The central mode in the critical scattering of X-rays by SrTiO₃

Abstract

The dynamics of the phase transition in SrTiO3 (Tc approximately 105K) is complicated by the presence of a central mode in the scattering at the R point of the cubic Brillouin zone in addition to the expected soft-mode scattering. Experiments using Mossbauer gamma rays are reported in which 'elastic' scattering with an energy change of less than 1.5*10-8 eV is detected at the R-point for temperatures as high as 170K. Furthermore, by using X-rays of different incident energy, it is shown that above Tc the scattering power of the crystal at the R point is depth-dependent. The intensity of the scattering above Tc can be altered by annealing the crystal. The results are interpreted in terms of a depth-dependent transition temperature; the crystal begins to transform to the lower-temperature phase first at the surface, and as Tc is approached from above the transformed surface region extends further into the bulk.