Recently, sophisticated theoretical computational studies have proposed several new crystal structures of carbon (e.g., bct-C₄, H-, M-, R-, S-, W-, and Z-carbon). However, until now, there lacked experimental evidence to verify the predicted high-pressure structures for cold-compressed elemental carbon at least up to 50 GPa. Here we present direct experimental evidence that this enigmatic high-pressure structure is currently only consistent with M-carbon, one of the proposed carbon structures. Furthermore, we show that this phase transition is extremely sluggish, which led to the observed broad x-ray diffraction peaks in previous studies and hindered the proper identification of the post-graphite phase in cold-compressed carbon.

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.

High-pressure Raman spectroscopy and x-ray diffraction of Sb₂S₃ up to 53 GPa reveals two phase transitions at 5 GPa and 15 GPa. The first transition is evidenced by noticeable compressibility changes in distinct Raman-active modes, in the lattice parameter axial ratios, the unit cell volume, as well as in specific interatomic bond lengths and bond angles. By taking into account relevant results from the literature, we assign these effects to a second-order isostructural transition arising from an electronic topological transition in Sb₂S₃ near 5 GPa. Close comparison between Sb₂S₃ and Sb₂Se₃ up to 10 GPa reveals a slightly diverse structural behavior for these two compounds after the isostructural transition pressure. This structural diversity appears to account for the different pressure-induced electronic behavior of Sb₂S₃ and Sb₂Se₃ up to 10 GPa, i.e. the absence of an insulator-metal transition in Sb₂S₃ up to that pressure. Finally, the second high-pressure modification appearing above 15 GPa appears to trigger a structural disorder at ~20 GPa; full decompression from 53 GPa leads to the recovery of an amorphous state.

We have explored the high-pressure structural, vibrational, electronic, and magnetic behavior of the multiferroic $\mathrm{CoC}{\mathrm{r}}_2{\mathrm{O}}_4$ spinel up to 30 GPa by means of x-ray diffraction, Raman spectroscopy, and density functional theory calculations. Our investigations revealed a reversible tetragonal distortion of the starting cubic structure above 16 GPa. We suggest that the structural modification is mainly driven by magnetic effects induced under pressure. The results obtained are compared with the high-pressure behavior of related spinels.

A large volume press coupled with in-situ energy-dispersive synchrotron X-ray was used to probe the change of silicon carbide (SiC) under high pressure and temperature (P-T) up to 8.1 GPa and 1100 K. The obtained pressure–volume–temperature data were fitted to a modified high-T Birch-Murnaghan equation of state, yielding values of a series of thermo-elastic parameters, such as the ambient bulk modulus K_To = 237(2) GPa, temperature derivative of the bulk modulus at a constant pressure (∂K/∂T)_P = −0.037(4) GPa K⁻¹, volumetric thermal expansivity α(0, T) = a + bT with a = 5.77(1) × 10⁻⁶ K⁻¹ and b = 1.36(2) × 10⁻⁸ K⁻², and pressure derivative of the thermal expansion at a constant temperature (∂α/∂P)_T = 6.53 ± 0.64 × 10⁻⁷ K⁻¹GPa⁻¹. Furthermore, we found the temperature derivative of the bulk modulus at a constant volume, (∂K_T/∂T)_V, equal to −0.028(4) GPa K⁻¹ by using a thermal pressure approach. In addition, the elastic properties of SiC were determined by density functional theory through the calculation of Helmholtz free energy. The computed results generally agree well with the experimentally determined values.