In this study, thermodynamic properties of BC2N under extreme conditions have been reported by using first-principle calculations and quasi-harmonic Debye model. Isochoric heat capacity (Cv) of BC2N at normal temperature and pressure is 23.15 kJ mol−1 K−1 and it increases with the temperature and decreases with the pressure. In the low temperature region, pressure has no obvious influence on phonons and thus the decrease of Cv is very slow. In the medium temperature region, the decrease of Cv becomes steep. The reason is that high pressure plays an important role in controlling the vibration of atoms. In the high temperature region, the decrease of Cv becomes slow. Debye temperature (θ) decreases with the temperature. However, the tendency is not obvious in the low temperature region but very clear in high temperature. Moreover, θ increases with pressure and the amplitude is larger in higher temperature. Because of the four covalent bonds with different strength and distribution asymmetric thermal expansion along different axes occurs. The value of thermal expansion coefficient along c axis is more than that of along a and b axes.

In this work, pressure-induced phase transition and elastic properties of BeS II–VI compounds semiconductor are investigated by first-principle method. Phase transition of BeS from direct gap semiconductor phase (ZB) to indirect-gap semiconductor phase (RS) occurs at 51.45 GPa accompanied by 11.23% volume collapse. Phase transition is due to S atom’s weakened electron attraction. Once the shared charge center of shared electron reaches 0.58 of the distance between Be and S, will the phase be unstable. Moreover, the broadened anti-bonding state and reduced bonding state display that tetrahedral Be-S covalent bonds are weakened. And then, the ZB structure is destroyed and rebuilt to RS structure. Furthermore, changes of covalent bond would cause evident variation of elastic constants and shears on {1 0 0} and {1 1 0} planes.