Competing structural instabilities in cubic perovskites.

We study the antiferrodistortive instability and its interaction with ferroelectricity in cubic perovskite compounds. Our first-principles calculations show that coexistence of both instabilities is very common. We develop a firstprinciples scheme to study the thermodynamics of these compounds when both instabilities are present, and apply it to SrTiO3. We find that increased pressure enhances the antiferrodistortive instability while suppressing the ferroelectric one. Moreover, the presence of one instability tends to suppress the other. A very rich P–T phase diagram results. Submitted to Phys. Rev. Lett. 77.80.Bh, 61.50.Lt, 64.60.Cn, 64.70.-p Typeset using REVTEX