Structural Evolution and Optical Properties of Hydroxyfluorooxoborates MM′2n‑1[B3O3F4(OH)]n (M = K, Cs, and Rb; M′ = K, NH4, and Cs; n = 1, 2, and 3)

Four hydroxyfluorooxoborates, α-K2[B3O3F4(OH)] (I), β-K2[B3O3F4(OH)] (II), Cs­(NH4)3[B3O3F4(OH)]2 (III), and RbCs5[B3O3F4(OH)]3 (IV), with identical fundamental building block (FBB) [B3O3F4(OH)]2–, were obtained successfully and characterized comprehensively. A phase transition between I and II was detected, which led to a space group evolution from centrosymmetric to noncentrosymmetric. The F– and OH– substitution effect was studied based on II and its hydroxyborate derivative K2[B3O3(OH)5], which reveals the corresponding changes in the band gap and the nonlinear optical properties. In addition, the cationic-size effect on the arrangement of the [B3O3F4(OH)]2– FBBs and the birefringent properties were discussed for the title compounds. This study contributes to the advancement of structural chemistry and sheds some light on the structure–property relationship studies of hydroxyfluorooxoborates.