Orbital molecules in vanadium oxide spinels

Abstract

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.