The structural and vibrational properties of bismuth selenide (Bi2Se3) have been studied by means of x-ray diffraction and Raman scattering measurements up to 20 and 30~GPa, respectively. The measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Our experimental results evidence a phase transition from the low-pressure rhombohedral (R-3m) phase ($\alpha${}-Bi2Se3) with sixfold coordination for Bi to a monoclinic C2/m structure ($\beta${}-Bi2Se3) with sevenfold coordination for Bi above 10~GPa. The equation of state and the pressure dependence of the lattice parameters and volume of $\alpha${} and $\beta${} phases of Bi2Se3 are reported. Furthermore, the presence of a pressure-induced electronic topological phase transition in $\alpha${}-Bi2Se3 is discussed. Raman measurements evidence that Bi2Se3 undergoes two additional phase transitions around 20 and 28~GPa, likely toward a monoclinic C2/c and a disordered body-centered cubic structure with 8-fold and 9- or 10-fold coordination, respectively. These two high-pressure structures are the same as those recently found at high pressures in Bi2Te3 and Sb2Te3. On pressure release, Bi2Se3 reverts to the original rhombohedral phase after considerable hysteresis. Symmetries, frequencies, and pressure coefficients of the Raman and infrared modes in the different phases are reported and discussed.

We report an experimental and theoretical lattice dynamics study of antimony telluride (Sb2Te3) up to 26 GPa together with a theoretical study of its structural stability under pressure. Raman-active modes of the low-pressure rhombohedral (R-3m) phase were observed up to 7.7 GPa. Changes of the frequencies and linewidths were observed around 3.5 GPa where an electronic topological transition was previously found. Raman-mode changes evidence phase transitions at 7.7, 14.5, and 25~GPa. The frequencies and pressure coefficients of the new phases above 7.7 and 14.5 GPa agree with those calculated for the monoclinic C2/m and C2/c structures recently observed at high pressures in Bi2Te3 and also for the C2/m phase in the case of Bi2Se3 and Sb2Te3. Above 25 GPa no Raman-active modes are observed in Sb2Te3, similarly to the case of Bi2Te3 and Bi2Se3. Therefore, it is possible that the structure of Sb2Te3 above 25 GPa is the same disordered bcc phase already found in Bi2Te3 by x-ray diffraction studies. Upon pressure release, Sb2Te3 reverts back to the original rhombohedral phase after considerable hysteresis. Raman- and IR-mode symmetries, frequencies, and pressure coefficients in the different phases are reported and discussed.

We report an experimental and theoretical lattice dynamics study of bismuth telluride (Bi${}_2$Te${}_3$) up to 23 GPa together with an experimental and theoretical study of the optical absorption and reflection up to 10 GPa. The indirect bandgap of the low-pressure rhombohedral (R-3$m$) phase (α-Bi${}_2$Te${}_3$) was observed to decrease with pressure at a rate of −6 meV/GPa. In regard to lattice dynamics, Raman-active modes of α-Bi${}_2$Te${}_3$ were observed up to 7.4 GPa. The pressure dependence of their frequency and width provides evidence of the presence of an electronic-topological transition around 4.0 GPa. Above 7.4 GPa a phase transition is detected to the $C$2/m structure. On further increasing pressure two additional phase transitions, attributed to the C2/c and disordered bcc (Im-3$m$) phases, have been observed near 15.5 and 21.6 GPa in good agreement with the structures recently observed by means of x-ray diffraction at high pressures in Bi${}_2$Te${}_3$. After release of pressure the sample reverts back to the original rhombohedral phase after considerable hysteresis. Raman- and IR-mode symmetries, frequencies, and pressure coefficients in the different phases are reported and discussed.

We have studied by means of high-pressure x-ray diffraction the structural stability of Ni2Mo3N, Co3Mo3N, and Fe3Mo3N. We also report ab initio computing modeling of the high-pressure properties of these compounds, Pd2Mo3N and Pt2Mo3N. We have found that the nitrides remain stable in the ambient-pressure cubic structure at least up to 50 GPa and determined their equation of state. All of them have a bulk modulus larger than 300 GPa. Single-crystal elastic constants have been calculated in order to quantify the stiffness of the investigated nitrides. We found that they should have a Vickers hardness similar to that of cubic spinel nitrides such as $\gamma${}-Si3N4.

Raman scattering measurements and lattice-dynamics calculations were performed on magnesium tungstate (MgWO${}_4$) under high pressure up to 41 GPa. Experiments were carried out on a selection of pressure media. The influence of nonhydrostaticity on the structural properties of MgWO${}_4$ and isomorphic compounds is examined. Under quasihydrostatic conditions, a phase transition was found at 26 GPa in MgWO${}_4$. The high-pressure phase is tentatively assigned to a triclinic structure similar to that of CuWO${}_4$. We also report and discuss the Raman symmetries, frequencies, and pressure coefficients in the low- and high-pressure phases. In addition, the Raman frequencies for different wolframites are compared and the variation of the mode frequency with the reduced mass across the family is investigated. Finally, the accuracy of theoretical calculations is systematically discussed for MgWO${}_4$, MnWO${}_4$, FeWO${}_4$, CoWO${}_4$, NiWO${}_4$, ZnWO${}_4$, and CdWO${}_4$.