Ferroelectric phase transition in thiourea:KBr composites studied with optical second-harmonic generation

We present results of an experimental study on the effect of the inclusion of ferroelectric thiourea particles in paraelectric amorphous KBr on the ferroelectric phase transition. The phase transition as a function of temperature has been observed with optical second-harmonic generation (SHG) of infrared light. The Curie temperature T_c shows a minimum as a function of the volume fraction x of thiourea in the composites at x = 0.4. We describe the experimentally observed density dependence of the Curie temperature in a phenomenological mean-field model.