Search

Raman scattering measurements have been performed in cadmium digallium sulphide (CdGa2S4) with defect chalcopyrite structure up to 25 GPa in order to study its pressure-induced phase transitions. These measurements have been complemented and compared with lattice-dynamics ab initio calculations including the TO-LO splitting at high pressures in order to provide a better assignment of experimental Raman modes. In addition, experimental and theoretical Grüneisen parameters have been reported in order to calculate the molar heat capacity and thermal expansion coefficient of CdGa2S4. Our measurements provide evidence that CdGa2S4 undergoes an irreversible phase transition above 15 GPa to a Raman-inactive phase, likely with a disordered rock salt structure. Moreover, the Raman spectrum observed on downstroke from 25 GPa to 2 GPa has been attributed to a new phase, tentatively identified as a disordered zinc blende structure, that undergoes a reversible phase transition to the Raman-inactive phase above 10 GPa. [ABSTRACT FROM AUTHOR]

We report a joint experimental and theoretical high-pressure study of the structural and vibrational properties of tetradymite-like (R3¯m) β-As2Te3. Two samples have been characterized by angle-dispersive synchrotron powder X-ray diffraction and Raman scattering measurements under hydrostatic conditions with the help of ab initio calculations. One sample was synthesized at high pressure and high-temperature conditions with a Paris-Edinburg cell and the other by the melt-quenching technique. Both β-As2Te3 samples show the same properties and exhibit two isostructural phase transitions of order higher than 2, i.e. of electronic origin, near 2.0(2) and 6.0(5) GPa that are compatible with the changes predicted by recent electronic band structure calculations. The first isostructural phase transition can be attributed to the topological quantum phase transition from a trivial insulator to a topological insulator, passing through a 3D Dirac topological semimetal. This topological transition, specific to β-As2Te3, is not observed in isostructural Te-based sesquichalcogenides α-Sb2Te3 and α-Bi2Te3 that are topological insulators at room conditions. The second isostructural phase transition is likely related to an insulator-metal transition. Additionally, we have observed two partially reversible first-order phase transitions in β-As2Te3 above 10 and 17 GPa. We have found a high anharmonic behavior of the two Raman-active modes with the lowest frequencies in β-As2Te3 that explains the already reported ultra-low lattice thermal conductivity of β-As2Te3. Moreover, we have studied the similarities of β-As2Te3 with α-Sb2Te3 and α-Bi2Te3 (two of the best thermoelectric materials), thus providing insights into the origin of the ultra-low lattice thermal conductivity values in these compounds related to unconventional chemical bonds present in these isostructural materials. [ABSTRACT FROM AUTHOR]

We report a joint experimental and theoretical study of the low-pressure phase of α′-Ga2S3 under compression. Theoretical ab initio calculations have been compared to X-ray diffraction and Raman scattering measurements under high pressure carried out up to 17.5 and 16.1 GPa, respectively. In addition, we report Raman scattering measurements of α′-Ga2S3 at high temperature that have allowed us to study its anharmonic properties. To understand better the compression of this compound, we have evaluated the topological properties of the electron density, the electron localization function, and the electronic properties as a function of pressure. As a result, we shed light on the role of the Ga–S bonds, the van der Waals interactions inside the channels of the crystalline structure, and the single and double lone electron pairs of the sulphur atoms in the anisotropic compression of α′-Ga2S3. We found that the structural channels are responsible for the anisotropic properties of α′-Ga2S3 and the A′(6) phonon, known as the breathing mode and associated with these channels, exhibits the highest anharmonic behaviour. Finally, we report calculations of the electronic band structure of α′-Ga2S3 at different pressures and find a nonlinear pressure behaviour of the direct band gap and a pressure-induced direct-to-indirect band gap crossover that is similar to the behaviour previously reported in other ordered-vacancy compounds, including β-Ga2Se3. The importance of the single and, more specially, the double lone electron pairs of sulphur in the pressure dependence of the topmost valence band of α′-Ga2S3 is stressed. [ABSTRACT FROM AUTHOR]

The Na-W-H and Na-Re-H ternary systems were studied in a diamond anvil cell through X-ray diffraction and Raman spectroscopy, supported by density functional theory and molecular dynamics calculations. Na 3 WH 9 can be synthesized above 7.8 GPa and 1400 K, remaining stable between at least 0.1 and 42.1 GPa. The rhenium analogue Na 3 ReH 8 can form at 10.1 GPa upon laser heating, being stable between at least 0.3 and 32.5 GPa. Na 3 WH 9 and Na 3 ReH 8 host [WH 9 ] 3- and [ReH 8 ] 3- anions, respectively, forming homoleptic 18-electron complexes in both cases. Both ternary hydrides show similar structural types and pressure dependent phase transitions. At the highest pressures they adopt a distorted fcc Heusler structure (Na 3 WH 9 -II' and Na 3 ReH 8 -II') while upon decompression the structure symmetrizes becoming fcc between ∼6.4 and 10 GPa for Na 3 WH 9 -II and at 17 GPa for Na 3 ReH 8 -II. On further pressure release, the fcc phases transform into variants of a (quasi-) hexagonal structure at ∼3 GPa, Na 3 WH 9 -I and Na 3 ReH 8 -I.

The high-pressure and high-temperature structural, mechanical, and dinamical stability of rhodium has been investigated via synchrotron X-ray diffraction using a resistively heated diamond anvil cell and density functional theory. The isothermal compression data have been fitted with a Rydberg-Vinet equation of state (EoS) with best-fitting parameters V 0 =55.046(16) Å 3 , K 0 = 251(3) GPa, and K 0 ' = 5.7(2). The thermal equation of state has been determined based upon the data collected following four different isotherms and has been fitted to a Holland and Powell thermal equation-of-state model with α 0 = 3.36(7)x10 - 5 K - 1 . The measured equation of state and structural parameters have been compared to the results of ab initio simulations. The agreement between theory and experiments is generally quite good. The present results solve controversies between previous studies which reported values of the bulk modulus from 240 to 300 GPa., (© 2024. The Author(s).)

The effect of pressure on the structural, vibrational, and optical properties of lead thiogallate, PbGa 2 S 4 , crystallizing under room conditions in the orthorhombic EuGa 2 S 4 -type structure (space group Fddd ), is investigated. The results from X-ray diffraction, Raman scattering, and optical-absorption measurements at a high pressure beyond 20 GPa are reported and compared not only to ab initio calculations, but also to the related compounds α'-Ga 2 S 3 , CdGa 2 S 4 , and HgGa 2 S 4 . Evidence of a partially reversible pressure-induced decomposition of PbGa 2 S 4 into a mixture of Pb 6 Ga 10 S 21 and Ga 2 S 3 above 15 GPa is reported. Thus, our measurements and calculations show a route for the high-pressure synthesis of Pb 6 Ga 10 S 21 , which is isostructural to the stable Pb 6 In 10 S 21 compound at room pressure., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

This joint experimental and theoretical study of the structural and vibrational properties of β-In 2 S 3 upon compression shows that this tetragonal defect spinel undergoes two reversible pressure-induced order-disorder transitions up to 20 GPa. We propose that the first high-pressure phase above 5.0 GPa has the cubic defect spinel structure of α-In 2 S 3 and the second high-pressure phase (ϕ-In 2 S 3 ) above 10.5 GPa has a defect α-NaFeO 2 -type ( R 3̄ m ) structure. This phase, related to the NaCl structure, has not been previously observed in spinels under compression and is related to both the tetradymite structure of topological insulators and to the defect LiTiO 2 phase observed at high pressure in other thiospinels. Structural characterization of the three phases shows that α-In 2 S 3 is softer than β-In 2 S 3 while ϕ-In 2 S 3 is harder than β-In 2 S 3 . Vibrational characterization of the three phases is also provided, and their Raman-active modes are tentatively assigned. Our work shows that the metastable α phase of In 2 S 3 can be accessed not only by high temperature or varying composition, but also by high pressure. On top of that, the pressure-induced β-α-ϕ sequence of phase transitions evidences that β-In 2 S 3 , a BIII2XV3 compound with an intriguing structure typical of A II BIII2XVI4 compounds (intermediate between thiospinels and ordered-vacancy compounds) undergoes: (i) a first phase transition at ambient pressure to a disordered spinel-type structure (α-In 2 S 3 ), isostructural with those found at high pressure and high temperature in other BIII2XV3 compounds; and (ii) a second phase transition to the defect α-NaFeO 2 -type structure (ϕ-In 2 S 3 ), a distorted NaCl-type structure that is related to the defect NaCl phase found at high pressure in A II BIII2XVI4 ordered-vacancy compounds and to the defect LiTiO 2 -type phase found at high pressure in A II BIII2XVI4 thiospinels. This result shows that In 2 S 3 (with its intrinsic vacancies) has a similar pressure behaviour to thiospinels and ordered-vacancy compounds of the A II BIII2XVI4 family, making β-In 2 S 3 the union link between such families of compounds and showing that group-13 thiospinels have more in common with ordered-vacancy compounds than with oxospinels and thiospinels with transition metals.

We report a joint experimental and theoretical study of the low-pressure phase of α'-Ga 2 S 3 under compression. Theoretical ab initio calculations have been compared to X-ray diffraction and Raman scattering measurements under high pressure carried out up to 17.5 and 16.1 GPa, respectively. In addition, we report Raman scattering measurements of α'-Ga 2 S 3 at high temperature that have allowed us to study its anharmonic properties. To understand better the compression of this compound, we have evaluated the topological properties of the electron density, the electron localization function, and the electronic properties as a function of pressure. As a result, we shed light on the role of the Ga-S bonds, the van der Waals interactions inside the channels of the crystalline structure, and the single and double lone electron pairs of the sulphur atoms in the anisotropic compression of α'-Ga 2 S 3 . We found that the structural channels are responsible for the anisotropic properties of α'-Ga 2 S 3 and the A'(6) phonon, known as the breathing mode and associated with these channels, exhibits the highest anharmonic behaviour. Finally, we report calculations of the electronic band structure of α'-Ga 2 S 3 at different pressures and find a nonlinear pressure behaviour of the direct band gap and a pressure-induced direct-to-indirect band gap crossover that is similar to the behaviour previously reported in other ordered-vacancy compounds, including β-Ga 2 Se 3 . The importance of the single and, more specially, the double lone electron pairs of sulphur in the pressure dependence of the topmost valence band of α'-Ga 2 S 3 is stressed.

The effects of pressure on the crystal structure of aurophilic tetragonal gold iodide have been studied by means of powder X-ray diffraction up to 13.5 GPa. We found evidence of the onset of a phase transition at 1.5 GPa that is more significant from 3.8 GPa. The low- and high-pressure phases coexist up to 10.7 GPa. Beyond 10.7 GPa, an irreversible process of amorphization takes place. We determined the axial and bulk compressibility of the ambient-pressure tetragonal phase of gold iodide up to 3.3 GPa. This is extremely compressible with a bulk modulus of 18.1(8) GPa, being as soft as a rare gas, molecular solids, or organometallic compounds. Moreover, its response to pressure is anisotropic.