Transformation of Ammonium Dicyanamide into Dicyandiamide in the Solid

Ammonium dicyanamide NH4[N(CN)2] was synthesized through
aqueous ion exchange. The crystal structure was investigated by
single-crystal X-ray diffraction (P21/c, a = 378.67(6) pm, b =
1240.9(3) pm, c = 911.84(14) pm, β = 91.488(18)°, Z = 4). It
derives from the CsCl structure type. Medium strong hydrogen
bonds between NH4+ and [N(CN)2]- ions are indicative of the
observed formation of dicyandiamide H4C2N4 during heating.
According to DSC and temperature-dependent X-ray powder
diffractometry, this isomerization is exothermic and occurs between
102 and 106 °C in the solid. The reaction represents the isolobal
analogue to the classical synthesis of urea by heating NH4OCN.
While other alkali and alkaline earth dicyanamides undergo
trimerization or polymerization of their anions during heating,
ammonium dicyanamide thus shows a different reactivity.