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.