First-principles study of PbTiO3 under uniaxial strains and stresses

The behavior of PbTiO3 under uniaxial strains and stresses is investigated from first-principles calculations within density functional theory. We show that irrespective of the uniaxial mechanical constraint applied, the system keeps a purely ferroelectric ground state, with the polarization aligned either along the constraint direction (FEz phase) or along one of the pseudocubic axes perpendicular to it (FEx phase). This contrasts with the cases of isotropic and biaxialmechanical constraints for which novel phases combining ferroelectric and antiferrodistortive motions have been previously reported. Under uniaxial strain, PbTiO3 switched from an FEx ground state under compressive strain to an FEz ground state under tensile strain beyond a critical strain ηc zz ≈ +1%. Under uniaxial stress, PbTiO3 exhibits either an FEx ground state under compression (σzz < 0) or an FEz ground state under tension (σzz > 0). Here, however, an abrupt jump of the structural parameters is also predicted under both compressive and tensile stresses at critical values σzz ≈ +2 and −8 GPa. This behavior appears to be similar to that predicted under negative isotropic pressure and might turn out to be practically useful for enhancing the piezoelectric response in nanodevices.