Orbital structures in spinel vanadates AV2O4 (A = Fe, Mn)

Spinel FeV2O4 exhibits successive structural phase transitions, reflecting the interplay between the Fe2+ (3d(6)) and V3+ (3d(2)) ions, both of which have orbital and spin degrees of freedom. The temperature-dependent orbital shapes of Fe2+ and V3+ were investigated by means of single-crystal structure analysis, and were compared with those in MnV2O4, where only the V3+ ions are Jahn-Teller active. The highest-temperature transition from the cubic to the high-temperature tetragonal phase was driven by a ferroic Fe2+ 3z(2)-r(2) orbital order (OO). At 110 K, where the ferrimagnetic transition takes place, the magnetic order modified the orbital shape through intratomic spin-orbit coupling, causing an orthorhombic distortion. The V3+ orbital order (V-OO) contributed to the lowest temperature transition from the orthorhombic to the low-temperature tetragonal phase. The V-OO in FeV2O4 was qualitatively different from that in MnV2O4. We propose that ferro-OO contains a complex orbital in FeV2O4 in contrast to the V-OO of real orbitals observed in MnV2O4.