Structural phase transitions in guanidinium nitrate

Transformations of NH ... O hydrogen bonds at two structural phase transitions in guanidinium nitrate (GN) crystals have been studied by X-ray diffraction, calorimetric measurements and energy calculations of crystal cohesion forces. The symmetry of the crystal phases and the structure have been determined between 128 and 420 K; preliminary results of neutron-diffraction studies between 4 and 395 K are also reported. The structure undergoes a first-order phase transition at T-12 = 296 K, and a continuous phase transition at T-23 = 384 K. Mechanisms of the two structural phase transitions reveal the interdependence between the transformations of the NH ... O hydrogen bonds and the crystal structures: the low-temperature phase is destabilized by electrostatic interactions between sheets of hydrogen-bonded ions leading to a rearrangement of the hydrogen-bonded aggregates at T-12, while the increased thermal vibrations change the balance between the hydrogen bonds and electrostatic forces within the sheets, leading to dynamic reorientations of the [C(NH2)(3)](+) and NO3- ions and to a continuous phase transition at T-23. Unusually strong crystal strain accompanying the transition at T-12 and resulting in elongation of the samples by over 44% is described in terms of the crystal lattice transformations following a rearrangement of hydrogen-bonded supramolecules, while a negative volume change at T-12 is shown to result from the increased attractive electrostatic forces between aggregates of hydrogen-bonded ions after their rearrangement.