Prepd from the elements: Sacc et al., cited in Mellor's vol. X, 894 (1930); by the action of PCl5 on selenium, on the selenide of phosphorus or antimony, on SeO2 or on SeOCl2: Baudrimont, Ann. Chim. Phys. [4] 2, 5 (1864); Michaelis, Z. Chem. [2] 6, 460 (1870); by the action of HCl on a soln of selenium in fuming sulfuric acid: Divers, Shimose, J. Chem. Soc. 45, 194, 198, 201 (1884); from Se, SeO2 and HCl: Lenher, Kao, J. Am. Chem. Soc. 47, 772 (1925); 48, 1550 (1926).