The hydrostatic pressure dependence of photoluminescence, dEPL/dp, of InxGa1−xN epilayers has been measured in the full composition range 0<x0.4 and a relatively steep dependence for x<0.4. On the basis of the agreement of the observed PL pressure coefficient with our calculations, we confirm that band-to-band recombination processes are responsible for PL emission and that no localized states are involved. Moreover, the good agreement between the experimentally determined dEPL/dp and the theoretical curve of dEG/dp indicates that the hydrostatic pressure dependence of PL measurements can be used to quantify changes of the band gap of the InGaN ternary alloy under pressure, demonstrating that the disorder-related Stokes shift in InGaN does not induce a significant difference between dEPL/dp and dEG/dp. This information is highly relevant for the correct analysis of pressure measurements.

The results of high-pressure spectroscopic studies of near-stoichiometric ${\mathrm{LiNbO}}_3:\mathrm{C}\mathrm{r}(0.1\%)\mathrm{}$ and ${\mathrm{LiNbO}}_3:\mathrm{M}\mathrm{g}\mathrm{O},\mathrm{C}\mathrm{r}(0.1\%)\mathrm{}$ crystals are reported. Much lower inhomogeneous broadening of electronic transitions in near-stoichiometric crystals than in congruent ones allows us to obtain better resolution of the spectroscopic measurements. The results of measurements show that ${\mathrm{Cr}}^{3+}$ ions occupy mostly the same centers in near-stoichiometric crystals and in the congruent ones. In near-stoichiometric crystals doped only with chromium, the ${\mathrm{Cr}}^{3+}$ ions substitute the ${\mathrm{Li}}^{+}$ sites in the ${\mathrm{LiNbO}}_3$ host forming one major center (denoted as center γ) and two lower concentration centers, denoted by α and β. In near-stoichiometric crystals doped additionally with a proper amount of MgO, the additional low-strength crystal-field center δ dominates the spectrum. This center corresponds to the ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Nb}}^{5+}$ sites. The hydrostatic pressure above 75 kbar transforms this center into the high-strength crystal-field center. Due to better resolution of measurements, it was possible to determine the splitting of the ${}^{2}E$ level for the δ center, which is equal to about 17 ${\mathrm{cm}}^{\mathrm{-}1}$. Estimated values of the energy difference between the position of the ${}^4{T}_2$ and the ${}^{2}E$ levels for centers β, γ, and δ in these near-stoichiometric ${\mathrm{LiNbO}}_3$ crystals are equal to -20, -242, and more than -1000 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively.

Studies of the spectroscopic properties of Ce3+ dopant in bulk Gd3Ga5O12:Ce crystal under pressure are presented. In spite of strong intershell 4f$\rightarrow${}5d absorption bands at ambient pressure, the cerium luminescence in Gd3Ga5O12 is entirely quenched even at low temperature. It has been shown that applying pressure allows for recovery of the 5d$\rightarrow${}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 5d energy level of the Ce3+ 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 5d state and high vibrational levels of the ground 4f 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 Ce3+ 5d electronic levels with respect to the host conduction band. In such a situation, the ionization of Ce3+ to Ce4+ occurs, accompanied by large lattice relaxation, which enables the nonradiative recombination to the Ce 4f state. The pressure-induced approach of the conduction-band bottom of the host crystal by the excited 5d energy level of the Ce3+ results in mixing between conduction band states and the 5d 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 Ce3+5d$\rightarrow${}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.

Spontaneous and piezoelectric polarization in the nitrides is analyzed. The slab model was designed and proved to be appropriate to obtain the spontaneous polarization in AlN, GaN and InN. The spontaneous polarization and polarization related electric fields in AlN, GaN and InN were determined using DFT slab calculations. The procedure generates single value of spontaneous polarization in the nitrides. It was shown that Berry phase polarization may be applied to determination of spontaneous polarization by appropriate addition of polarization induced electric fields. The electric fields obtained from slab model are consistent with the Berry phase results of Bernardini et al. The obtained spontaneous polarization values are: 8.69*10^-3 C/m², 1.88*10^-3 C/m², and 1.96*10^-3 C/m² for AlN, GaN and InN respectively. The related Berry phase polarization values are 8.69*10^-2 C/m², 1.92*10^-2 C/m², and 2.86*10^-2 C/m², for these three compounds, respectively. The GaN/AlN multiquantum wells (MQWs) were simulated using ab intio calculations. The obtained electric fields are in good agreement with those derived from bulk polarization values. GaN/AlN MQWs structures, obtained by MBE growth were characterized by TEM and X-ray measurements. Time dependent photoluminescence measurements were used to determine optical transition energies in these structures. The PL obtained energies are in good agreement with ab initio data confirming overall agreement between theoretical and experimental data.