Ferroelectric (Pb,Ba)Nb₂O₆ near the morphotropic phase boundary

Measurements of the optic index of refraction, along the c axis, are presented for three representative single crystals in the (PbxBa 1-x) Nb2O6 ferroelectric system. In particular, we show that as the temperature is lowered, crystals at the morphotropic phase boundary (x=0.63±0.03) first develop a polarization along the c axis and then, at lower temperature, the polarization rotates to the ab plane.