Coordination Site Disorder in Spinel-Type LiMnTiO₄

LiMnTiO₄ was prepared through solid-state syntheses employing different heating and cooling regimes. Synchrotron X-ray and neutron powder diffraction data found quenched LiMnTiO₄ to form as single phase disordered spinel (space group <i>Fd</i>3̅<i>m</i>), whereas slowly cooled LiMnTiO₄ underwent partial phase transition from <i>Fd</i>3̅<i>m</i> to <i>P</i>4₃32. The phase behavior of quenched and slowly cooled LiMnTiO₄ was confirmed through variable-temperature synchrotron X-ray and neutron powder diffraction measurements. The distribution of Li between tetrahedral and octahedral sites was determined from diffraction data. Analysis of the Mn/Ti distribution in addition required Mn and Ti K-edge X-ray absorption near-edge structure spectra. These revealed the presence of Mn³⁺ in primarily octahedral and Ti⁴⁺ in octahedral and tetrahedral environments, with very slight variations depending on the synthesis conditions. Magnetic measurements indicated the dominance of antiferromagnetic interactions in both the slowly cooled and quenched samples below 4.5 K

Impact of Cu Doping on the Structure and Electronic Properties of LaCr_1–<i>y</i>Cu_<i>y</i>O₃

Oxides of the type LaCr_1–<i>y</i>Cu_<i>y</i>O₃ have been prepared using solid-state methods and their crystal structures refined using synchrotron X-ray powder diffraction. The solubility limit of Cu was found to be around <i>y</i> = 0.2, and such oxides are orthorhombic in space group <i>Pbnm</i>. X-ray absorption spectroscopy measurements at the Cr and Cu L-edges demonstrated that the Cr remains trivalent upon Cu doping, with the Cu being present as Cu­(III). The oxides are found to be antiferromagnets, and the Néel temperature, <i>T</i>_N, decreases as the Cu content is increased. The crystal and magnetic structures of one example La­(Cr_0.85Cu_0.15)­O₃ have been investigated between 3 and 350 K by neutron powder diffraction. The samples are semiconductors

Gradual Structural Evolution from Pyrochlore to Defect-Fluorite in Y₂Sn_2–<i>x</i>Zr_<i>x</i>O₇: Average vs Local Structure

We have studied the long-range average and local structures in Y₂Sn_2–<i>x</i>Zr_<i>x</i>O₇ (<i>x</i> = 0–2.0) using synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, respectively, and by theoretical methods. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect-fluorite at <i>x</i> ∼ 1.0–1.2, X-ray absorption near-edge structure (XANES) results at the Zr L₃- and Y L₂-edges reveal a gradual structural evolution across the whole compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary, as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defect-fluorite region. The Zr and Y L-edge spectra are very sensitive to changes in the local structure; such sensitivity enables us to reveal the progressive nature of the phase transition. Experimental results are supported by <i>ab initio</i> atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures are predicted, and the trends agree well with the experimental XANES results. The calculations predict that the coordination of cations in the Y₂Zr₂O₇ defect-fluorite (normally considered to be 7 for all cations) varies depending on the species with the average coordination of Y and Zr being 7.2 and 6.8, respectively

Probing Long- and Short-Range Disorder in Y₂Ti_2–<i>x</i>Hf_<i>x</i>O₇ by Diffraction and Spectroscopy Techniques

We studied the long-range average and short-range local structures in Y₂Ti_2–<i>x</i>Hf<i>_x</i>O₇ (<i>x</i> = 0–2.0) using diffraction and spectroscopy techniques, respectively. Both neutron and synchrotron X-ray powder diffraction data show a clear phase transition of the average structure from ordered pyrochlore to disordered defect-fluorite at <i>x</i> ≈ 1.6; the long-range anion disorder appears to develop gradually throughout the entire pyrochlore region in contrast to the rapid loss of cation ordering from <i>x</i> = 1.4 to 1.6. The commonly observed two-phase region around the pyrochlore/defect-fluorite phase boundary is absent in this system, demonstrating high sample quality. X-ray absorption near-edge structure (XANES) results at the Y L₂-, Ti K- and L_3,2-, Hf L₃-, and O K-edges indicate a gradual local structural evolution across the whole compositional range; the Y coordination number (CN) decreases and the CN around Ti and Hf increases with increasing Hf content (<i>x</i>). The spectroscopic results suggest that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary as determined by diffraction, and this disorder evolves continuously from short- to medium- and eventually to long-range detectable by diffraction. This study highlights the complex disordering process in pyrochlore oxides and the importance of a multitechnique approach to tackle disorder over different length scales and in the anion and cation sublattices, respectively. The results are important in the context of potential applications of these oxides such as ionic conductors and radiation-resistant nuclear waste forms

Giant Magnetoelastic Effect at the Opening of a Spin-Gap in Ba₃BiIr₂O₉

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⁺ (<i>S</i> = 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 <i>T</i>* = 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)

Giant Magnetoelastic Effect at the Opening of a Spin-Gap in Ba₃BiIr₂O₉

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⁺ (<i>S</i> = 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 <i>T</i>* = 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)

Key Role of Bismuth in the Magnetoelastic Transitions of Ba₃BiIr₂O₉ and Ba₃BiRu₂O₉ As Revealed by Chemical Doping

The 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