Prediction of a low-temperature ferroelectric instability in antiphase domain boundaries of strontium titanate

The structure of the antiphase domain boundaries of strontium titanate SrTiO₃ that exist at temperatures below the antiferrodistortive transition, is analyzed. It is found that some boundaries are similar to N\'eel domain walls, as an additional component of the order parameter develops within them. We show quantitatively that at low temperatures, typically below ~40 K, such boundaries become unstable with respect to the development of a ferroelectric polarization. This ferroelectric transition might provide the much needed explanation for several anomalies that are experimentally observed in SrTiO₃ in this temperature region.