Lattice dynamics investigation of different transition behaviors of cubic BaTiO3and SrTiO3by first-principles calculations

The lattice dynamics of cubic BaTiO 3 and SrTiO 3 at different crystal lattice volumes were investigated using first-principles density functional theory calculation. The computational results indicate that the Ti–O and A–O (A = Sr or Ba) interactions are responsible for the unstable ferroelectric Γ 15 and antiferrodistortive R 25 phonons, respectively. With decreasing volume, the Γ 15 phonon behaviors of cubic SrTiO 3 and BaTiO 3 show a similar trend and the Ti–O repulsions are significantly enhanced, leading to the disappearance of ferroelectric instability. However, for BaTiO 3 the large ionic radius of Ba with respect to Sr results in a repulsive Ba–O interaction and thereby the disappearance of the R 25 instability, while the long-range characteristic of Sr–O pair are crucial for the antiferrodistortive instability of cubic SrTiO 3 . By analyzing the real-space interatomic force constants, it can be confirmed that the different antiferrodistortive behaviors of the two compounds are dictated by different short-range repulsions of A–O pairs. Moreover, owing to the smaller ionic radius of Sr relative to Ba, the lattice constant of SrTiO 3 can be significantly reduced, leading to the absence of ferroelectric instability for SrTiO 3 . The substitution and different crystal volumes make the phase transitions of the two compounds entirely different.