Cation Coordination Control of Anionic Group Alignment to Maximize SHG Effects in the BaMBO3F (M = Zn, Mg) Series

Two new noncentrosymmtric fluoroborates, BaZnBO3F and BaMgBO3F, have been synthesized and characterized. It is found that BaZnBO3F possesses a P space group with cell parameters of a= 5,0657(1) angstrom, c=4.2800(1) angstrom, and Z= 1. The structure is built up by five coordinated trigonal bipyramidal ZnO3F2 polyhedra and triangular BO3 groups. The ZnO3F2 bipyramid shares its three equatorial oxygen atoms with three separate BO3 groups to form a ZnO3-BO3 layer, and the layers are linked by the apical fluorine atoms in the third dimension. Owing to its special coordination, ZnO3F2 forces its three neighboring BO3 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 LiB3O5. In contrast, in BaMgBO3F, which crystallizes in space group Cc with a = 17.614(3) angstrom, b = 30.546(6) angstrom, c = 8.060(2) angstrom, and beta = 90.008(2)degrees, Mg coordinates to four oxygen and two fluorine atoms to form a severely distorted MgO4F2 octahedron. It also uses four equatorial oxygen atoms, sharing with three BO3 groups to form a MgO4-BO3 layer, and again the layers are linked by the apical F atoms. Every Mg atom links with one edge-sharing and two corner-sharing BO3 groups, leaving the BO3 group misaligned and resulting in most of their contributions to NLO effects being canceled.