Relationship between lattice deformation and polarization in BaTiO3

The relationship between lattice deformation and electrical polarization in tetragonal BaTiO3 is investigated. The density functional theory within th e local density approximation using the full-potential-linear-augmented-pla ne-wave (FLAPW) method is adopted to obtain internal atom positions and one -electron wave functions. Electric polarization is calculated using the Ber ry-phase theory. We have found that a lattice strain of the order of 1 % al ong the c-axis enhances polarization considerably. However, for that of the order of 0.1 %, polarization hardly changes. We assume that these response s of the polarization to lattice strain are related to the stress sensitivi ty of the polarization in ferroelectric-thin films through nanoscale domain s, especially ferroelectric-90-degree domains. We have also found that the polarization of BaTiO3 can be scaled linearly by the distance between Ti an d its nearest-neighbor oxygen (apical site in oxygen octahedron). This indi cates that the covalency between Ti and the apical oxygen is the only drivi ng force for the ferroelectricity in BaTiO3, We suggest that this covalency softens Young's modulus of BaTiO3 in the ferroelectric states compared to the paraelectric states through the increase of the degree of freedom for a tomic displacements in a unit cell.