Synthesis and Polymorphism of Mixed Aluminum–Gallium Oxides

The synthesis of a new solid solution of the oxyhydroxide Ga_5–xAl_xO₇(OH) is investigated via solvothermal reaction between gallium acetylacetonate and aluminum isopropoxide in 1,4-butanediol at 240 °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 Al content. Solid-state ²⁷Al and ⁷¹Ga 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, γ-Ga_2–xAl_xO₃ defect spinel solid solutions with x ≤ 1.8 can be prepared at 240 °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:Al content, and solid-state NMR spectroscopy again shows a preference for Al to occupy the octahedral sites. Thermal decomposition of Ga_5–xAl_xO₇(OH) occurs via poorly ordered materials that resemble ε-Ga_2–xAl_xO₃ and κ-Ga_2–xAl_xO₃, but γ-Ga_2–xAl_xO₃ transforms above 750 °C to monoclinic β-Ga_2–xAl_xO₃ for 0 ≤ x ≤ 1.3 and to hexagonal α-Ga_2–xAl_xO₃ for x = 1.8, with intermediate compositions of 1.3 < x < 1.8 giving mixtures of the α- and β-polymorphs. Solid-state NMR spectroscopy shows only the expected octahedral Al for α-Ga_2–xAl_xO₃, and for β-Ga_2–xAl_xO₃, the ∼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 ∼5.2 kJ mol^–1 penalty for tetrahedral rather than octahedral Al in Ga_5–xAl_xO₇(OH), whereas this penalty is much smaller (∼2.0 kJ mol^–1) for β-Ga_2–xAl_xO₃, in good qualitative agreement with the experimental NMR spectra.