Orbital molecules in vanadium oxide spinels
Browne, Alexander James
MetadataShow full item record
Orbital molecules are clusters of transition metal cations, formed by orbital ordering in materials with extended structures that allow direct d-orbital interactions. Vanadium oxides exhibit an especially rich variety of orbital molecule states, with dimers and trimers identified in numerous systems. VO₂, in which V-V dimerisation accompanies a metal-insulator transition, is a particularly well-known example. Materials of general composition AB₂O₄ often adopt the spinel structure. As this structure features edge-sharing chains of BO₆ octahedra it is a good motif for orbital molecule formation, and the choice of A-site cation allows both the B-site oxidation state and the B-B separation to be varied. Unusually large V₇ ‘heptamer’ orbital molecules had been reported to form in the spinel AlV₂O₄ below an ordering transition at 700 K. Atomic pair distribution function analysis was used to investigate the V-V bonding in this material and reveals that the heptamers are actually ordered pairs of V₃ trimers and V₄ tetramers. Furthermore, these orbital molecules persist into a structurally disordered phase above the 700 K transition and remain well-defined to temperatures of at least 1100 K. Analogous behaviour is found in GaV₂O₄, a newly-synthesised spinel. It is isoelectronic with AlV₂O₄ and crystallographic and local-structure characterisation, complemented by magnetic and transport property measurements, reveals that it has the same V₃ and V₄ orbital molecule states but with a lower ordering temperature, of 415 K. In addition, quasi-elastic neutron scattering indicates that the orbital molecules in the high-temperature phase of GaV₂O₄ have static, rather than dynamic, disorder. By contrast, ZnV₂O₄ has an antiferromagnetic ground state without ordered orbital molecules. The nature of the orbital ordering in this state has been contentious, and has been investigated using X-ray total scattering for the first time. The ground state has a tetragonal structure consistent with long-range ferro-orbital ordering, and V-V bonding is not evident in either its average or local structures. The variation of electronic ordering in ZnₓGa₁₋ₓV₂O₄ solid solutions has also been explored. Whilst the structural and electronic perturbations induced by doping rapidly suppress the long-range ordering found in the two end members, local V-V bonding is remarkably stable with respect to these perturbations and is found in phases with x ≤ 0.875. Powder neutron diffraction and magnetometry measurements suggest that disordered orbital molecules are also present in Li₀.₅Ga₀.₅V₂O₄, another newly-synthesised material. A particularly interesting vanadium oxide is LiV₂O₄, which is one of very few delectron systems in which heavy-fermion behaviour has been found. Although V-V orbital interactions have been implicated in the microscopic origin of this behaviour, no orbital molecule-like distortions are found in the local structure of the heavy-fermion phase. LiV₂O₄ also exhibits a pressure-induced metal-insulator transition, and powder X-ray diffraction under low temperature-high pressure conditions reveals a concurrent cubic-monoclinic structural distortion that may be the result of orbital molecule ordering.