Specific Features of Synthesis and Luminescence for Lithium Aluminum Spinel LiAl5O8 Doped with Manganese Ions

Ceramic lithium aluminum spinels doped with manganese ions have been synthesized by a high-temperature solid-state reaction. Samples fabricated by annealing at 1000, 1100, 1200, or 1300 degrees C in air for 4 h have been identified as LiAl5O8, which is cubic spinel belonging to the space group P4(1)32. After annealing at 1000-1200 degrees C, samples with the lattice parameter a = 7.923-7.925 angstrom show intense red luminescence of Mn4+ ions, which has a narrow luminescence band with a maximum at 662 nm. The excess charge of Mn4+ ions replacing Al3+ ions in the octahedral sites of the LiAl5O8 lattice is compensated for by an excess amount of lithium ions (compared to the stoichiometry) substituting for Al3+ ions adjacent to Mn4+. However, annealing at a higher temperature (1300 degrees C) leads to the almost complete disappearance of the Mn4+ luminescence at 662 nm, although X-ray powder diffraction shows that the crystal structure of these ceramics remains the same one (space group P4(1)32), but the lattice parameter a becomes 7.908 angstrom, which exactly corresponds to the lattice parameter of the LiAl5O8 single crystal. It is suggested that ceramics with a larger lattice parameter obtained at lower temperatures are solid solutions in which some of the Al3+ ions are replaced by larger Li+ ions. Such solid solutions are not stable and lose Li+ ions upon annealing at a high temperature to convert to the stoichiometric compound LiAl5O8. In stoichiometric LiAl5O8, Mn4+ ions cannot be stabilized in the octahedral sites due to the lack of a charge compensation mechanism, and there is no red luminescence of Mn4+ ions in such samples.