  The pressure effect on the fluoride scheelite laser host $LiYF_4$ is studied
at room temperature up to 26 GPa by Raman scattering and up to 40 GPa by
$P^{3+}$ 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 $P^{3+}$ are collected in the energy range corresponding to
following transitions $^3P_{0,1}--^3H_{4,5,6}$, $^1D_2--^3H_4$ and
$^3P_0--^3F_2$. 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 $^4F_2$ configuration of $Pr^{3+}$.
These singularities suggest possible phase transformations leading to lowering
of the lattice symmetry.
