THE PHASE-TRANSITION SEQUENCE IN THIOUREA (SC(NH2)(2)) - A HEAT-CAPACITY STUDY

A careful adiabatic calorimetric study of the phase transitions sequen ce in thiourea has been made. Phase transitions at 169 K, 171 K and 20 0 K have been studied. The associated enthalpies are Delta H-169 (K)/R = 3.167 K, Delta H-171 (K)/R = 0.103 K, Delta H-200 (K)/R = 13.547 K. UP to the experimental resolution of the system, there is no evidence for the existence of calorimetric anomalies at 161 K or 177 K, as sug gested by dielectric techniques carried out at non-zero electric field s. Making use of the published temperature dependent Raman spectroscop ic data, the different contributions to the heat capacity have been de termined. This method permits one to assign very accurate values for t he thermodynamic functions of the various phase transitions undergone by the crystal.