Abstract
Using the first-principles linear response method implemented with an LAPW basis [1], we have investigated the lattice dynamics of cubic SrTiO3 [2] at the LDA equilibrium lattice parameter. Consistent with the antiferrodistortive (AFD) phase transition to the tetragonal structure at 105 K, we found phonon instabilities along the R-M-R line in the Brillouin zone. In addition, a ferroelectric-type (FE) instability was found at the Γ-point, although no FE structural phase transition is observed. Working in the tetragonal phase, we determine the theoretical equilibrium twist angle for the oxygen octahedra, and show that as the lattice parameter is reduced, the AFD instability is enhanced, while the FE instability is diminished. Furthermore, we find that the tetragonal structure is marginally stable against FE distortions along the c-axis.
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