Synthesis and Polymorphism of Mixed Aluminum-Gallium Oxides

The synthesis of a new solid solution of the oxyhydroxide Ga5-xAlxO7(OH) is investigated via solvothermal reaction between gallium acetylacetonate and aluminum isopropoxide in 1,4-butanediol at 240 degrees C. A limited compositional range of 0 <= x <= 1.5 is produced, with the hexagonal unit cell parameters refined from powder X-ray diffraction (XRD) showing a linear contraction in unit cell volume with an increase in AI content. Solid-state Al-27 and Ga-71 nuclear magnetic resonance (NMR) spectroscopies show a strong preference for Ga to occupy the tetrahedral sites and Al to occupy the octahedral sites. Using isopropanol as the solvent, gamma-Ga2-xAlxO3 defect spinel solid solutions with x <= 1.8 can be prepared at 240 degrees C in 24 h. These materials are nanocrystalline, as evidenced by their broad diffraction profiles; however, the refined cubic lattice parameter shows a linear relationship with the Ga:AI content, and solid-state NMR spectroscopy again shows a preference for AI to occupy the octahedral sites. Thermal decomposition of Ga5-xAlxO7(OH) occurs via poorly ordered materials that resemble epsilon-Ga2-xAlxO3 and kappa-Ga2-xAlxO3, but gamma-Ga2-xAlxO3 transforms above 750 degrees C to monoclinic beta-Ga2-xAlxO3 for 0 <= x <= 1.3 and to hexagonal alpha-Ga2-xAlxO3 for x = 1.8, with intermediate compositions of 1.3 < x < 1.8 giving mixtures of the a- and beta-polymorphs. Solid-state NMR spectroscopy shows only the expected octahedral Al for alpha-Ga2-xAlxO3, and for beta-Ga2-xAlxO3, the similar to 1:2 tetrahedral:octahedral Al ratio is in good agreement with the results of Rietveld analysis of the average structures against powder XRD data. Relative energies calculated by periodic density functional theory confirm that there is an similar to 5.2 kJ mol(-1) penalty for tetrahedral rather than octahedral Al in Ga5-xAlxO7(OH), whereas this penalty is much smaller (similar to 2.0 kJ mol(-1)) for beta-Ga2-xAlxO3, in good qualitative agreement with the experimental NMR spectra.