Glassy polarization in the ferroelectric tungsten bronze (Ba,Sr)Nb2O6

We report accurate temperature dependent measurements of optic indices of refraction, the birefringence, and the strain in the ferroelectric tungsten bronze crystals Ba0.25Sr0.75Nb2O6 and two compositions of (Ba2−xSrx)2(K1−yNay)2(NbO3)10. These results are compared to our previous results in Ba0.4Sr0.6Nb2O6. From the experimental data, it appears that far above the ferroelectric Tc, up to a temperature Td, these crystals possess a local, randomly oriented polarization, Pd, with similar Td values, irrespective of their chemical composition and Tc. Various aspects of our understanding of the polarization behavior and other effects in this ferroelectric system are discussed.