Vanadium 3d charge and orbital states in V2OPO4 probed by x-ray absorption spectroscopy

V 3d charge and orbital states in V2OPO4 have been investigated by means of x-ray absorption spectroscopy (XAS). The electronic structure of V2OPO4 is very unique in that the charge transfer between V2+ and V3+ in face-sharing VO6 chains provides negative thermal expansion as reported by E. Pachoud [J. Am. Chem. Soc. 140, 636 (2018)].JACSAT0002-786310.1021/jacs.7b09441 The near-edge region of O 1s XAS exhibits the three features which can be assigned to transitions to O 2p mixed into the unoccupied V 3dt2g and eg orbitals of V2+ and V3+. The V 2p XAS line shape can be reproduced by multiplet calculations for a mixed-valence state with V2+ and V3+. The polarization dependence of the O 1s and V 2p XAS spectra indicates V 3d orbital order in which xy and yz (or zx) orbitals are occupied at the V3+ site in the face-sharing chains. The occupied xy orbital is essential for the antiferromagnetic coupling between the V2+ and V3+ sites along the chains, while the occupied yz (or zx) orbital provides the antiferromagnetic coupling between the V2+ and V3+ sites between the chains.