INVESTIGATION OF PHASE-TRANSITIONS IN GUANIDINIUM NITRATE CRYSTALS

Calorimetry, high-pressure dilatometry, x-ray powder diffraction and e lectric conduction studies were applied to investigate the phase trans itions and physical properties of guanidinium nitrate, [C(NH2)3]+NO3-. Two thermal anomalies related to the first-order and second-order pha se transitions were observed at T12 = 296 K and T23 = 384 K, respectiv ely. The transformation at T23 indicates the existence of a new high-t emperature phase of guanidinium nitrate, which has been confirmed by t he x-ray powder diffraction study. The transition entropies were found to be DELTAS12 = 5.7 J mol-1 K and DELTAS23 = 1.5 J mol-1 K. The magn itude of the change in the entropy at the 296 K phase transition indic ates its order-disorder character. The change in the specific volume a t T12 is negative, i.e. DELTAV12 = -1.6 cm3 mol-1, and T12 linearly de creases with increasing pressure, with dT12/dp = -24.3 K kbar-1. The d euterization has only a slight effect on the temperatures of the obser ved phase transitions. Above 320 K an increase in electric conductivit y has been noted in the polycrystalline samples of the compound studie d. The results of the studies performed suggest that the mechanism of the phase transition at 296 K, accompanied by unusual deformation of t he crystals, may be of an electrostatic nature.