V 3$d$ charge and orbital states in V${}_2$OPO${}_4$ probed by x-ray absorption spectroscopy

Abstract

V 3$d$ charge and orbital states in V${}_2$OPO${}_4$ have been investigated by means of x-ray absorption spectroscopy (XAS). The electronic structure of V${}_2$OPO${}_4$ is very unique in that the charge transfer between V${}^{2+}$ and V${}^{3+}$ in face sharing VO${}_6$ chains provides negative thermal expansion as reported by Pachoud {\it et al.} [J. Am. Chem. Soc. {\bf 140}, 636 (2018).] The near edge region of O 1$s$ XAS exhibits the three features which can be assigned to transitions to O 2$p$ mixed into the unoccupied V 3$d$ $t_{2g}$ and $e_g$ orbitals of V${}^{2+}$ and V${}^{3+}$. The V 2$p$ XAS line shape can be reproduced by multiplet calculations for a mixed valence state with V${}^{2+}$ and V${}^{3+}$. The polarization dependence of the O 1$s$ and V 2$p$ XAS spectra indicates V 3$d$ orbital order in which $xy$ and $yz$ (or $zx$) orbitals are occupied at the V${}^{3+}$ site in the face sharing chains. The occupied $xy$ orbital is essential for the antiferromagnetic coupling between the V${}^{2+}$ and V${}^{3+}$ sites along the chains while the occupied $yz$ (or $zx$) orbital provides the antiferromagnetic coupling between the V${}^{2+}$ and V${}^{3+}$ sites between the chains.

18 pages, 7 figures