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 °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-Ga2–xAlxO3 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 Ga5–xAlxO7(OH) occurs via poorly ordered materials that resemble ε-Ga2–xAlxO3 and κ-Ga2–xAlxO3, but γ-Ga2–xAlxO3 transforms above 750 °C to monoclinic β-Ga2–xAlxO3 for 0 ≤ x ≤ 1.3 and to hexagonal α-Ga2–xAlxO3 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-Ga2–xAlxO3, and for β-Ga2–xAlxO3, 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 Ga5–xAlxO7(OH), whereas this penalty is much smaller (∼2.0 kJ mol–1) for β-Ga2–xAlxO3, in good qualitative agreement with the experimental NMR spectra.
Solvothermal reactions in 1,4-butanediol or isopropanol yield a new solid solution of the oxyhydroxide Ga5−xAlxO7(OH) or the spinel series γ-Ga2−xAlxO3, respectively. We have explored the possible composition range and the thermal stability of the materials. For γ-Ga2−xAlxO3 (x ≤ 1.3), transformation to the monoclinic β-polymorph occurs above 1000 °C, providing a convenient route to this material of interest for electronic applications.