New observations on the pressure dependence of luminescence from Eu ²⁺-doped MF₂ (M = Ca, Sr, Ba) fluorides

The luminescence from Eu²⁺ ions in MF₂ (M = Ca, Sr, Ba) fluorides has been investigated under the pressure range of 0-8 GPa. The emission band originating from the 4f⁶d¹ → 4f ⁷ transition of Eu²⁺ ions in CaF₂ and SrF ₂ shows the red-shift as increasing pressure with pressure coefficients of -17 meV/GPa for CaF₂ and -18 meV/GPa for SrF ₂. At atmospheric pressure, the emission spectrum of BaF ₂:Eu²⁺ comprises two peaks at 2.20 and 2.75 eV from the impurity trapped exciten (ITE) and the self-trapped exciton (STE), respectively. As the pressure is increased, both emission peaks shift to higher energies, and the shifting rate is slowed by the phase transition from the cubic to orthorhombic phase at 4 GPa. Due to the phase transition at 4-5 GPa pressure, the ITE emission disappears gradually, and the STE emission is gradually replaced by the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺. Above 5 GPa, the pressure behavior of the 4f ⁶5d¹ → 4f⁷ transition of Eu²⁺ in BaF₂: Eu²⁺ is the same as the normal emission of Eu²⁺ in CaF₂ and SrF₂ phosphors.