A new phase of ThC at high pressure predicted from a first-principles study

•A new phase (P4/nmmP4/nmm) of ThC at high pressure has been predicted.•Dynamical stability for the P4/nmmP4/nmm and B2 phases has been investigated.•The transition path and energy barrier between B  1 and P4/nmmP4/nmm have been studied.•Transformation from B  1 to P4/nmmP4/nmm is a first-order transition.•Transformation from P4/nmmP4/nmm to B2 is a second-order transition.

The phase transition of thorium monocarbide (ThC) at high pressure has been studied by means of density functional theory. Through structure search, a new phase with space group P4/nmmP4/nmm has been predicted. The calculated phonons demonstrate that this new phase and the previous B2 phase are dynamically stable as the external pressure is greater than 60 GPa and 120 GPa, respectively. The transformation from B  1 to P4/nmmP4/nmm is predicted to be a first-order transition, while that from P4/nmmP4/nmm to B2 is found to be a second-order transition.