Pressure-induced phase transition and shock response of Ti3SiC2

Detailed first-principles electronic band structure calculations have been performed on Ti3SiC2 to understand its behavior under high static and shock loading. The ab initio calculations in conjunction with the evolutionary structure prediction method have predicted α→P1¯ structural phase transition at ∼100 GPa under hydrostatic compression at 0 K. Here, α and P1¯ phase correspond to hexagonal and triclinic structures. The isothermal equation of state derived up to 200 GPa displays a volume discontinuity around the transition pressure, indicating the first-order nature of this transition. Further, the derived isotherm agrees well with the static experimental data of Onodera et al. To understand the structural stability under shock: (i) the P–T phase diagram for α→P1¯ transition has been calculated and (ii) the Hugoniot has been derived from the theoretical 0 K isotherm in conjunction with lattice dynamical calculations and the Rankine–Hugoniot relation. The crossover of the P–T Hugoniot curve with phase boundary of α→P1¯ transition around 90 GPa with a corresponding shock temperature of ∼104 K indicates that this transition also occurs under shock loading. This not only complies with reported dense phase around 90–120 GPa in shock experiments by Jordan et al. but also identifies the structure of this phase. Further, the electronic density of state calculations reveal the metallic nature of α-Ti3SiC2 at 0 GPa as well as at high pressure. Our calculations further suggest a brittle to ductile transition in this material around 175 GPa. The mechanical and lattice dynamical stability of energetically competing structures has been examined employing the elastic constants and phonon spectrum derived as a function of pressure. The predicted high-pressure phases have displayed both lattice dynamical and elastic stability in the pressure regime of their structural stability but the α→P1¯ phase transition could neither be associated with elastic instability nor with phonon softening. Additionally, employing the lattice dynamical calculations, several thermophysical parameters such as equilibrium volume, bulk modulus, Debye temperature, thermal expansion coefficient, Grüneisen parameter, heat capacity, and lattice thermal conductivity of α-Ti3SiC2 have been determined at normal conditions and compared with the available experimental data.