FIRST-PRINCIPLES STUDIES OF PRESSURE-INDUCED STRUCTURAL AND INSULATOR-TO-METAL TRANSITIONS IN ALKALINE-EARTH-METAL DICARBIDES MC2 (M = Ca, Sr AND Ba)

Pressure-induced phase transitions in MC2 (M = Ca, Sr and Ba) are investigated by using the first-principles plane wave pseudopotential method within the generalized gradient approximation. The first-order phase transition from tetragonal phase (CaC2-type, space group I4/mmm) to rhombohedral (CsCl-type, space group R3m) structure is predicted to occur at 22.2, 10.0 and 3.6 GPa, respectively, and transition pressure point of BaC2 agrees well with recent theoretical works. Based on the electronic analysis, the ionic Ca-C bond character becomes stronger with increasing pressure in both I4/mmm and R3m phases. In particular, there will occur a transition from insulator to metal with increasing pressure due to the reason that the calculated band gap gets narrower and finally closes at some high pressure.