The system Ga2O3(Al2O3)–GeO2(SiO2) studied by NMR, XRD, IR and DTA

The formation of germanate phases with the mullite structure [Ga6Ge2O13, Al6Ge2O13 and (Ga,Al)6Ge2O13] from oxide mixtures and hybrid sol–gel precursors was studied by thermal analysis, XRD, IR spectroscopy and multinuclear MAS NMR. The course of the thermal reactions in all the Ga-containing oxide mixtures, irrespective of the starting composition, is dominated by the intermediate formation below 1150 °C of α- and β-Ga4GeO8 and by the volatilization of GeO2 at higher temperatures. In oxide mixtures containing alumina, Ga incorporation in the mullite-type phase occurs only above 1150 °C. IR spectroscopy confirms the incorporation of both Al and Ga into the mullite-type structure at higher temperature. No reaction with Ga2O3 occurred where the GeO2 was replaced by SiO2. Thermal treatment of gel precursors produces intermediate amorphous phases in which the Al is in typical sites with a 27Al MAS NMR resonance at 33 ppm, in addition to tetrahedral and octahedral sites. Crystallization of the amorphous intermediates below 1300 °C produces mixtures of the mullite phase and Ga4GeO8. Many of the 71Ga MAS NMR spectra are dominated by the quadrupolar lineshape of β-Ga2O3 from the structural units of α-Ga4GeO8 or unreacted starting material. The 71Ga MAS NMR spectrum of Ga6Ge2O13 is broad and featureless, as are the 69Ga MAS NMR spectra of these systems. The 17O NMR spectra of an isotopically enriched gel and its reaction products are also reported.