Multiphonon radiationless phenomena in Mn²⁺-Doped Ca_1-xSr_xF₂ and BaF₂ Fluorites

This work investigates the surprising disappearance of the Mn²⁺ photoluminescence on passing from CaF₂:Mn²⁺ to SrF₂:Mn²⁺ or BaF₂:Mn²⁺ using pressure spectroscopy. We show that the loss of luminescence in these systems is associated with non-radiative thermally activated processes whose activation energy and pre-exponential rates strongly depend on the crystal volume irrespective of the chemical composition. A salient feature is the PL enhancement observed in the cotunnite high-pressure phase along the series. 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 sites leading to an increase of the radiative transition rate by the electric-dipole mechanism.

Keywords:

• High-pressure Spectroscopy
• Photoluminescence
• Radiationless Process
• Mn2+-doped Fluorites
• Fluorite-Cotunnite Phase Transition