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

The polymorphs alpha-, beta-, and gamma-LiAlO2 were synthesized by a hydrot hermal method. The as-prepared product obtained at 240 degrees C was beta-L iAlO2, which converted completely to gamma-LiAlO2 above 1000 degrees C. alp ha-LiAlO2 was obtained by the decomposition of LiAl(OH)(4). H2O prepared by imbibition of LiOH into LiAl2(OH)(7). 2H(2)O hydrothermally at 140 degrees C. Solid state MAS NMR (magic-angle spinning nuclear magnetic resonance) s tudies indicate that Li+ uniquely occupies octahedral sites in all the poly morphs. This observation indicates that the results of XRD and crystal stru cture studies on beta- and gamma-LiAlO2 reported in the literature that ind icate the presence of Li+ in the tetrahedral site are apparently in error w ith respect to Li+ coordination. The other cation, Al3+ occupied octahedral sites in alpha-LiAlO2 and tetrahedral sites in the beta- and gamma-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 eac h polymorph. The difference in the luminescence characteristics among the p olymorphic forms is due to the change in the site symmetry because of the d istortion of the octahedra occupied by Fe3+ across the phase transition. Th e infrared spectrum indicates that site symmetry is lowered during the phas e transition, (C) 1999 Elsevier Science Ltd.