Vibrational spectroscopy, X-ray diffraction, and electron microscopy have been applied to investigate the crystallization of concentrated amorphous nitric acid (0.5 ≤ x ≤ 0.95). Crystalline nitric acid monohydrate (HNO3 · H2O) is the major hydrate phase detected together with traces of metastable nitric acid dihydrate (HNO3 · 2H2O) and pure crystalline nitric acid. Nitric acid tetratohydrate (4HNO3 · H2O) was not achievable. The diffraction data show nitric acid monohydrate as a highly symmetrical structure, which bestows clearly laid out infrared, Raman and inelastic neutron scattering spectra. The low frequency region is still a matter of discussion since experiment and theory are not in accordance. The microscopic picture underlines the model character of nitric acid monohydrate, since the crystalline particles adopt a spherical shape, on various agglomeration stages, which minimizes the interference of scattering on the spectroscopic data.

This research project has been supported (in part) by the European Commission under the 7th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract n : CP-CSA_INFRA-2008-1.1.1 Number 226507-NMI3. The STFC Rutherford Appleton Laboratory is thanked for access to neutron beam facilities. SFP is grateful for a lecture invitation by the Austrian Chemical Society (GÖCh). RE and BML acknowledge funding from the Spanish Ministry of Education, Project FIS2007-61686. RE also acknowledges funding from Project FIS2010-16455 and Sabbatical Project PR2010-0012.

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