A first-principles investigation into the ferroelectric and antiferrodistortive instabilities of cubic SrTiO₃

The lattice dynamics and potential energy curves of cubic SrTiO₃ as a function of lattice volume were investigated by density functional theory (DFT) calculations. The calculated results indicate that the lowest optical phonon in the Brillouin-zone center is real and an unstable R₂₅ mode, corresponding to the antiferrodistortive transition, is found at the equilibrium volume. The results are consistent with previous experimental predictions. Moreover, increasing volume can improve the ferroelectric instability and restrain the antiferrodistortive one. A competition between the ferroelectric and antiferrodistortive instabilities does exist. The smaller lattice constant of cubic SrTiO₃ than of BaTiO₃ is crucial both for the appearance of the antiferrodistortive instability and the disappearance of the ferroelectric instability, which makes the transition behaviors of the two compounds entirely different.