PHASE-TRANSITIONS OF (THIOUREA)(3)-CCL4 CLATHRATE COMPOUND

An X-ray diffraction study of a clathrate compound, (thiourea)(3)-CCl4 , was performed to elucidate the mechanism of phase transitions which are characterized by the orientational ordering of CCl4 molecules. A d iscontinuous change of lattice constants was observed at (39 +/- 2) K indicating the second-order phase transition and a continuous change a t (64 +/- 2) K, the first-order one. Below the upper transition temper ature, the space group of the lattice changed from R3c to Cc. The lowe r two phases belong to the same space group Cc, though the phase trans ition occurred. The orientation of CCl4 molecules deduced from the str uctural symmetry agrees with that proposed from the previous heat capa city and NQR measurements.