Fluctuations in SrTiO3 near the 105-K phase transition

Abstract

The temperature dependence of the EPR linewidth of the Fe3+ -VO pair center in SrTiO3 has been investigated above and below Tc=105.6 K. It is shown from the angular dependence that the pronounced line broadening near Tc results from rotational fluctuations of the oxygen octahedra. The analysis yields a changeover from a fast-to a slow-fluctuation regime for T→Tc+. The linewidth has a cusp-shaped behavior close to Tc and is finite at Tc. From measurements in samples transforming to monodomains below Tc the anisotropy of the fluctuations is investigated. It is found that they are anisotropic even for temperatures within 10 K above Tc, where the point symmetry of the crystal is cubic. In this region, the anisotropy is connected with the increasing regions of correlated motions. Well below Tc it is related to the two distinct soft-mode branches. The results are analyzed in terms of the dynamical form factor S(q→,. Within a certain approximation they are found to be consistent, for temperatures above and below Tc, with a value of the critical exponent ν (for the correlation length ξ) of ν0.65, together with a two-dimensionally anisotropic static-order-parameter susceptibility χ(q→, T) with an anisotropy parameter Δ of the order of 140. The changeover from fast- to slow-fluctuation regime yields an estimate of the half-width of the central peak in S(q→, of 70 MHz at T=Tc+0.8±0.3 K. © 1973 The American Physical Society.