Pressure induced structural phase transformations of transition metal carbides

In this study, the pressure induced phase transformations of some transition metal carbides have been investigated using SIESTA package program. The studies were performed using the generalized gradient approximation (GGA in the context of density functional theory (DFT). Pressures were applied in gradual increments starting from 0 GPa to the materials. For each applied pressure value, the space groups and lattice parameters of the materials were determined. Two high pressure phases were found for each of ZrC and HfC and one high pressure phase was found for each of NbC and TaC. These phases for ZrC and HfC possess the space group Fm3 ̅m at 80 GPa and 60 GPa while for ZrC, HfC, NbC and TaC possess the space group C2/m at 320, 300, 60 GPa and 62 GPa, respectively. During these phase transformations, there is one intermediate phase with space group Imm2 for ZrC, there are five different intermediate phases with space groups Cmc2_1, P21, P1 ̅, C2/c, Fm3 ̅m for NbC and there are three different intermediate phases with space groups Ama2, Amm2, Fm3 ̅m for TaC. In order compare phase transition pressures with the experimental results, total energy and enthalpy calculations were carried out.