High‐temperature raman spectroscopy of SiO2 and GeO2 Polymorphs: Anharmonicity and thermodynamic properties at high‐temperatures

Abstract

The temperature dependence of the room pressure Raman spectra of the GeO2 (hexagonal and tetragonal) and SiO2 (quartz, coesite, and stishovite) polymorphs is presented. Several transformations upon heating are reported: α‐β quartz, coesite‐cristobalite, stishovite‐silica glass (with Si in four‐fold coordination). For all the compounds, Raman frequencies decrease linearly with temperature and the measured shifts are used in conjunction with available high‐pressure Raman data to calculate for each compound intrinsic mode anharmonicity through the parameter ai=(∂lnVi/∂T)v. In quartz the modes related to the α‐β transition (128 and 207 cm−1 modes) are highly anharmonic (ai=−20×10−5 K−11). In coesite many bands are unaffected by temperature and only two modes (77 and 116 cm−1) show significant anharmonic behaviour. Tetragonal GeO2 and stishovite behave similarly under temperature. Vibrational modeling of the specific heat and entropy, including anharmonic corrections deduced from the ai parameters, are presented and compared to available calorimetric measurements. For the GeO2 polymorphs the agreement between calculated and measured values is within 1–3% in the temperature range 100 to 1500 K. For quartz the agreement is similar over the range 50–850 K. For coesite and stishovite the models reproduce existing low temperature measurements. High‐temperature values of Cp and entropy are proposed for coesite and stishovite.