Synthesis and polymorphism of mixed aluminium-gallium oxides
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The synthesis of a new solidsolution of the oxyhydroxide Ga5–xAlxO7(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-Ga2–xAlxO3defect 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 Ga5–xAlxO7(OH)occurs via poorly ordered materials that resemble e-Ga2–xAlxO3and k-Ga2–xAlxO3,but g-Ga2–xAlxO3transforms above 750 °C to monoclinic b-Ga2–xAlxO3for 0 ≤ x ≤ 1.3 and to hexagonal a-Ga2–xAlxO3for 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-Ga2–xAlxO3and, for b-Ga2–xAlxO3,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 inGa5–xAlxO7(OH), whereas this penalty is muchlower (~2.0 kJ mol–1) for b-Ga2–xAlxO3,in good qualitative agreement with the experimental NMR spectra.
Cook , D S , Hooper , J E , Dawson , D M , Fisher , J M , Thompsett , D , Ashbrook , S E & Walton , R I 2020 , ' Synthesis and polymorphism of mixed aluminium-gallium oxides ' , Inorganic Chemistry , vol. Article ASAP . https://doi.org/10.1021/acs.inorgchem.9b03459
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DescriptionDSC is grateful to the EPSRC for award of an industrial CASE studentship, partly funded by Johnson Matthey plc. SEA, DMD and JEH thank the ERC (EU FP7 Consolidator Grant 614290 “EXONMR”) for funding. SEA would also like to thank the Royal Society and Wolfson Foundation for a merit award.
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