Showing papers by "Brendan J. Kennedy published in 2012"
TL;DR: These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation.
Abstract: The zirconates Ln2Zr2O7 (Ln = lanthanoid) have been studied using a combination of Zr L-edge X-ray absorption near edge structure (XANES) and synchrotron X-ray and neutron powder diffraction methods. These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation. The Zr L-edge spectrum is found to be extremely sensitive to changes in the local coordination environment and demonstrates an increase in local disorder across the pyrochlore oxides. The sensitivity of the XANES measurements enables us to identify the progressive nature of the transition that could not be detected using bulk diffraction techniques.
103 citations
TL;DR: It is shown that the 6H-perovskite Ba(3)BiIr(2)O(9) bioctahedra, which contains 5d Ir(4+) (S = 1/2) dimerized into isolated face-sharing Ir(2), exhibits a giant magnetoelastic effect, the largest of any known 5d compound, associated with the opening of a spin-gap at T* = 74 K.
Abstract: As 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).
38 citations
TL;DR: In this paper, the authors presented the neutron and synchrotron x-ray diffraction and near-edge structure results for polycrystalline La{}_{2}$Ce${}_{ 2}$O${}{7}$.
Abstract: We present the neutron and synchrotron x-ray diffraction and Ce ${L}_{3}$-edge x-ray absorption near-edge structure results for polycrystalline La${}_{2}$Ce${}_{2}$O${}_{7}$. We have found that this compound adopts a disordered defect fluorite structure with diffuse features evident in the neutron-diffraction pattern indicative of anion disorder. Our experimental results are discussed in the context of a recent theoretical paper on this material.
37 citations
TL;DR: The mixed-valence manganite Sr0.65Pr 0.35MnO3 has been prepared and its crystal and magnetic structure investigated between 7 and 1200 K using high-resolution powder neutron diffraction as discussed by the authors.
Abstract: The mixed-valence manganite Sr0.65Pr0.35MnO3 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 MnO6 octahedra about the c axis and a large Jahn–Teller-type distortion due to the presence of Mn3+. The sample exhibits a reversible phase transition from the cubic Pm3 m perovskite to a tetragonal I4/mcm structure at 750 K. The Pm3 m ↔ I4/mcm phase transition is continuous, and the tetragonal strain, which is dominated by the Jahn–Teller-type distortion of the MnO6 octahedra, exhibits an unusual et0.5 ∝ (Tc − T) temperature dependence. At low temperatures, a C-type antiferromagnetic structure develops with a Neel temperature TN of 250 K. The Mn magnetic moment at 7 K is 2.99(2) μB/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.
22 citations
TL;DR: In this paper, two double perovskites Ba2CeMO6 (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.
Abstract: Two Ce3+ containing double perovskites Ba2CeMO6 (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 → R3 and a continuous R3 → Fm3m transition. Analysis of the first order I2/m → R3 transitions is impeded by the complex peak shapes and diffuse scattering evident in the synchrotron powder diffraction data because of domain wall effects.
19 citations
TL;DR: In this article, the authors studied the crystal structure of mixed-valence Sr0.7Ce0.3MnO3 from 4.2 to 973 K using high-resolution neutron powder diffraction.
Abstract: We have studied the crystal structure of mixed-valence Sr0.7Ce0.3MnO3 from 4.2 to 973 K using high-resolution neutron powder diffraction. The crystal structure is tetragonal in space group I4/mcm at 4.2–923 K and cubic in Pm3 m at T ≥ 948 K. Lattice parameters and Mn-O bond distances, obtained by Rietveld refinement, have been used to derive the spontaneous strains and MnO6 octahedral distortion, which are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a Pm3 m reference structure with two instabilities (R4+ and Γ3+). Two phase transitions were proposed: an octahedral tilting transition at Tc,φ ∼ 938 K (Pm3 m↔ I4/mcm, R4+), and an isosymmetric, electronically driven (Jahn-Teller–like) transition at Tc,JT ∼ 770 K (I4/mcm, R4+ ↔ I4/mcm, R4+ and Γ3+). The nature of the tilting transition appears to be tricritical, while that of the Jahn-Teller–like transition is second order. In addition to the contributions from octahedral tilting and Jahn-Teller–like distortions, there is an excess octahedral distortion at temperatures below 250 K; this is speculated to be associated with an anomaly observed over the temperature range of 275–300 K in the heat-capacity measurements.
16 citations
TL;DR: In this article, the structure of double perovskites Ba2-xSrxInTaO6 was studied from room temperature to 1000°C by high-resolution synchrotron X-ray powder diffraction.
Abstract: The utilization of the high resolution powder diffractometer at the Australian Synchrotron to obtain accurate and precise structures of some perovskite-type oxides is described. The structure of CdTiO3 has been studied from room temperature to 1000°C by high-resolution synchrotron X-ray powder diffraction. It was found that CdTiO3 remains orthorhombic in the Pbnm space group over the entire temperature range, with the expansion in the cell volume well fitted to the expression . The magnitudes of the TiO6 tilts are estimated from the refined structural parameters and these progressively reduce as the temperature is increased. The effect of Sr content on the room temperature structure of the double perovskites Ba2–xSrxInTaO6 is also described. At room temperature Ba2InTaO6 crystallizes in a cubic structure in space group . Doping with Sr results in tilting of the corner sharing octahedra with a concurrent lowering of symmetry with the sequence of structures being
5 citations
TL;DR: In this article, the manganite perovskites Ca 0.2 Sr 0.6 Nd 0. 2 Mn 1− x Cr x O 3, 0.1 and 0.2 were synthesized by solid state methods, and their room temperature structures determined using synchrotron X-ray powder diffraction.
2 citations