Color properties of the model spin chain materialsVOHPO4⋅12H2Oand(VO)2P2O7: Spectroscopy and electronic structure calculations

Abstract

We report the optical properties of two prototypical $S=1∕2$ magnetic materials: vanadyl (IV) hydrogen-phosphate hemihydrate $\mathrm{V}\mathrm{O}\mathrm{H}\mathrm{P}{\mathrm{O}}_{4}\ensuremath{\cdot}\frac{1}{2}{\mathrm{H}}_{2}\mathrm{O}$ and its derivative vanadyl pyrophosphate ${(\mathrm{V}\mathrm{O})}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}$. Local density approximation electronic structure calculations are used to identify and evaluate correct structures, assign the observed excitations, and quantify bonding and hybridization effects in both materials. $\mathrm{V}\mathrm{O}\mathrm{H}\mathrm{P}{\mathrm{O}}_{4}\ensuremath{\cdot}\frac{1}{2}{\mathrm{H}}_{2}\mathrm{O}$ displays a strong color band that is derived from ${\mathrm{V}}^{4+}\phantom{\rule{0.2em}{0ex}}d\ensuremath{\rightarrow}d$ excitations. It is sensitive to both the local vanadium environment and the enhanced low-temperature hydrogen bonding between layers. In contrast, ${(\mathrm{V}\mathrm{O})}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}$ displays a diffuse and gradually rising near infrared absorption in all directions. The $\mathrm{O}\phantom{\rule{0.2em}{0ex}}p\ensuremath{\rightarrow}\mathrm{V}\phantom{\rule{0.2em}{0ex}}d$ charge transfer gaps in both materials are similar. We predict that the on-site excitations of the transition metal centers may be sensitive to a magnetic state via a magnetic field control of $p\text{\ensuremath{-}}d$ hybridization.