STRUCTURAL PHASE-TRANSITIONS IN GUANIDINIUM NITRATE

Transformations of NH ... O hydrogen bonds at two structural phase tra nsitions in guanidinium nitrate (GN) crystals have been studied by X-r ay diffraction, calorimetric measurements and energy calculations of c rystal cohesion forces. The symmetry of the crystal phases and the str ucture 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 th e two structural phase transitions reveal the interdependence between the transformations of the NH ... O hydrogen bonds and the crystal str uctures: the low-temperature phase is destabilized by electrostatic in teractions between sheets of hydrogen-bonded ions leading to a rearran gement of the hydrogen-bonded aggregates at T-12, while the increased thermal vibrations change the balance between the hydrogen bonds and e lectrostatic forces within the sheets, leading to dynamic reorientatio ns of the [C(NH2)(3)](+) and NO3- ions and to a continuous phase trans ition at T-23. Unusually strong crystal strain accompanying the transi tion 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 vol ume change at T-12 is shown to result from the increased attractive el ectrostatic forces between aggregates of hydrogen-bonded ions after th eir rearrangement.