Effect of Hydrostatic Pressure on the Phase Transitions in Thiourea

The low frequency dielectric constant of thiourea was measured along the b -direction in the temperature region from 25°C to -120°C at various hydrostatic pressure up to 7 kbar. The pressure-temperature phase diagram was obtained. The initial pressure coefficients for the V-IV transition temperature (-72.0°C), the IV-III transition temperature (-94.0°C), the III-II transition temperature (-96.8°C) and for the II-I transition temperature (-103.0°C) are -19±2°C kbar -1 , -34±1°C kbar -1 , -31±1°C kbar -1 and -26±1°C kbar, respectively. the V-VI transition pressure, which is 3.3 kbar at room temperature, decreases with lowering temperature. The phase VI is not identical with any of low temperature ferroelectric phases.