Investigating the phase transition and properties of CaSiN2 under pressure based on first-principles calculations

In this study, we apply first-principles calculations to examine the pressure-induced phase transformation of CaSiN2 in a range of pressure of 0–100 GPa. Its pressure-induced transitions at 1.3 GPa, 15.3 GPa, and 55.8 GPa followed the order of α- CaSiN2 → β- CaSiN2 → α- CaSiN2 → γ- CaSiN2, for α- CaSiN2 → β- CaSiN2, β- CaSiN2 → α- CaSiN2, and α- CaSiN2 → γ- CaSiN2, respectively. The stability of the phases of CaSiN2 was confirmed based on calculations of the Born criterion of elastic stability. Its behavior transitioned in the sequence of brittle (0–1.3 GPa) → ductile (1.3–55.8 GPa) → brittle (55.8–100 GPa). The structure of its projected orbital band reflected insulating behavior by CaSiN2 under a range of pressure of 0–55.8 GPa with a direct band gap, which transformed into metallic behavior by the γ- CaSiN2 phase under pressures higher than 55.8 GPa, due to a shift in energy to higher levels around the Γ point of the N p orbitals and Si p orbitals. The Si-N bonds in CaSiN2 were found to be covalent, while ionic bonding dominated the Ca-Si and Ca-N bonds in the range of pressure of 0–100 GPa. We also investigate and discuss its mechanical properties, Vickers hardness (Hv), average sound velocity vm, and Debye temperature (θD).