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X-ray powder diffraction experiments are performed to prove the possible crystallization of nitric acid dihydrate (HNO(3).2H(2)O, further denoted NAD) and to determine the best thermal conditions for growing a single crystal. It is shown that the kinetic behaviour of NAD strongly depends on the preliminary thermal treatment. One good single crystal obtained by an in situ adapted Bridgman method procedure enabled determination of the crystal structure. The intensities of diffracted lines with h odd are all very weak. The H atom of nitric acid is delocalized to one water molecule leading to an association of equimolar nitrate (NO(3)(-)) and an H(5)O(2)(+) ionic group. The asymmetric unit contains two such molecules. These two molecules are related by a pseudo a/2 translation (with a 0.3 A mean atomic distance difference), except for one H atom of the water molecules (0.86 A) because of their different orientations in the two molecules. The two molecules, linked by very strong hydrogen bonds, are arranged in layers. Two layers which are linked by weaker hydrogen bonds are approximately oriented along the c axis. The structure may be described by translations of this set of two layers along the c axis without hydrogen bonds leading to a two-dimensional hydrogen-bond network. The structures of the monohydrate (NAM) and trihydrate (NAT) are re-determined for comparisons. These structures may be described by one- and three-dimensional hydrogen-bond networks, respectively.

The first-order transformation of the supercooled liquid into the glacial state of triphenyl phosphite was investigated by differential scanning calorimetry (DSC) using two different thermal procedures. In the first procedure the transformation was analyzed by heating run DSC experiments. The glaciation process was interpreted as an aborted crystallization because of a high nucleation rate in a temperature range where the crystal growth is low. This relative separation between the nucleation- and growth-rate curves suggests that the glacial state can be described as a supercooled liquid–nanocrystalline mixed phase, characterized by a high-density nucleation which frustrates further crystallization. In a second procedure, DSC experiments were carried out during isothermal transformations of the supercooled liquid into the glacial state. The sigmoidal shapes of DSC isotherms are interpreted as transient kinetics of nucleation rather than nucleation and growth processes.

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