Germanium mullite: Structure and vibrational spectra of gels, glasses and ceramics

The Raman spectra of mullite phases in GeO2-Al2O3 and GeO2-Ga2O3 systems are compared with those of silicon mullite and sillimanite. Replacement of silicon by germanium and aluminium by gallium modifies the wavenumbers of certain lines, especially at high frequency. However, all germanium mullites present similar features which reflect their disordered character. Even for compositions Al2GeO5 and Ga2GeO5 which are homologous to that of sillimanite, Raman spectra consist of broad bands, as previously observed in silicon mullites. The evolution of spectra is discussed for a series of Al4+2xGe2-2xO10-x mullite phases with 0 < x < 0.47. The spectra of amorphous gels and glasses prepared by slow hydrolysis-polycondensation of aluminium and germanium (silicon) alkoxide mixtures show rather narrow bands. This indicates that the polymeric arrangement of molecular entities is less distorted in non-crystalline precursors than in crystalline mullite. Finally, a phase relation diagram is proposed for the sol-gel route.