First-principles density functional study of polarization-strain coupling in bismuth titanate

The influence of uniaxial and biaxial strain (+/- 3%) on the spontaneous polarization of orthorhombic bismuth titanate (Bi4Ti3O12) is investigated using a first-principles density functional approach. Born effective charges are obtained using linear response theory. In unstrained bismuth titanate the calculated principal component of polarization along the a axis (P-a) is 0.46 C m(-2), which is close to the experimental measurement of 0.50 C m(-2). Uniaxial strain along this axis is more effective than along the c axis in improving this component of polarization, by up to 17% for a tensile strain of 3%. Compressive strain along the c axis also enhances Pa but to a lesser degree. Biaxial strain has a more significant effect on Pa than uniaxial strain. A simultaneous uniform tensile strain of 3% along the a and b axes enhances the principal component of polarization by 39% while a similar strain along the c and b axes produces an enhancement of 8%. These effects are explained in terms of the off-centre displacements of ions and have implications for device applications of bismuth titanate which use epitaxially strained thin-film heterostructures.