Search results for " quantum chemistry"
showing 10 items of 549 documents
Effect of pressure on the structural properties and electronic band structure of GaSe
2007
The structural properties of GaSe have been investigated up to 38 GPa by monochromatic X-ray diffraction. The onset of the phase transition from the e-GaSe to a disordered NaCi-type structural motif is observed near 21 GPa. Using the experimentally determined lattice parameters of the layered e-phase as input, constrained ab-initio total energy calculations were performed in order to optimize the internal structural parameters at different pressures. The results obtained for the nearest-neighbor Ga-Se distance agree with those derived from recent EXAFS measurements. In addition, information is obtained on the changes of Ga-Ga and Se-Se bond lengths which were not accessible to a direct expe…
Comment on “High-pressure x-ray diffraction study of YBO3/Eu3+, GdBO3, and EuBO3: Pressure-induced amorphization in GdBO3” [J. Appl. Phys. 115, 04350…
2014
High-pressure x-ray diffraction studies on vaterite-type borates were reported on the above paper and their room-temperature P-V equation of state (EOS) determined. YBO3/Eu3+ and GdBO3 were found to have bulk moduli around 320 GPa, 90% larger than the bulk modulus obtained for EuBO3. Consequently, it was stated that vaterite-type borates are as incompressible as cubic BN. Such a different compressional behavior of isomorphic borates contradicts the known systematic of related borates. Here, we show that the conclusions reported on the above article could be hindered by experimental errors and artifacts. Ab initio calculations support our criticism giving similar bulk moduli (130–141 GPa) in…
High-pressure structural behaviour of HoVO4: combined XRD experiments and ab initio calculations.
2014
We report a high-pressure experimental and theoretical investigation of the structural properties of zircon-type HoVO4. Angle-dispersive x-ray diffraction measurements were carried out under quasi-hydrostatic and partial non-hydrostatic conditions up to 28 and 23.7 GPa, respectively. In the first case, an irreversible phase transition is found at 8.2 GPa. In the second case, the onset of the transition is detected at 4.5 GPa, a second (reversible) transition is found at 20.4 GPa, and a partial decomposition of HoVO4 was observed. The structures of the different phases have been assigned and their equations of state (EOS) determined. Experimental results have also been compared to theoretica…
High-pressure phase transitions and compressibility of wolframite-type tungstates
2010
This paper reports an investigation on the phase diagram and compressibility of wolframite-type tungstates by means of x-ray powder diffraction and absorption in a diamond-anvil cell and ab initio calculations. The diffraction experiments show that monoclinic wolframite-type MgWO4 suffers at least two phase transitions, the first one being to a triclinic polymorph with a structure similar to that of CuWO4 and FeMoO4-II. The onset of each transition is detected at 17.1 and 31 GPa. In ZnWO4 the onset of the monoclinic-triclinic transition has been also found at 15.1 GPa. These findings are supported by density-functional theory calculations, which predict the occurrence of additional transiti…
Structural, vibrational and electrical study of compressed BiTeBr
2016
Compresed BiTeBr has been studied from a joint experimental and theoretical perspective. Room-temperature x-ray diffraction, Raman scattering, and transport measurements at high pressures have been performed in this layered semiconductor and interpreted with the help of ab initio calculations. A reversible first-order phase transition has been observed above 6–7 GPa, but changes in structural, vibrational, and electrical properties have also been noted near 2 GPa. Structural and vibrational changes are likely due to the hardening of interlayer forces rather than to a second-order isostructural phase transition while electrical changes are mainly attributed to changes in the electron mobilit…
Structural and electrical study of the topological insulator SnBi2Te4 at high pressures
2016
We report high-pressure X-ray diffraction and electrical measurements of the topological insulator SnBi2Te4 at room temperature. The pressure dependence of the structural properties of the most stable phase of SnBi2Te4 at ambient conditions (trigonal phase) have been experimentally determined and compared with results of our ab initio calculations. Furthermore, a comparison of SnBi2Te4 with the parent compound Bi2Te3 shows that the central TeSnTe trilayer, which substitutes the Te layer at the center of the TeBiTeBiTe layers of Bi2Te3, plays a minor role in the compression of SnBi2Te4. Similar to Bi2Te3, our resistance measurements and electronic band structure simulations in SnBi2Te4 at hi…
Thallium under extreme compression
2016
We present a combined theoretical and experimental study of the high-pressure behavior of thallium. X-ray diffraction experiments have been carried out at room temperature up to 125 GPa using diamond-anvil cells, nearly doubling the pressure range of previous experiments. We have confirmed the hcp-fcc transition at 3.5 GPa and determined that the fcc structure remains stable up to the highest pressure attained in the experiments. In addition, HP-HT experiments have been performed up to 8 GPa and 700 K by using a combination of x-ray diffraction and a resistively heated diamond-anvil cell. Information on the phase boundaries is obtained, as well as crystallographic information on the HT bcc …
Pressure-induced amorphization of the Y3Ga5O12 garnet studied to 1 Mbar
2020
We use micro-beam synchrotron x-ray diffraction to study the pressure-induced amorphization of nano-sized and single crystals of Y3Ga5O12 up to pressures exceeding 1 Mbar in static compression. The abrupt pressure-induced amorphization found for both 56 nm and bulk micrometric crystals at around 76 GPa independently of the pressure transmitting medium employed demonstrates its intrinsic nature, previously predicted at 79 GPa by ab initio calculations. The single crystal structural solution at 50 GPa shows that the contraction of the unit-cell, mostly accommodated by the compressible YO8 dodecahedra, gives rise to a regularization and tilting increase of the GaO6 polyhedra with the Y?O-Ga an…
The Poisson Ratio in CoFe2O4Spinel Thin Films
2012
The response of epitaxial CoFe2O4 thin films to biaxial compressive stress imposed by MgAl2O4 and SrTiO3 single crystalline substrates is studied using X-ray diffraction and Raman spectroscopy. It is found that the Poisson ratio ν signals a non-auxetic behavior and depends on the substrate used. The Raman modes show an increase in frequency when increasing compressive strain by reducing film thickness; this is due to the shrinking of the unit cell volume. Such behavior is in qualitative agreement with recent ab initio calculations, although the measured values are significantly smaller than predictions. In contrast, the measured Poisson ratio is found to be in good agreement with expectatio…
GdBO3 and YBO3 crystals under compression
2021
High-pressure X-ray diffraction studies on nanocrystals of the GdBO3 and YBO3 rare-earth orthoborates are herein reported up to 17.4(2) and 13.4(2) GPa respectively. The subsequent determination of the room-temperature pressure-volume equations of state is presented and discussed in the context of contemporary publications which contradict the findings of this work. In particular, the isothermal bulk moduli of GdBO3 and YBO3 are found to be 170(13) and 163(13) GPa respectively, almost 50% smaller than recent findings. Our experimental results provide an accurate revision of the high-pressure compressibility behaviour of GdBO3 and YBO3 which is consistent with the known systematics in isomor…