Concentration dependence of the Bi3+ luminescence in LnPO4 (Ln = Y3+, Lu3+)

•The concentration dependence of the Bi3+ luminescence in YPO4 and LuPO4 is examined.•The high energy emission band is assigned to isolated Bi3+ ion.•The low energy emission band is assigned to Bi3+ pairs/clusters.•A Bi3+ → Y3+ charge transfer does not provide a complete description for the nature of the low energy band.

The spectroscopic properties of the Bi3+ ion (6s2) as a function of its concentration is examined to clarify the nature of optical transitions that are responsible for the low- and high-energy bands in the emission spectra of LnPO4:Bi3+ (Ln3+ = Y3+, Lu3+). Our investigations conclude that (1) the high-energy emission band is due to isolated Bi3+ ions (3P0,1 → 1S0 transition), and (2) pairs or cluster of Bi3+ ions is responsible for the low-energy emission band. We discuss our results in view of a recent interpretation, which tentatively assigns the low-energy emission band to a metal-to-metal (Bi3+–Y3+) charge transfer transition. The Stokes shift of Bi3+ luminescence in a site with dodecahedral coordination is examined in this work.