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.