Prepd by the action of an excess of chlorine on selenium: Berzelius, cited in Mellor's vol X, 898 (1930); by the action of thionyl chloride or phosphorus trichloride on selenium oxychloride and by the action of phosphorus pentachloride on selenium oxide: Michaelis, Z. Chem. [2] 6, 460 (1870); by the action of anhydr selenic acid on acetyl chloride at 0°: Lamb, Am. Chem. J. 30, 209 (1903). Crystal structure: Shoemaker, Abrahams, Acta Crystallogr. 18, 296 (1965).