both oxides adopt a monoclinic structure in I2/m at room temperature and undergo two phase transitions upon heating, a discontinuous I2/m -> R (3) over bar and a continuous R (3) over bar -> Fm (3) over barm transition. Analysis of the first order I2/m -> R (3) over bar transitions is impeded by the complex peak shapes and diffuse scattering evident in the synchrotron powder diffraction data because of domain wall effects. © 2012, American Chemical Society.
- ItemYCa3(VO)3(BO3)4: a kagom compound based on vanadium(III) with a highly frustrated ground state(American Chemical Society, 2011-01-31) Miiller, W; Christensen, M; Khan, A; Sharma, N; Macquart, RB; Avdeev, M; McIntyre, GJ; Piltz, RO; Ling, CDA new S = 1 kagomé compound based on vanadium(III) is reported. The structure was refined simultaneously against single-crystal neutron and X-ray diffraction data, as a gaudefroyite-type with a new supercell (a′ = 2a + b, b′ = −a − 2b, c′ = c) driven by the ordering of columns of isolated triangular BO3(3)− ions. Low-temperature neutron powder diffraction and magnetic (dc and ac susceptibility) data rule out the presence of long-range magnetic order above at least 1.5 K, but specific heat data suggest that the ground state involves short-range magnetic order, which is frustrated by the coexistence/competition of FM and AFM correlations, together with the characteristic geometric frustration of the kagomé lattice. Magnetic susceptibility data rule out a spin-glass state, pointing to an exotic ground state comparable to the spin-ice or spin-liquid states. This makes YCa3(VO)3(BO3)4 one of the most highly frustrated experimental realizations of the kagomé lattice yet discovered. © 2011, American Chemical Society