Cation coordination and Fe3+ luminescence in LiAlO2 polymorphs prepared by a hydrothermal method

The polymorphs α-, β-, and γ- LiAlO2 were synthesized by a hydrothermal method. The as-prepared product obtained at 240°C was β-LiAlO2, which converted completely to γ-LiAlO2 above 1000°C. α-LiAlO2 was obtained by the decomposition of LiAl(OH)4 · H2O prepared by imbibition of LiOH into LiAl2(OH)7 · 2H2O hydrothermally at 140°C. Solid state MAS NMR (magic-angle spinning nuclear magnetic resonance) studies indicate that Li+ uniquely occupies octahedral sites in all the polymorphs. This observation indicates that the results of XRD and crystal structure studies on β- and γ-LiAlO2 reported in the literature that indicate the presence of Li+ in the tetrahedral site are apparently in error with respect to Li+ coordination. The other cation, Al3+, occupied octahedral sites in α-LiAlO2 and tetrahedral sites in the β- and γ-LiAlO2. The Fe3+ doped in the various polymorphic forms of this compound was found to uniquely occupy the octahedral Li+ site. EPR spectrum of the Fe3+ doped in these polymorphs indicates that during the transformation this site is distorted. The Fe3+ photoluminescent emission maximum was different for each polymorph. The difference in the luminescence characteristics among the polymorphic forms is due to the change in the site symmetry because of the distortion of the octahedra occupied by Fe3+ across the phase transition. The infrared spectrum indicates that site symmetry is lowered during the phase transition.