Phonon Spectroscopy in Antimony and Tellurium Oxides

alpha-Sb2O3 (senarmontite), beta-Sb2O3 (valentinite), and alpha-TeO2 (paratellurite) are compounds with pronounced stereochemically active Sb and Te lone pairs. The vibrational and lattice properties of each have been previously studied but often lead to incomplete or unreliable results due to modes being inactive in infrared or Raman spectroscopy. Here, we present a study of the relationship between bonding and lattice dynamics of these compounds. Mossbauer spectroscopy is used to study the structure of Sb in alpha-Sb2O3 and beta-Sb2O3, whereas the vibrational modes of Sb and Te for each oxide are investigated using nuclear inelastic scattering, and further information on O vibrational modes is obtained using inelastic neutron scattering. Additionally, vibrational frequencies obtained by density functional theory (DFT) calculations are compared with experimental results in order to assess the validity of the utilized functional. Good agreement was found between DFT-calculated and experimental density of phonon states with a 7% scaling factor. The Sb-O-Sb wagging mode of alpha-Sb2O3 whose frequency was not clear in most previous studies is experimentally observed for the first time at similar to 340 cm(-1). Softer lattice vibrational modes occur in orthorhombic beta-Sb2O3 compared to cubic alpha-Sb2O3, indicating that the antimony bonds are weakened upon transforming from the molecular alpha phase to the layer-chained beta structure. The resulting vibrational entropy increase of 0.45 +/- 0.1 k B /Sb2O3 at 880 K accounts for about half of the alpha-beta transition entropy. The comparison of experimental and theoretical approaches presented here provides a detailed picture of the lattice dynamics in these oxides beyond the zone center and shows that the accuracy of DFT is sufficient for future calculations of similar material structures.