Observation of a sample-dependent 37K anomaly on the lattice parameters of strontium titanate

– The lattice parameters of tetragonal strontium titanate are determined to about 1 part in 10 with penetrating X-rays. While no peculiarity is detected in a bulk sample, a distinct anomaly at 37K is observed in an optically polished thin platelet derived from the same single crystal. This suggests that the anomaly is related to a high density of dislocations. A possible mechanism is that dislocations stabilize transverse antiphase boundaries which undergo a ferroelectric transition near 37K. If so, this transition stresses the boundaries which then strain the bulk leading to the observed anomaly. The perovskites, ABO3, form a large family of compounds with a rich variety of properties finding many applications, in particular dielectric ones. Strontium titanate, SrTiO3, is often considered as the paradigm of perovskites. It is cubic at room temperature (T ) and the ionic radii ideally match the structure, the tolerance factor [1] being equal to 1 to better than 1%. The interest in perovskites does not lie in perfect cubic structures, but rather in the many instabilities that the structure is prone to. Considering crystalline —or ferroic— changes in pure compounds, the instabilities are mainly of two types: i) the ferrodistortive ones that preserve the number Z of molecular units per primitive cell, and ii) the antiferrodistortive (AFD) ones for which Z is multiplied by an integer greater than 1. SrTiO3 exhibits both types of instabilities. It remains cubic with Z = 1 down to Ta ∼= 105K, where it first undergoes an AFD transition with cell doubling [2]. The latter results from alternate rotations of the quite rigid TiO6 octahedra around one of the three cubic axes, which becomes thereby the tetragonal (∗) Present address: Institut für Werkstofforschung, GKSS-Forschungszentrum Max-Planck-Strasse, D-21502 Geesthacht, Germany.