Electrooptical effect and birefringence in pyroniobate cadmium crystals

The temperature dependences of electrooptical effect and spontaneous birefringence in Cd2Nb2O7 fermoelectric are investigated both in the absence of and under applied permanent bias fields. The curves exhibited anomalous behaviour at consequent phase transitions at 205, 201 and approximately 190 K. The contribution to birefringence from a spontaneous polarization under an improper-phase transition at 205 K is evaluated. The temperature dependence of the linear electrooptical coefficients and their changes at phase transitions in Cd2Nb2O7 are qualitatively interpreted