Benedikt Klobes

α-Sb2O3 (senarmontite), β-Sb2O3 (valentinite), and α-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 led to incomplete or unreliable results due to modes being inactive in infrared (IR) or Raman spectroscopy. Here, we present a study of the relationship between bonding and lattice dynamics of these compounds. M¨ossbauer spectroscopy (MS) is used to study the structure of Sb in α-Sb2O3 and β-Sb2O3, whereas the vibrational modes of Sb and Te for each oxide are investigated using nuclear inelastic scattering (NIS) and further information on O vibrational modes is obtained using inelastic neutron scattering (INS). 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 α-Sb2O3 whose frequency was not clear in most previous studies is experimentally observed for the first time at ∼ 340 cm−1. Softer lattice vibrational modes occur in orthorhombic β-Sb2O3 compared to cubic α-Sb2O3, indicating that the antimony bonds are weakened upon transforming from the molecular α phase to the layer-chained β structure. The resulting vibrational entropy increase of 0.45 ± 0.1 kB/Sb2O3 at 880 K accounts for about half of the α-β 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.

Resonant ultrasound spectroscopy is a non-destructive technique for determining the elastic constants of a material. In the context of thermoelectric materials, elastic constants can be used to investigate the speed of sound and, thus, the lattice thermal conductivity of a substance. For Mg2Si-Mg2Sn solid solutions, a possible connection between shear modulus and band convergence can be assumed in that way. Moreover, the rather low speed of sound in this system points towards a high contribution of optical phonons to thermal conductivity. Additionally, significant resonance frequency shifts are observed during natural aging of technical Al-Cu-Mg and Al-Mg-Si alloys, which are proposed to assist investigation of the early stages of clustering in the future.