Studies of tetragonal PbTiO₃ subjected to uniaxial stress along the c-axis

Tetragonal PbTiO₃ under uniaxial stress along the c-axis is investigated from first-principles. The structural parameters, polarization, and squares of the lowest optical phonon frequencies for E(1TO) and A ₁(1TO) modes at Γ show abrupt changes near a stress σ_c of 1.04GPa, which is related to the dramatic change of elastic constant c₃₃ resulting from the uniaxial stress applied along the c-axis. We also find that the uniaxial compressive stress could enhance the piezoelectric stress coefficients, whereas the uniaxial tensile stress could enhance the piezoelectric strain coefficients. It is also found that when the magnitude of uniaxial compressive stress σ₃₃ is greater than 12GPa, PbTiO₃ is transformed to the paraelectric tetragonal phase.