A pressure-induced phase transition, associated with an increase of the coordination number of In and Ta, is detected beyond 13 GPa in InTaO 4 by combining synchrotron x-ray diffraction and Raman measurements in a diamond-anvil cell with ab initio calculations. High-pressure optical-absorption measurements were also carried out. The high-pressure phase has a monoclinic structure that shares the same space group with the low-pressure phase (P 2/c). The structure of the high-pressure phase can be considered as a slight distortion of an orthorhombic structure described by space group Pcna. The phase transition occurs together with a unit-cell volume collapse and an electronic band-gap collapse observed by experiments and calculations. Additionally, a band crossing is found to occur in the low-pressure phase near 7 GPa. The pressure dependence of all the Raman-active modes is reported for both phases as well as the pressure dependence of unit-cell parameters and the equations of state. Calculations also provide information on infrared-active phonons and bond distances. These findings provide insights into the effects of pressure on the physical properties of InTaO 4 .

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.

Antimony trisulfide (Sb 2 S 3 ), found in nature as the mineral stibnite, has been studied under compression at room temperature from a joint experimental and theoretical perspective. X-ray diffraction and Raman scattering measurements are complemented with ab initio total-energy, lattice-dynamics, and electronic structure calculations. The continuous changes observed in the volume, lattice parameters, axial ratios, bond lengths, and Raman mode frequencies as a function of pressure can be attributed to the different compressibility along the three orthorhombic axes in different pressure ranges, which in turn are related to the different compressibility of several interatomic bond distances in different pressure ranges. The structural and vibrational properties of Sb 2 S 3 under compression are compared and discussed in relation to isostructural Bi 2 S 3 and Sb 2 Se 3 . No first-order phase transition has been observed in Sb 2 S 3 up to 25 GPa, in agreement with the stability of the Pnma structure in Bi 2 S 3 and Sb 2 Se 3 previously reported up to 50 GPa. Our measurements and calculations do not show evidence either for a pressure-induced secondorder isostructural phase transition or for an electronic topological transition in Sb 2 S 3 .

We report a study of the structural, vibrational, and electronic properties of layered monoclinic arsenic telluride (α-As 2 Te 3 ) at high pressures. Powder x-ray diffraction and Raman scattering measurements up to 17 GPa have been complemented with ab initio total-energy, lattice dynamics, and electronic band structure calculations. Our measurements, which include previously unreported Raman scattering measurements for crystalline α-As 2 Te 3 , show that this compound undergoes a reversible phase transition above 14 GPa at room temperature. The monoclinic crystalline structure of α-As 2 Te 3 and its behavior under compression are analysed by means of the compressibility tensor. Major structural and vibrational changes are observed in the range between 2 and 4 GPa and can be ascribed to the strengthening of interlayer bonds. No evidence of any isostructural phase transition has been observed in α-As 2 Te 3 . A comparison with other group-15 sesquichalcogenides allows understanding the structure of α-As 2 Te 3 and its behavior under compression based on the activity of the cation lone electron pair in these compounds. Finally, our electronic band structure calculations show that α-As 2 Te 3 is a semiconductor at 1 atm, which undergoes a trivial semiconducting-metal transition above 4 GPa.The absence of a pressure-induced electronic topological transition in α-As 2 Te 3 is discussed.

The high-pressure behavior of technologically important visible-light photocatalytic semiconductor InNbO, adopting a monoclinic wolframite-type structure at ambient conditions, was investigated using synchrotron-based X-ray diffraction, Raman spectroscopic measurements, and first-principles calculations. The experimental results indicate the occurrence of a pressure-induced isostructural phase transition in the studied compound beyond 10.8 GPa. The large volume collapse associated with the phase transition and the coexistence of two phases observed over a wide range of pressure shows the nature of transition to be first-order. There is an increase in the oxygen anion coordination number around In and Nb cations from six to eight at the phase transition. The ambient-pressure phase has been recovered on pressure release. The experimental pressure-volume data when fitted to a Birch-Murnaghan equation of states yields the value of ambient pressure bulk modulus as 179(2) and 231(4) GPa for the low- and high-pressure phases, respectively. The pressure dependence of the Raman mode frequencies and Grüneisen parameters was determined for both phases by experimental and theoretical methods. The same information is obtained for the infrared modes from first-principles calculations. Results from theoretical calculations corroborate the experimental findings. They also provide information on the compressibility of interatomic bonds, which is correlated with the macroscopic properties of InNbO.