Understanding the Gap in Polyoxovanadate Molecular-Based Magnets

We report a joint experimental and theoretical investigation of the transport gap, optical properties, and electronic structure of two chemically similar, inhomogeneously mixed-valent polyoxavanadate molecule-based magnets. We attribute the substantial gap in [NHEt$_3$]$_4$[V$_{8}$$^{IV}$V$_{4}$$^{V}$As$_8$O$_{40}$(H$_2 $O))]$^.$H$_2$O to weak $p$-$d$ hybridization and a large on-site Coulomb repulsion ($U$ = 5 eV). The reduced gap in [NHEt$_3$]$_3$[V$_{6}$$^{IV}$V$_{6}$$^{V}$As$_8$O$_{40}$(HCO$_2 $)]$\cdot$2H$_2$O is associated with a smaller value of $U$, at least from a molecular point of view, although the transport properties also reflect subtle organization of the molecular structure and the difference between direct and indirect intermolecular charge transfer. A detailed analysis of the vibrational response supports the important role of local molecular distortion and hydrogen bonding in the intramolecular and intermolecular charge transport in [NHEt$_3$]$_4$[V$_{8}$$^{IV}$V$_{4}$$^{V}$As$_8$O$_{40}$(H$_2 $O))]$^.$H$_2$O. This work is supported by PRF and the U.S. Department of Energy.