Crystal Structures and Mechanical Properties of Ca₂C at High Pressure.

Recently, a new high-pressure semiconductor phase of Ca₂C (space group ) was successfully synthesized, it has a low-pressure metallic phase (space group 2/). 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 2/-Ca₂C to -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 2/-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 -Ca₂C and the metallicity of 2/-Ca₂C have been demonstrated by the electronic band structure calculations. The quasi-direct band gap of -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.