The structural and vibrational properties of bismuth selenide (Bi 2 Se 3 ) 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 (α-Bi 2 Se 3 ) with sixfold coordination for Bi to a monoclinic C2/m structure (β-Bi 2 Se 3 ) with sevenfold coordination for Bi above 10 GPa. The equation of state and the pressure dependence of the lattice parameters and volume of α and β phases of Bi 2 Se 3 are reported. Furthermore, the presence of a pressure-induced electronic topological phase transition in α-Bi 2 Se 3 is discussed. Raman measurements evidence that Bi 2 Se 3 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 Bi 2 Te 3 and Sb 2 Te 3 . On pressure release, Bi 2 Se 3 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 (Sb 2 Te 3 ) 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 Bi 2 Te 3 , and also for the C2/m phase in the case of Bi 2 Se 3 and Sb 2 Te 3 . Above 25 GPa no Raman-active modes are observed in Sb 2 Te 3 similarly to the case of Bi 2 Te 3 and Bi 2 Se 3 . Therefore, it is possible that the structure of Sb 2 Te 3 above 25 GPa is the same disordered bcc phase already found in Bi 2 Te 3 by x-ray diffraction studies. Upon pressure release, Sb 2 Te 3 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.

Spectrum of -Bi 2 Te 3 at 13.3 GPa and the corresponding fit of Voigt profiles corresponding to the Raman-active modes of the C2/m structure.

High pressure structural stability of BaLiF3 J. Appl. Phys. 110, 123505 (2011) Pressure effects on the transitions between disordered phases in supercooled liquid silicon J. Chem. Phys. 135, 204508 (2011) Microfabrication of controlled-geometry samples for the laser-heated diamond-anvil cell using focused ion beam technology Rev. Sci. Instrum. 82, 115106 (2011) First-principles investigations of elastic stability and electronic structure of cubic platinum carbide under pressure J. Appl. Phys. 110, 103507 (2011) Additional information on J. Appl. Phys. High-pressure optical absorption and Raman scattering measurements have been performed in defect chalcopyrite (DC) CdGa 2 Se 4 up to 22 GPa during two pressure cycles to investigate the pressure-induced order-disorder phase transitions taking place in this ordered-vacancy compound. Our measurements reveal that on decreasing pressure from 22 GPa, the sample does not revert to the initial phase but likely to a disordered zinc blende (DZ) structure the direct bandgap and Raman-active modes of which have been measured during a second upstroke. Our measurements have been complemented with electronic structure and lattice dynamical ab initio calculations. Lattice dynamical calculations have helped us to discuss and assign the symmetries of the Raman modes of the DC phase. Additionally, our electronic band structure calculations have helped us in discussing the order-disorder effects taking place above 6-8 GPa during the first upstroke.

31We report on high-pressure x-ray diffraction measurements up to 17.

A strong nonlinear pressure dependence of the optical absorption edge has been measured in defect chalcopyrites CdGa 2 Se 4 and HgGa 2 Se 4 . The behavior is due to the nonlinear pressure dependence of the direct band-gap energy in these compounds as confirmed by ab initio calculations. Our calculations for CdGa 2 Se 4 , HgGa 2 Se 4 and monoclinic ␤-Ga 2 Se 3 provide evidence that the nonlinear pressure dependence of the direct band-gap energy is a general feature of adamantine ordered-vacancy compounds irrespective of their composition and crystalline structure. The nonlinear behavior is due to a conduction band anticrossing at the ⌫ point of the Brillouin zone caused by the presence of ordered vacancies in the unit cell of these tetrahedrally coordinated compounds.

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 on Raman scattering measurements in mercury digallium selenide (HgGa 2 Se 4) up to 25 GPa. We also performed, for the low-pressure defect-chalcopyrite structure, lattice-dynamics ab initio calculations at high pressures which agree with experiments. Measurements evidence that the semiconductor HgGa 2 Se 4 exhibits a pressure-induced phase transition above 19 GPa to a previously undetected structure. This transition is followed by a transformation to a Raman-inactive phase above 23.4 GPa. On downstroke from 25 GPa till 2.5 GPa a broad Raman spectrum was observed, which has been attributed to a fourth phase, and whose pressure dependence was followed during a second upstroke. Candidate structures for the three phases detected under compression are proposed. Finally, we also report and discuss the decomposition of the sample by laser heating at pressures close to 19 GPa. As possible products of decomposition we have identified at least the formation of trigonal selenium nanoclusters and cinnabar-type HgSe.