Selected members of the A2B3 (A = Sb, Bi; B = Se, Te) family are topological insulators. The Sb2Se3 compound does not exhibit any topological properties at ambient conditions; a recent high-pressure study, however, indicated that pressure transforms Sb2Se3 from a band insulator into a topological insulator above ~2 GPa; in addition, three structural transitions were proposed to occur up to 25 GPa. Partly motivated by these results, we have performed x-ray diffraction and Raman spectroscopy investigations on Sb2Se3 under pressure up to 65 GPa. We have identified only one reversible structural transition: the initial Pnma structure transforms into a disordered cubic bcc alloy above 51 GPa. On the other hand, our high-pressure Raman study did not reproduce the previous results; we attribute the discrepancies to the effects of the different pressure transmitting media used in the high-pressure experiments. We discuss the structural behavior of Sb2Se3 within the A2B3 (A = Sb, Bi; B = Se, Te) series.

We have conducted high-pressure x-ray diffraction and Raman spectroscopic studies on the CdCr 2 Se 4 spinel at room temperature up to 42 GPa. We have resolved three structural transitions up to 42 GPa, i.e. the starting Fd3 m phase transforms at ~11 GPa into a tetragonal I4 1 /amd structure, an orthorhombic distortion was observed at ~15 GPa, whereas structural disorder initiates beyond 25 GPa. Our ab initio DFT studies successfully reproduced the observed crystalline-to-crystalline structural transitions. In addition, our calculations propose an anti-ferromagnetic ordering as a potential magnetic ground state for the high-pressure tetragonal and orthorhombic modifications, as compared to the starting ferromagnetic phase. Furthermore, the computational results indicate that all phases remain insulating in their stability pressure range, with a direct-to-indirect band gap transition for the Fd3 m phase taking place at 5 GPa.We attempted also to offer an explanation behind the peculiar first-order character of the Fd3 m (cubic)→I4 1 /amd (tetragonal) transition observed for several relevant Cr-spinels, i.e. the sizeable volume change at the transition point, which is not expected from space group symmetry considerations. We detected a clear correlation between the cubic-tetragonal transition pressures and the next-nearest-neighbor magnetic exchange interactions for the Crbearing sulphide and selenide members, a strong indication that the cubic-tetragonal transitions in these systems are principally governed by magnetic effects.