Eu²⁺luminescence in M₅(PO₄)₃X apatites, where M is Ca²⁺, Sr²⁺and Ba²⁺, and X is F^-, Cl^-, Br^-and OH^$

Abstract

M5(PO4)3X apatites are well known for their technological importance as phosphors, laser hosts and biocompatible materials. Divalent-europium-activated alkaline earth chloroapatites are of special importance for their application as the blue component in high-efficiency trichromatic fluorescent lamps. In these apatites, Eu2+ yields narrow-band emission in the blue region corresponding to the 4f65d to 8S7/2 allowed electric dipole transition. It has been found that the dominant emission band observed can be assigned to Eu2+ occupying MII sites of the apatite system having C1h symmetry. In the case of barium chloroapatites one could also observe Eu2+ emission from a second type of site (MI) that is available in the apatite system. The weak emission observed in the latter can be attributed to the intense spectral overlap with the former site(s) and thermal quenching effects. The exchange coupling between the 4f6 and 5d electrons of Eu2+ manifests itself in 'stair-case' features in the excitation spectrum and the strength of the exchange interaction is highly dependent on the host apatite(s). Also, in the Eu2+, Mn2+ co-doped system, energy transfer is found to occur from the former to the latter and the transfer is regulated by an exchange process. Various results based on these facts are discussed in detail.