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

Abstract

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°C in air for 4 h have been identified as LiAl₅O₈, which is cubic spinel belonging to the space group P4₁32. After annealing at 1000–1200°C, samples with the lattice parameter a = 7.923–7.925 Å 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°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 P4₁32), but the lattice parameter a becomes 7.908 Å, 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.