LOCAL DISORDER STUDIED IN SRTIO3 AT LOW-TEMPERATURE BY EXAFS SPECTROSCOPY

The temperature dependence of the local distortions in SrTiO3 has been studied by EXAFS spectroscopy at the titanium K edge (4982 eV). The oxygen-ion Debye-Waller factor al has been determined from 4.5 to 240 K. The antiferrodistortive transition at 105 K is evidenced by a step in this Debye-Waller factor. At about 31 K, a maximum of sigma0(2) is detected and the EXAFS oscillations due to the first oxygen shell increase. This is the signature of a maximum disorder in the lattice vibrations in this temperature range. A quasiharmonic model with a sinusoidal modulation of the Ti-O distance cannot account for these observations. The lattice and electronic excitations which could support our data are qualitatively described.