The Ce3+ → Tb3+ transfer in phosphate host lattices

A Ce³⁺ → Tb³⁺ transfer has been shown to appear in various phosphates belonging to different structural types and particularly in some new compounds: Na₃Ln(PO₄)₂:Ce, Tb; Na_xSr_3?2xLn_x(PO₄)₂:Ce, Tb and KCaLn(PO₄)₂:Ce, Tb.The presence of PO₄ groups, in which the P-O bonds are strongly covalent, leads to a relatively weak crystal field at the rare-earth sites. The best overlapping between cerium emission and terbium excitation spectra is obtained for phosphates containing alkali ions. This result can be explained by a lowering of the emitting level of the cerium 5d configuration due to stronger covalency of the CeO bond and to an increasing crystal field at the rare-earth sites.