Red Mn2+-luminescence in hexagonal aluminates

On UV excitation, a red luminescence band peaking at 690 nm is observed in Eu, Mn doped aluminate lattices containing large trivalent cations and high Mn²⁺-concentrations. The green Mn²⁺ luminescence band occuring at lower Mn²⁺ contents is quenched by the appearance of the red band. The excitation spectra of the red luminescence show that energy transfer takes place from the green Mn²⁺ centres which are located at tetrahedrally surrounded sites to the red emitting Mn²⁺ centres. The last-named are most probably Mn²⁺ ions surrounded octahedrally by oxygen ions. One of these oxygen ions is located at the crystallographic position normally occupied by the large trivalent cation. The dependence of the intensity of the red luminescence on the Mn²⁺ concentration in La-aluminate samples can be explained if it is assumed that the Mn²⁺ ions in tetrahedral sites (the green emitting ones) are all located near the oxygen ions at the large cation sites.