Understanding the gap in polyoxovanadate molecule-based magnets

Abstract

We report a joint experimental and theoretical investigation of the transport gap, optical properties, and electronic structure of two chemically similar, inhomogeneously mixed-valent polyoxovanadate molecule-based magnets. We attribute the substantial gap in $[{\mathrm{NHEt}}_{3}{]}_{4}[{\mathrm{V}}_{8}^{\mathrm{IV}}{\mathrm{V}}_{4}^{\mathrm{V}}{\mathrm{As}}_{8}{\mathrm{O}}_{40}({\mathrm{H}}_{2}\mathrm{O})]∙{\mathrm{H}}_{2}\mathrm{O}$ to weak $p\text{\ensuremath{-}}d$ hybridization and a large on-site Coulomb repulsion $(U=6\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. The reduced gap in $[{\mathrm{NHEt}}_{3}{]}_{3}[{\mathrm{V}}_{6}^{\mathrm{IV}}{\mathrm{V}}_{6}^{\mathrm{V}}{\mathrm{As}}_{8}{\mathrm{O}}_{40}({\mathrm{HCO}}_{2})]∙2{\mathrm{H}}_{2}\mathrm{O}$ is associated with a smaller value of $U$ $(4\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, at least from a molecular point of view, although the transport properties also reflect subtle organization of the molecular structure and differences between direct and indirect intermolecular charge transfer. A detailed analysis of the vibrational response supports the important role of local molecular distortions and hydrogen bonding in the intramolecular and intermolecular charge transport of $[{\mathrm{NHEt}}_{3}{]}_{4}[{\mathrm{V}}_{8}^{\mathrm{IV}}{\mathrm{V}}_{4}^{\mathrm{V}}{\mathrm{As}}_{8}{\mathrm{O}}_{40}({\mathrm{H}}_{2}\mathrm{O})]∙{\mathrm{H}}_{2}\mathrm{O}$.