Usually prepd by heating dicyandiamide, H2NC(=NH)NHC≡N, under pressure: Mackay, US 2737513 (1956 to Am. Cyanamid). Alternate methods starting with urea: Mackey, US 2760961 (1956 to Am. Cyanamid); Pomot et al., US 3111519 (1963 to Office Natl. Ind. de l'Azote). X-ray and neutron crystal structure: J. N. Varghese et al., Acta Crystallogr. 33B, 2102 (1977). Review of mfg processes: Faith, Keyes & Clark's Industrial Chemicals, F. A. Lowenheim, M. K. Moran, Eds. (Wiley-Interscience, New York, 4th ed., 1975) pp 519-523.