A High-Resolution Diffraction and Spectroscopic Study of the Low-Temperature Phase Transformation of Hexagonal to Tetragonal GeO2 with and without Alkali Hydroxide Promotion

摘要

Pure and RbOH-impregnated hexagonal GeO2 were studied by high-resolution and -temperature X-ray diffraction, thermal gravimetric analysis coupled with mass spectrometry, and high-temperature Raman spectroscopy in the temperature range 313-1300 K for a direct comparison of the phase transformation from hexagonal to tetragonal GeO2. On both samples, the phase transformation was observed to follow through distinct reaction steps. At low temperatures, a partial transformation to tetragonal GeO2 occurred on both materials likely induced by the presence of strongly bound water. The removal of the latter is accompanied by substantial anisotropic lattice distortions and yields at maximum ~12% tetragonal GeO2. This partial phase transformation appears to be almost independent of the alkali hydroxide addition. In contrast, the final high-temperature phase transformation around 1100 K results in full conversion to tetragonal GeO2 on the RbOH-impregnated GeO2 material, whereas, on pure hexagonal GeO2, only partial transformation to the tetragonal phase was obtained.