Birefringence above Tc in single crystals of barium titanate

The temperature dependence of the birefringence above Tc in high quality BaTiO3 single crystals has been measured with very high accuracy. On the basis of literature data, the behaviour of the anomalous birefringence can be explained by the existence of polar clusters (microregions/nanoregions) connected with correlated jumps between the off-centre positions of Ti ions inside the oxygen octahedra. The underlying origin of the polar clusters is an intrinsic effect connected with the interdependence of the structural and polar soft modes (Bussmann-Holder et al 2007 Phys. Rev. Lett. 99 167603). These clusters locally break the intrinsic cubic symmetry and the phase above Tc becomes birefringent. A birefringence Δn of the order of 10-5 has been found up to about 300 above Tc. In this temperature range Tc<T<Tc+30 K, the function Δn(T) exhibits a thermal hysteresis: the values of Δn were higher on heating than on cooling. A characteristic feature is the existence of a local Δn(T) maximum at a temperature close to Tc on the high temperature side that appears due to a specific cluster-cluster interaction. Since the values of Δn(T) runs were not dependent on the crystal thickness, one can conclude that in this limited temperature range above Tc the birefringence is an inherent feature of the crystal. (fast track communication)