Noncentrosymmetric Rb3(COOH)3(H3BO3)2 vs Centrosymmetric Cs3(COOH)3(H3BO3)2

Two new alkali metal formic-borates named Rb3(COOH)3(H3BO3)2 and Cs3(COOH)3(H3BO3)2 have been successfully synthesized under mild conditions. Influenced by the different cation sizes, these two stoichiometrically equivalent compounds reveal discrepant symmetries: Rb3(COOH)3(H3BO3)2 is noncentrosymmetric (NCS) while Cs3(COOH)3(H3BO3)2 is centrosymmetric (CS). Especially, these two title compounds exhibit unique planar [(COOH)3(H3BO3)2]3– layers constructed from two kinds of planar π-conjugated asymmetric BO3 and COOH units, which induce the large birefringence, 0.09@1064 nm for Rb3(COOH)3(H3BO3)2 and 0.10@1064 nm for Cs3(COOH)3(H3BO3)2, respectively. Optical property measurements indicate that these two compounds display short UV absorption edges, suggesting that they are promising short-wave UV birefringent materials. A detailed theoretical analysis confirmed that the excellent linear optical property was derived from the synergistic effect of the two kinds of π-conjugated planar groups.