Dynamic Disorder in Perovskites: Local Symmetry Breaking at the Sr Site in SrTiO3

Abstract

Perovskites such as SrTiO3 and BaTiO3 have been considered to be classical displacive systems where the phase transitions are connected with a soft mode instability. Only recently a disorder component has been detected in addition to the soft mode. It has been shown [1–3] by quadrupole perturbed Ti (I = 5/2) and Ti (I = 7/2) NMR that in the high temperature cubic phases of BaTiO3 and SrTiO3 the Ti ions are dynamically disordered between 8 off-center sites so that the ideal Oh symmetry is locally broken. The electric field gradient (EFG) tensor is non-zero at the Ti sites whereas it should be zero by symmetry in the ideal perovskite lattice with the Ti ion in the center of the oxygen octahedron. The Ti off-center sites are located at the cubic body diagonals in agreement with the extended X-ray absorption fine structure (EXAFS) data [4, 5], the early X-ray work of Comes et al. [6], and the theoretical model of Chaves et al. [7] (Fig. 1). The off-centering of the Ti ions is accompanied by a local tetragonal distribution of the unit cell, so that fluctuating tetragonal nanodomains (+a, − a, + b, − b, + c, − c) are formed in the cubic phase (Fig. 2). The macroscopic cubic symmetry is the result of space and time averaging. Below the cubic-tetragonal transition long range correlations appear and the nanodomains transform into macroscopic 90◦ domains. ∗corresponding author