Investigation of phase transitions in guanidinium nitrate crystals

Calorimetry, high-pressure dilatometry, X-ray powder diffraction and electric conduction studies were applied to investigate the phase transitions and physical properties of guanidinium nitrate, (C(NH2)3)+NO3-. Two thermal anomalies related to the first-order and second-order phase transitions were observed at T12 = 296 K and T23 = 384 K, respectively. The transformation at T23 indicates the existence of a new high-temperature phase of guanidinium nitrate, which has been confirmed by the X-ray powder diffraction study. The transition entropies were found to be Delta S12 = 5.7 J mol-1 K and Delta S23 = 1.5 J mol-1 K. The magnitude of the change in the entropy at the 296 K phase transition indicates its order-disorder character. The change in the specific volume at T12 is negative, i.e. Delta V12 = -1.6 cm3 mol-1, and T12 linearly decreases with increasing pressure, with dT12/dp = -24.3 K kbar-1. The deuterization has only a slight effect on the temperatures of the observed phase transitions. Above 320 K an increase in electric conductivity has been noted in the polycrystalline samples of the compound studied. The results of the studies performed suggest that the mechanism of the phase transition at 296 K, accompanied by unusual deformation of the crystals, may be of an electrostatic nature.