Thermal Expansion of Thiourea in the Range of Phase Transition

By means of a quartz dilatometer three principal expansion coefficients have been determined in the temperature range 100–273°K, and their anisotropy within (100) plane. The appearance of singular points, corresponding to phase transformations, depends on direction of measurements but most drastic changes are observed along the crystallographic c-axis, in agreement with the direction of the weakest intermolecular interactions. The calculated pressure necessary to shift the phase equilibrium I/II to room temperature is in good agreement with the value observed in a direct experiment.¹¹