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, alpha-K2[B3O3F4(OH)] (I), beta-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.