Universal phase transitions of B1-structured stoichiometric transition metal carbides.

The high-pressure phase transitions of B1-structured stoichiometric transition metal carbides (TMCs, TM = Ti, Zr, Hf, V, Nb, and Ta) were systematically investigated using ab initio calculations. These carbides underwent universal phase transitions along two novel phase-transition routes, namely, B1 → distorted TlI (TlI') → TlI and/or B1 → distorted TiB (TiB') → TiB, when subjected to pressure. The two routes can coexist possibly because of the tiny enthalpy differences between the new phases under corresponding pressures. Four new phases result from atomic slips of the B1-structured parent phases under pressure. After completely releasing the pressure, taking TiC as representative of TMCs, only its new TlI'-type phase is mechanically and dynamically stable, and may be recovered.