Kinetic behaviour investigations and crystal structure of nitric acid dihydrate

X-ray powder diffraction experiments are performed to prove the possible cr ystallization of nitric acid dihydrate (HNO3. 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 preli minary thermal treatment. One good single crystal obtained by an in situ ad apted Bridgman method procedure enabled determination of the crystal struct ure. 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 as sociation of equimolar nitrate (NO3-) and an H5O2+ ionic group. The asymmet ric unit contains two such molecules. These two molecules are related by a pseudo a/2 translation (with a 0.3 Angstrom mean atomic distance difference ), except for one H atom of the water molecules (0.86 Angstrom) because of their different orientations in the two molecules. The two molecules, linke d by very strong hydrogen bonds, are arranged in layers. Two layers which a re linked by weaker hydrogen bonds are approximately oriented along the c a xis. The structure may be described by translations of this set of two laye rs along the c axis without hydrogen bonds leading to a two-dimensional hyd rogen-bond network. The structures of the monohydrate (NAM) and trihydrate (NAT) are redetermined for comparisons. These structures may be described b y one- and three-dimensional hydrogen-bond networks, respectively.