Pressure induced Raman and fluorescence singularities in LiYF₄

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The pressure effect on the fluoride scheelite laser host LiYF₄ is studied at room temperature up to 26 GPa by Raman scattering and up to 40 GPa by P³⁺ fluorescence of doped sample. The Raman spectra exhibit three singularities at the vicinity of 6 GPa, 10-12 GPa and 16-17 GPa. The samples pressurized to 21 GPa or higher do not recover the original phase after being released, giving more Raman lines than original samples. The luminescence spectra of P³⁺ are collected in the energy range corresponding to following transitions ³P₀,₁--³H₄,₅,₆, ¹D₂--³H₄ and ³P₀--³F₂. Singularities are observed in the vicinity of 6 GPa, 10 GPa, 16 GPa, 23 GPa in agreement with the Raman study. Moreover, an irreversible transition occurs at 23 GPa. The samples pressurized to above 26 GPa become amorphous when released and all the sharp lines disappear. Above 31 GPa, the spectra at high pressures show only some broad bands corresponding to transitions between two multiplets of the ⁴F₂ configuration of Pr³⁺. These singularities suggest possible phase transformations leading to lowering of the lattice symmetry.