Characterization of the Thermally Induced Topochemical Solid-State Transformation of NH4[N(CN)2] into NCNC(NH2)2 by Means of X-ray and Neutron Diffraction as Well as Raman and Solid-State NMR Spectroscopy

The mechanism of the solid−solid transformation of NH4[N(CN)2] into NCNC(NH2)2, which represents the isolobal analogue of Wohler's historic conversion of ammonium cyanate into urea, has been investigated by temperature-dependent single-crystal and powder X-ray diffraction, neutron powder diffraction, and Raman and solid-state NMR spectroscopy as well as thermoanalytical measurements. The transformation of the ionic dicyanamide into its molecular isomer upon controlled thermal treatment was found to proceed topochemically in the solid state with little molecular motion, giving rise to a single-crystal to single-crystal transformation which manifests itself by a defined metric relation between the unit cells of the two isomers. The exothermic phase transition is thermally activated and was observed to commence at temperatures ≥80 °C. The pronounced temperature dependence of the onset of the transformation may be assessed as an indication for the metastability of ammonium dicyanamide at elevated temperatures...