Measurements of strain and the optical indices in the ferroelectric ${\mathrm{Ba}}_{0.4}$${\mathrm{Sr}}_{0.6}$${\mathrm{Nb}}_2$${\mathrm{O}}_6$: Polarization effects

We report accurate temperature-dependent measurements of the optic index of refraction, the birefringence, and the strain in the ferroelectric, tungsten-bronze crystal ${\mathrm{Ba}}_{0.4}$ ${\mathrm{Sr}}_{0.6}$ ${\mathrm{Nb}}_2$ ${\mathrm{O}}_6$. This composition congruently melts, so large homogeneous high-quality crystals can be grown. From the experimental data, it appears that far above the ferroelectric phase transition temperature ($T_c$≊88 °C), up to a temperature $T_d$ (≊300 °C), the crystals appear to possess a local, randomly oriented (up or down), polarization $P_d$. The values of $T_d$ obtained from the index and completely independent strain measurements are in very good agreement with each other, as are the values of $P_d$. Various aspects of our understanding of the polarization behavior and other effects in this ferroelectric system are discussed.