Phase transition in PbTiO3 under pressure studied by the first-principles method

The pressure-induced tetragonal-to-cubic phase transition of PbTiO3 is investigated in the pressure range of −5 to 35 GPa using the density-functional theory with the local density approximation. The spontaneous polarization and spontaneous strain, calculated as the order parameters, are found to decrease abruptly at 32 GPa, while the volume of the unit cell to decrease continuously, displaying a non-purely first-order character consistent with the experimental results. The structural explanation for the crossover in the order of the phase transition in PbTiO3 (from first to second order) is generally discussed in terms of the bonding structure.