Quantum mechanical calculations of the vibrational spectra of quartz- and rutile-type GeO2

Heat-treatment and stepwise cooling of as-delivered, water-containing quartz-type GeO₂ powder resulted in transformation into a water-free form. A rutile-type modification could be prepared by impregnation of the quartz-type phase with RbOH solutions, drying and annealing. Raman- and FTIR-absorption spectra of quartz- and rutile-type GeO₂ were measured and compared to quantum-mechanical ab initio calculations based on a hybrid functional using the Perdew–Burke–Ernzerhof correlation functional with 16.7% Hartree–Fock exchange density functional. Maximum and mean deviations between measured spectral bands and assigned vibrational modes are 14 and ±8 cm⁻¹ for the quartz-type and 30 and ±13 cm⁻¹ for the rutile-type polymorphic form. Water is incorporated into GeO₄ entities of quartz-type GeO₂; a water-free and structurally stable form can be prepared by a heating up to 1,425 K, tempering at 1,323 K and stepwise cooling. Spectral bands not explained by the calculations suggest defects and distortions in both quartz- and rutile-type structures, in case of the quartz-type one by incomplete transformation into an ideal structure after removing the water, whereas the rutile-type modification most probably incorporates Rb during its synthesis.