Minimum ion-ion distance, cross relaxation and excitonic behaviour in Ln 3+ stoichiometric materials

Abstract

Comparison of the self-quenching rates of Nd 3+( 4F 3/2) and Tb 3+( 5D 3) in two stoichiometric materials LnP 5O 14 and LnNa(WO 4) 2, shows opposite behaviour (Ln=lanthanides). For Nd 3+( 4F 3/2), LnNa(WO 4) 2 which has a small minimum Ln 3+ -Ln 3+ distance ( R min = 3.9 A ̊ ), exhibits a stronger rate than LnP 5O 14 ( R min = 5.2 A ̊ ) with respectively a quadratic and a linear concentration dependence. On the contrary, for Tb 3+( 5D 3), LnNa(WO 4) 2 exhibits a smaller rate than LnP 5O 14 with opposite concentration dependence. Showing that the coherent propagation is negligible, self-quenching is attributed essentially to cross-relaxation with fast or limited incoherent diffusion. Results clearly demonstrate that the existence of a structural isolation is only secondary to energy matching conditions. This in turn confirms the important role of the crystal field strength as exemplified by comparing NaLn(WO 4) 2 and KLn(WO 4) 2.