•Three pressure-induced first-order phase transformations were predicted for Mg3P2.•Metallization was predicted at the phase transition of P-3m1→P63/mmc at 35 GPa.•Pressure-induced charge transfer is responsible for the phase transformations.The phase behavior of Mg3P2 was investigated at increasing pressure from ambient to 100 GPa using first-principles calculations. It transformed first from anti-C (Ia-3) to anti-A (P-3m1) at 2.5 GPa, and then to antipost-perovskite (P63/mmc) at 35 GPa. Remarkably, we found a new low-enthalpy structure with space group C2/c to be favored above 65 GPa. The electronic calculations predict Mg3P2 to be initially a semiconductor, but to become a metal at 35 GPa in the P63/mmc structure, and remain so in the new C2/c structure at 65 GPa. Phonon dispersions demonstrate that the four phases are dynamically stable in their respective low-enthalpy pressure ranges. Charge transfers in the phases are also calculated and discussed.

Until now, it has been a challenge both in experiment and in theory to design new superhard materials with high hardness values that are comparable to that of diamond. Here, by using first-principles calculations, we have introduced two new phases for a carbon-rich C–N compound with stoichiometry C3N, which is predicted to be energetically stable or metastable with respect to graphite and solid N2 at ambient pressure. It is found that C3N has a layered structure containing graphitic layers sandwiched with freely rotated N2 molecules. The layer-structured C3N is calculated to transform into a three-dimensional C2221 structure at 9 GPa with sp3-hybridized C atoms and sp2-hybridized N atoms. Phonon dispersion and elastic constant calculations reveal the dynamical and mechanical stability of the C2221 phase of C3N at ambient pressure. Significantly, first-principles ideal strength calculations indicate that the C2221 phase of C3N is a superhard material with an estimated Vickers hardness (∼76 GPa) comparable to that of diamond (60–120 GPa). The present results shed strong light on designing new superhard materials in the C–N system.

Ab initio calculations were performed on CrO2 to study its behavior and possible similarity to silica under high pressures. At the rutile→CaCl2-type phase transition, the lattice constants, cell volume and total energy change continuously, indicating the second-order nature of the phase transition, consistent with the experimental observations. The current calculations have demonstrated that the rutile→CaCl2-type phase transition is driven by the softening of the Raman active B1g mode, weakly coupling with the elastic shear modulus Cs. Further phase transitions of CrO2 to denser packed phases of α-PbO2-type and pyrite have been well predicted by total energy calculations. Our electronic calculations revealed that CrO2 is still a half-metallic ferromagnet up to pressure of 95 GPa. The present results confirm the analogy of the phase sequence between silica and CrO2 at high pressures.Highlights► Two new phases was predicted for CrO2 at high pressures for the first time. ► The mechanism of the rutile→CaCl2-type transition is revealed. ► CrO2 is still a half-metallic ferromagnet up to pressure of 95 GPa.