Abstract
Ceramic samples of lithium aluminates doped with manganese ions have been synthesized by a solid-state reaction method. The ceramics sintered at temperatures of 1000, 1100, 1200 or 1300 °C for 4 h in air are identified as LiAl5O8 compounds belonging to a spinel-like crystal structure with the P4132 space group. Nonetheless, ceramic compositions obtained by annealing at temperatures 1000, 1100 or 1200 °C have larger lattice parameters than that of a LiAl5O8 single crystal and are considered as metastable LiAl5O8 phases having partial disorder for the occupation of two inequivalent octahedral sites by Li+ and Al3+ ions, i.e. possessing Li - Al inversion. Such Mn doped disordered compositions show typical Mn4+ red luminescence with an emission peak at ∼662 nm. The charge compensation of Mn4+ replacing Al3+ ions in octahedral sites is achieved by Li+ ions entering some adjacent Al3+ sites due to inversion. On the other hand, annealing at a higher temperature (1300 °C) or prolonged (170 h) annealing at a lower temperature (1100 °C) leads to complete ordering of Li+ and Al3+ in ratio 1:3 at different octahedral sites in the LiAl5O8 structure in which Mn4+ luminescence is not observed due to the lack of a charge compensation mechanism. Doping Mn ions in such an ordered structure are realized in di- and trivalent states causing green and far-red luminescence, respectively. Additional annealing of the mixtures of the Mn-doped ordered LiAl5O8 composition and Li2CO3 at 1300 °C in air results in the production of phosphors which again show intense Mn4+ red luminescence and have the spinel-like crystal structure with the Fd3‾m space group due to random occupation of cationic sites by Li+ and Al3+ ions.
Published Version
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