title: Cation coordination control of anionic group alignment to maximize SHG effects in the BaMBO(3)F (M = Zn, Mg) series.
abstract: Two new noncentrosymmtric fluoroborates, BaZnBO(3)F and BaMgBO(3)F, have been synthesized and characterized. It is found that BaZnBO(3)F possesses a P6 space group with cell parameters of a = 5.0657(1) A, c = 4.2800(1) A, and Z = 1. The structure is built up by five coordinated trigonal bipyramidal ZnO(3)F(2) polyhedra and triangular BO(3) groups. The ZnO(3)F(2) bipyramid shares its three equatorial oxygen atoms with three separate BO(3) groups to form a ZnO(3)-BO(3) layer, and the layers are linked by the apical fluorine atoms in the third dimension. Owing to its special coordination, ZnO(3)F(2) forces its three neighboring BO(3) groups to arrange into a perfect parallel alignment in the plane to give maximum contribution to the nonlinear optical (NLO) effect. Both calculation and powder second harmonic generation tests show that its effective NLO coefficient is on about the same order as that of LiB(3)O(5). In contrast, in BaMgBO(3)F, which crystallizes in space group Cc with a = 17.614(3) A, b = 30.546(6) A, c = 8.060(2) A, and beta = 90.008(2) degrees , Mg coordinates to four oxygen and two fluorine atoms to form a severely distorted MgO(4)F(2) octahedron. It also uses four equatorial oxygen atoms, sharing with three BO(3) groups to form a MgO(4)-BO(3) layer, and again the layers are linked by the apical F atoms. Every Mg atom links with one edge-sharing and two corner-sharing BO(3) groups, leaving the BO(3) group misaligned and resulting in most of their contributions to NLO effects being canceled.
authors.affiliation: Beijing Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. rkli@mail.ipc.ac.cn
authors.forename: R K
authors.initials: RK
authors.lastname: Li
authors.forename: Peng
authors.initials: P
authors.lastname: Chen