AB-INITIO LINEAR-RESPONSE STUDY OF SRTIO3

The lattice instabilities of perovskite structure oxides are responsib le for many of their interesting properties, such as temperature-depen dent ferroelectric phase transitions. First-principles calculations us ing linear response theory provide an accurate means to determine the lattice dynamics throughout the entire Brillouin zone (BZ), Using the LAPW linear response method([1]), we have previously carried out such a study on ferroelectric KNbO3[2]. We present here the results of a si milar investigation for cubic SrTiO3. While KNbO3 has only ferroelectr ic-type instabilities, SrTiO3 exhibits both ferroelectric (FE) and ant iferrodistortive (AFD) instabilities. We correctly predict the known i nstability at the R-point in the Brillouin zone, which is responsible for the AFD phase transition to the tetragonal structure at about 105 K. Furthermore, the phase space of the ferroelectric instability is gr eatly reduced compared to KNbO3. Antiferrodistortive instabilities exi st in one-dimensional cylindrical tubes extending along the entire R-M -R line in the Brillouin zone. The essentially 1-D character of these tubes corresponds to real-space planar instabilities characterized by rotations oi oxygen octahedra.