Tb3+-Eu3+ Energy Transfer in Tb-Based Phosphates: A Comparison of Two Extreme Cases

Abstract

Energy transfer processes involving trivalent lanthanide ions (Ln3+) are crucial for the development of innovative luminescent materials and devices such as phosphors, scintillators, materials for solar cells, nanothermometers or materials for optical bioimaging, among others. In the case of the Tb3+→Eu3+ energy transfer in Tb-based materials containing Eu3+ as a dopant, different types of behaviour can be observed in different systems due to structural peculiarities of the host. These differences determine the emission colour of the material, since this is strongly related to the presence or absence of energy transfer (both Tb3+→Tb3+ and Tb3+→Eu3+). In this contribution we present and discuss the luminescence properties of Ca9Tb(PO4)7 powders (whitlockite family, rhombohedral structure) and TbPO4 powders (tetragonal structure) doped with Eu3+. In the case of the Eu-doped TbPO4 powders, a very fast and efficient Tb3+→Eu3+ energy transfer is present upon near UV excitation in the Tb3+ ion. This changes the emission colour of the material from green to red even by addition of small amounts of europium ions. On the contrary, in the Eu-doped Ca9Tb(PO4)7 samples the Tb3+→Eu3+ energy transfer process is not efficient and the resulting emission colour of the Eu3+-doped materials appears to be yellow, even for high europium doping levels.