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Studies of the spectroscopic properties of Ce${}^{3+}$ dopant in bulk Gd${}_{3}$Ga${}_{5}$O${}_{12}$:Ce crystal under pressure are presented. In spite of strong intershell $4f\ensuremath{\rightarrow}5d$ absorption bands at ambient pressure, the cerium luminescence in Gd${}_{3}$Ga${}_{5}$O${}_{12}$ is entirely quenched even at low temperature. It has been shown that applying pressure allows for recovery of the $5d\ensuremath{\rightarrow}4f$ radiative transitions. Further increase of pressure improves the emission efficiency. This effect is analyzed in terms of two possible phenomena: (i) by pressure-induced electronic crossover of the excited 5$d$ energy level of the Ce${}^{3+}$ with the conduction band bottom of the host crystal, and (ii) by decrease of electron-lattice coupling with increasing pressure, resulting in reduction of the Stokes shift and nonradiative transitions between the low vibrational levels of the 5$d$ state and high vibrational levels of the ground 4$f$ state. The results of high-pressure absorption and luminescence measurements point out that the ambient-pressure luminescence quenching is caused by the donor-like charge transfer processes due to the resonant location of the Ce${}^{3+}$ 5$d$ electronic levels with respect to the host conduction band. In such a situation, the ionization of Ce${}^{3+}$ to Ce${}^{4+}$ occurs, accompanied by large lattice relaxation, which enables the nonradiative recombination to the Ce 4$f$ state. The pressure-induced approach of the conduction-band bottom of the host crystal by the excited 5$d$ energy level of the Ce${}^{3+}$ results in mixing between conduction band states and the 5$d$ state, and the broadband luminescence from this mixed state is observed at pressures from 30 up to about 60 kbar. Then, for pressures exceeding 60 kbar, this luminescence is replaced by the classic ${\mathrm{Ce}}^{3+}\phantom{\rule{0.16em}{0ex}}5d\ensuremath{\rightarrow}4f$ transitions. Using a simple anticrossing model and configuration coordinate model in the appropriate pressure range, quantitative description of the system has been carried out, and the pressure-induced decrease of electron-lattice coupling has been shown.

The results of high-pressure low-temperature optical measurements in diamond-anvil cell of bulk indium phosphide crystals doped with Yb are reported. The observed initial increase in energy of $f\text{\ensuremath{-}}f$ intrashell transitions between the ${^{2}F}_{5/2}$ excited and ${^{2}F}_{7/2}$ ground levels at pressure up to 6 GPa is interpreted as a result of a linear increase in the spin-orbit parameter of ${\text{Yb}}^{3+}$ ions with pressure coefficient equal to $0.0059\text{ }{\text{cm}}^{\ensuremath{-}1}/\text{GPa}$ due to the covalency effects and mixing of the ${\text{Yb}}^{3+}$ and phosphorus wave functions. Above 6 GPa the pressure dependences of the peak positions of all the luminescence lines flatten out and the values of pressure coefficients strongly decrease. A tentative explanation is provided in terms of the $f$ levels of ${\text{Yb}}^{3+}$ approaching of the top of the valence band of InP crystal host, which limits further increase in $f\text{\ensuremath{-}}f$ intrashell transitions energies above 6 GPa pressure.