Synthesis and polymorphism of mixed aluminium-gallium oxides

The synthesis of a new solidsolution of the oxyhydroxide Ga_5–xAl_xO₇(OH) isinvestigated via solvothermalreaction between gallium acetylacetonate and aluminium isopropoxide in1,4-butanediol at 240 °C. A limited compositional range 0 ≤ x ≤ 1.5 is produced, with the hexagonalunit cell parameters refined from powder X-ray diffraction (XRD) showing alinear contraction in unit cell volume with increasing Al content. Solid-state ²⁷Aland ⁷¹Ga NMR spectroscopy show a strong preference for Ga to occupythe tetrahedral sites and Al to occupy the octahedral sites. Using isopropanolas the solvent, g-Ga_2–xAl_xO₃defect spinel solid solutions with x ≤ 1.8 can be prepared at 240 °C in24 hours. These materials are nanocrystalline, as evidenced by their broaddiffraction profiles, but the refined cubic lattice parameter shows a linearrelationship with the Ga:Al content and solid-state NMR spectroscopy again showsa preference for Al to occupy the octahedral sites. Thermal decomposition ofthe Ga_5–xAl_xO₇(OH)occurs via poorly ordered materials that resemble e-Ga_2–xAl_xO₃and k-Ga_2–xAl_xO₃,but g-Ga_2–xAl_xO₃transforms above 750 °C to monoclinic b-Ga_2–xAl_xO₃for 0 ≤ x ≤ 1.3 and to hexagonal a-Ga_2–xAl_xO₃for x = 1.8, with intermediate compositions 1.3 < x < 1.8 giving mixturesof the aand b polymorphs.Solid-state NMR spectroscopy shows only the expected octahedral Al for a-Ga_2–xAl_xO₃and, for b-Ga_2–xAl_xO₃,the ~1:2 ratio of tetrahedral:octahedral Al is in good agreement with Rietveldanalysis of the average structures against powder XRD data. Relative energiescalculated by periodic density functional theory (DFT) confirm that there is a~5.2 kJ mol^–1 penalty for tetrahedral rather than octahedral Al inGa_5–xAl_xO₇(OH), whereas this penalty is muchlower (~2.0 kJ mol^–1) for b-Ga_2–xAl_xO₃,in good qualitative agreement with the experimental NMR spectra.