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Recently, a new high-pressure semiconductor phase of Ca₂C (space group Pnma ) was successfully synthesized, it has a low-pressure metallic phase (space group C 2/ m ). In this paper, a systematic investigation of the pressure-induced phase transition of Ca₂C is studied on the basis of first-principles calculations. The calculated enthalpy reveals that the phase transition which transforms from C 2/ m -Ca₂C to Pnma -Ca₂C occurs at 7.8 GPa, and it is a first-order phase transition with a volume drop of 26.7%. The calculated elastic constants show that C 2/ m -Ca₂C is mechanically unstable above 6.4 GPa, indicating that the structural phase transition is due to mechanical instability. Both of the two phases exhibit the elastic anisotropy. The semiconductivity of Pnma -Ca₂C and the metallicity of C 2/ m -Ca₂C have been demonstrated by the electronic band structure calculations. The quasi-direct band gap of Pnma -Ca₂C at 0 GPa is 0.86 eV. Furthermore, the detailed analysis of the total and partial density of states is performed to show the specific contribution to the Fermi level.

Crystal Structures and Mechanical Properties of Ca2C at High Pressure