Concentration dependence of the Bi 3+ luminescence in LnPO 4 (Ln=Y 3+ , Lu 3+ )

•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.