Vibrations and Phase Stability in Mixed Valence Antimony Oxide

α-Sb₂O₄ (cervantite) and β-Sb₂O₄ (clinocervantite) are mixed valence compounds with equal proportions of Sb^III and Sb^V as represented in the formula Sb^IIISb^VO₄. Their structure and properties can be difficult to calculate owing to the Sb^III lone-pair electrons. Here, we present a study of the lattice dynamics and vibrational properties using a combination of inelastic neutron scattering, Mössbauer spectroscopy, nuclear inelastic scattering, and density functional theory (DFT) calculations. DFT calculations that account for lone-pair electrons match the experimental densities of phonon states. Mössbauer spectroscopy reveals the β phase to be significantly harder than the α phase. Calculations with O vacancies reveal the possibility for nonstoichiometric proportions of Sb^III and Sb^V in both phases. An open question is what drives the stability of the α phase over the β phase, as the latter shows pronounced kinetic stability and lower symmetry despite being in the high-temperature phase. Since the vibrational entropy difference is small, it is unlikely to stabilize the α phase. Our results suggest that the α phase is more stable only because the material is not fully stoichiometric.