K2(BeS)O4F2: a novel fluorosulfate with unprecedented 1D [(BeO3F)–(SO3F)]∞chains exhibiting large birefringence

By introducing the strongly electronegative F − ions into [SO 4 ] tetrahedral units, polar NLO-active units [(BeS)O 4 F 2 ] were obtained, thus forming 1D zigzag [(BeO 3 F)–(SO 3 F)] ∞ chains.