Eu2+-luminescence was investigated in alkali metal magnesium monophosphates AMgPO(4) (A = Li, Na, and K). The samples were synthesized by solid-state reaction and emission and excitation spectra were measured at 18 K and room temperature. Unusual line emission due to the intraconfigurational 4f(7)(P-6(7/2)) -> 4f(7)(S-8(7/2)) transition (f-f transition) and the anomalous broadband emission at low energy side are observed together with the normal band emission due to the 4f(6)5d(1) -> 4f(7) (S-8(7/2)) transition (d-f transition). The anomalous band emission is attributed to the recombination of Eu2+-trapped exciton (Eu-TE). In the sequence of increasing ionic radius Li, Na, and K in AMgPO(4) the relative intensity of the f-f line is reduced and the d-f band shifts to red. The changes in spectral features are explained by the crystal-field strength acting on Eu2+ ion and substitutional sites for Eu2+ in AMgPO(4) (A = Li, Na, and K). The thermal activation between the lowest excited 4f(6)5d state and the Eu-TE state is responsible for the quenching of the d-f emission or the Eu-TE emission at room temperature.

Keywords: 4f-4f transition; 5d-4f transition; Eu2+; Impurity trapped exciton; Crystal field strength