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.

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.

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 (approximately 4100 K) transition in the shock experiment is from the bcc structure to a new, highly viscous, structured melt.

We have performed an experimental study of the crystal structure, latticedynamics, and optical properties of PbCrO 4 (the mineral crocoite) at ambient and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band-gap have been accurately determined. X-ray-diffraction, Raman, and opticalabsorption experiments have allowed us also to completely characterize two pressureinduced structural phase transitions. The first transition is isostructural, maintaining the monoclinic symmetry of the crystal, and having important consequences in the physical properties; among other a band-gap collapse is induced. The second one involves an increase of the symmetry of the crystal, a volume collapse, and probably the metallization of PbCrO 4 . The results are discussed in comparison with related compounds and the effects of pressure in the electronic structure explained. Finally, the room-temperature equation of state of the low-pressure phases is also obtained.The AXO 4 monazite-type compounds form an extended family of oxides [1]. Due to some interesting physical and chemical properties, several applications for these materials are already reported and under development [1]; e.g. coatings and diffusion barriers; geochronology; luminophors, lasers, and light emitters; ionic conductors; and matrix for radioactive waste management. Monazite-type compounds crystallize in a monoclinic lattice with space group P2 1 /n (Z = 4) which was first reported in the framework of the Manhattan project [2]. This structure (see Fig. 1a) has been accurately described by Ni et al. [3], who precisely determined the structure of monazite-type phosphates. The structural arrangement is based on the nine-fold coordination of the A cation and the four-fold coordination of the X cation. Monazites exist in Nature and are important accessory minerals in granitoids and rhyolites, and because of their incorporation of rare-earth elements they can effectively control the rare-earths distribution in igneous rocks [4]. In addition, they are a common accessory mineral in plutonic and metamorphic rocks. Therefore, the knowledge of the high-pressure (HP) behavior of monazites is very relevant not only for technological applications, but also for mineral physics, chemistry, and for petrology studies [5]. Monazite-type phosphates have been consequently studied under compression [6,7] being the crystalline structure stable up to approximately 30 GPa. Cation substitution has been shown to have a relevant influence on transition pressures in AXO 4 oxides related to monazite. In particular, magnetic cations, like Cr, reduce considerably the transition pressures in zircon-type oxides [8,9], favoring the study of HP phases. In this work, to gain further understanding of the structural properties of monazite-type oxides, HP x-ray diffraction, Raman, and optical-absorption experiments on monazite-type PbCrO 4 (the mineral crocoite) up to 18 GPa are reported. We detected the occurrence of two structural 3 changes and characterized the str...

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We have studied the structural behavior of bismuth phosphate under compression. We performed x-ray powder diffraction measurements up to 31.5 GPa and ab initio calculations. Experiments were carried out on different polymorphs: trigonal (phase I) and monoclinic (phases II and III). Phases I and III, at low pressure (P < 0.2 -0.8 GPa), transform into phase II, which has a monazite-type structure. At room temperature, this polymorph is stable up to 31.5 GPa. Calculations support these findings and predict the occurrence of an additional transition from the monoclinic monazite-type to a tetragonal scheelite-type structure (phase IV). This transition was experimentally found after the simultaneous application of pressure (28 GPa) and temperature (1500 K), suggesting that at room temperature the transition might by hindered by kinetic barriers.Calculations also predict an additional phase transition at 52 GPa, which exceeds the 2 maximum pressure achieved in the experiments. This transition is from phase IV to an orthorhombic barite-type structure (phase V). We also studied the axial and bulk compressibility of BiPO 4 . Room-temperature pressure-volume equations of state are reported. BiPO 4 was found to be more compressible than isomorphic rare-earth phosphates. The discovered phase IV was determined to be the less compressible polymorph of BiPO 4 . On the other hand, the theoretically predicted phase V has a bulk modulus comparable with that of monazite-type BiPO 4 . Finally, the isothermal compressibility tensor for the monazite-type structure is reported at 2.4 GPa showing that the direction of maximum compressibility is in the (0 1 0) plane at approximately 15º (21º) to the a axis for the case of our experimental (theoretical) study.

In this paper, we report the angle-dispersive x-ray diffraction data of barite, BaSO 4 , 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 BaSO 4 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 P2 1 2 1 2 1 structure. The pressure dependence of the structural parameters of both room-and HP phases of BaSO 4 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.

We report a joint experimental and theoretical study of the structural, vibrational, elastic, optical and electronic properties of the layered high-mobility semiconductor Bi 2 O 2 Se at high pressure. A good agreement between experiments and ab initio calculations is observed for the equation of state, the pressure coefficients of the Raman-active modes and the bandgap of the material. In particular, a detailed description of the vibrational properties is provided. Unlike other Sillén-type compounds which undergo a tetragonal to collapsed tetragonal pressure-induced phase transition at relatively low pressures, Bi 2 O 2 Se shows a remarkable structural stability up to 30 GPa; however, our results indicate that this compound exhibits considerable electronic changes around 4 GPa, likely related to the progressive shortening and hardening of the long and weak Bi-Se bonds linking the Bi 2 O 2 and Se atomic layers. Variations of the structural, vibrational, and electronic properties induced by these electronic changes are discussed.