Abstract

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.