Domain Redistribution and Ferroelectric Phase Transition in SrTiO3 Under the Influence of an Electric Field and Mechanical Stress

Strontium titanate (SrTiO 3) is one of the best studied substances showing a displacive phase transition [1]. At ambient temperature, it exhibits a cubic perovskite structure with the space group Pm3m. On cooling, a Brillouin zone-boundary softmode drives the second-order transition to an antiferrodis-tortive phase at T C = 105 K. The primary order parameter is the antiphase rotation of TiO 6-octahedra in adjacent unit cells. As a result, a paraelectric, tetragonal phase with the space group I4/mcm is formed. The tetragonal distortion c/a is slightly larger than unity and represents a secondary order parameter of the transition. Without an external perturbation none of the three possible tetragonal axes is favored over the others and the twin domains are evenly distributed. The isostuctural barium titanate (BaTiO 3) shows a completely different behavior: Here, the condensation of a Γ-point softmode leads to a ferroelectric low temperature phase. A softmode with the same polar eigenvector is also observed in strontium titanate but shows only an incomplete softening [2]. This incipient ferroelectricity is also revealed by the high dielectric permittivity at low temperatures. It is assumed that long range ferroelectric order is suppressed by quantum fluctuations [3]. However, a polar phase can be induced by the application of an external electric field larger than 2 kV/cm below 40 K [4]. Both, an electric field as well as mechanical load are able to affect the distribution of tetragonal domains in SrTiO 3. A quantitative relation is, however, not yet available. Therefore, we investigated the domain distribution in a single crystal of strontium titanate while applying uniaxial stresses and electric fields simultaneously. A pressure cell was constructed that enabled us to apply a uniaxial force of up to 2 kN and voltages up to several kV at low temperatures in a closed cycle cryostat. The lattice dynamics and the intensities of two tetragonal superlattice reflections were studied at the thermal three axis spectrometer PUMA at the FRM II neutron source over a wide range of temperatures. In addition a larger set of 15 superlattice reflections was measured at the hot neutron diffractometer HEIDI at the same facility to allow a quantitative determination of the domain fractions at selected temperatures. A model of the structure factor was employed to determine the influence of the three domains on the observed intensities. For example the reflection R1 (1/2-1/2 3/2) depends exclusively on the volume fractions of domains with …