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.

Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃ are narrow bandgap semiconductors with tetradymite crystal structure (R‐3m) which have been extensively studied along with their alloys due to their promising operation as thermoelectric materials in the temperature range between 300 and 500 K. Studies on these layered semiconductors have increased tremendously in the last years since they have been recently predicted and demonstrated to behave as 3D topological insulators. In particular, a number of high‐pressure studies have been done in the recent years in these materials. In this work we summarize the main results of the high‐pressure studies performed in this family of semiconductors to date. In particular, we review recent results that address the main characteristics of the pressure‐induced electronic topological transition and structural phase transitions observed in this family of compounds. Future high‐pressure studies to be performed on these 3D topological insulators are also commented.

We report an experimental and theoretical study of antimony oxide (Sb2O3) in its cubic phase (senarmontite) under high pressure. X-ray diffraction and Raman scattering measurements up to 18 and 25 GPa, respectively, have been complemented with ab initio total-energy and lattice-dynamics calculations. X-ray diffraction measurements do not provide evidence of a space-group symmetry change in senarmontite up to 18 GPa. However, Raman scattering measurements evidence changes in the pressure coefficients of the Raman mode frequencies at 3.5 and 10 GPa, respectively. The behavior of the Raman modes with increasing pressure up to 25 GPa is fully reproduced by the lattice-dynamics calculations in cubic Sb2O3. Therefore, the combined analysis of both experiments and lattice-dynamics calculations suggest the occurrence of two isostructural phase transformations at 3.5 and 10 GPa, respectively. Total-energy calculations show that the isostructural phase transformations occur through local atomic displacements in which senarmontite loses its molecular character to become a three-dimensional solid. In addition, our calculations provide evidence that cubic senarmontite cannot undergo a phase transition to orthorhombic valentinite at high pressure, and that a phase transition to a $\beta${}-Bi2O3-type structure is possible above 25 GPa.

In this paper, we report angle-dispersive X-ray diffraction data of molybdenum melting, measured in a double-sided laser-heated diamond-anvil cell up to a pressure of 119 GPa and temperatures up to 3400 K. The new melting temperatures are in excellent agreement with earlier measurements up to 90 GPa that relied on optical observations of melting and in strong contrast to most theoretical estimates. The X-ray measurements show that the solid melts from the bcc structure throughout the reported pressure range and provide no evidence for a high temperature transition from bcc to a close-packed structure, or to any other crystalline structure. This observation contradicts earlier interpretations of shock data arguing for such a transition. Instead, the values for the Poisson ratios of shock compressed Mo, obtained from the sound speed measurements, and the present X-ray evidence of loss of long-range order suggest that the 210 GPa (∼4100 K) transition in the shock experiment is from the bcc structure to a new, highly viscous, structured melt.

In this paper, we report the angle-dispersive x-ray diffraction data of barite, BaSO4, measured in a diamond-anvil cell up to a pressure of 48 GPa, using three different fluid pressure-transmitting media (methanol-ethanol mixture, silicone oil, and He). Our results show that BaSO4 exhibits a phase transition at pressures that range from 15 to 27 GPa, depending on the pressure media used. This indicates that nonhydrostatic stresses have a crucial role in the high-pressure behavior of this compound. The new high-pressure (HP) phase has been solved and refined from powder data, having an orthorhombic P212121 structure. The pressure dependence of the structural parameters of both room- and HP phases of BaSO4 is also discussed in light of our theoretical first-principles total-energy calculations. Finally, a comparison between the different equations of state obtained in our experiments is reported.