Photoluminescence and radiationless processes in Mn2+-doped Ca1−xSrxF2 fluorites as a function of pressure and temperature. A structural correlation study

Abstract

This work investigates the photoluminescence (PL) properties of Mn2+-doped fluorites in CaF2, BaF2, SrF2, and in solid solutions Ca1−xSrxF2. In particular, we focus on the radiationless processes leading to the surprising disappearance of the Mn2+ PL on passing from CaF2: Mn2+ to SrF2: Mn2+ or BaF2: Mn2+. For this purpose emission, excitation, lifetime, and time-resolved spectroscopy as a function of pressure and temperature are carried out in these compounds as well as in the Ca1−xSrxF2:Mn2+ (x=0–1) series using pressure spectroscopy. We show that the quenching of PL in these systems is associated with nonradiative thermal activated processes whose activation energy and pre-exponential rates strongly depend on the crystal volume irrespective of the chemical composition of the host crystal. A salient feature of this work deals with the increase of activation energy induced by pressure, whose variation with the lattice parameter is given by Ea(eV)=1.02–2.64 (a-5.46), with a(CaF2)=5.46 Å. It leads to a PL quantum yield enhancement, which favors appearance of Mn2+ PL even in the non-PL systems SrF2:Mn2+ and BaF2:Mn2+. Furthermore, the activation energy mainly depends on the crystal volume per molecule irrespective of the crystal structure or the local symmetry around the impurity. In this way, the relevance of the fluorite-to-cotunnite phase transition to enhance PL is analyzed. This enhancement is explained in terms of the large volume reduction at the phase transition, as well as by the presence of low-symmetry crystal fields attained at the cation site yielding an increase of the radiative transition rate by the electric-dipole mechanism.