Pressure-induced photoluminescence in Mn²⁺-doped BaF₂ and SrF₂ fluorites

This work reports an effective way for inducing room temperature photoluminescence (PL) in Mn²⁺-doped BaF₂ and SrF₂ using high-pressure techniques. The aim is to understand the surprising PL behavior exhibited by Mn²⁺ at the cubal site of the fluorite structure. While Mn²⁺-doped CaF₂ shows a green PL with quantum yield close to 1 at room temperature, Mn²⁺-doped MF₂ (M=Ba,Sr) is not PL either at room temperature (SrF₂) or at any temperature (BaF₂) at ambient pressure. We associate the loss of Mn²⁺ PL on passing from CaF₂ to SrF₂ or BaF₂ with nonradiative multiphonon relaxation whose thermal activation energy decreases along the series CaF₂→SrF₂→BaF₂. A salient feature of this work deals with the increase of activation energy induced by pressure. It leads to a quantum yield enhancement, which favors PL recovery. 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 is analyzed in connection with the PL properties of the investigated compounds. The PL spectrum and the corresponding lifetime are reported for both structural phases as a function of pressure.