Luminescence of Mn ions in ordered and disordered LiAl5O8

The phosphorescence and excitation spectra of Mn ions in the ordered and disordered phases of LiAl5O8 have been measured. In both phases Mn2+ ions substitute for Al3+ ions in two different tetrahedral sites of the LiAl5O8 lattice. In both sites in the ordered phase, sharp zero-phonon transitions have been observed in the low temperature phosphorescence and excitation spectra - these transitions were considerably broadened in the disordered phase due to crystal field inhomogeneity in that phase. The deviation from neutrality caused by the Mn2+ ions in the ordered phase is largely compensated by Mn4+ ions occupying octahedral Al3+ sites. On disordering, a large proportion of Mn4+ is reduced to Mn2+, while the remainder takes up a site with a higher proportion of Li+ ions as next nearest neighbours. This leads to an increase in the ionicity of the Mn4+ site in the disordered phase and hence to a larger value of the Racah parameter B.