偏硼酸钡低温相的晶体结构

本文报道了新型信频材料——偏硼酸钡Ba₃(B₃O₆)₂]低温相的晶体结构。晶体属三方晶系,空间群为C₃⁴-R3,取六方坐标系,晶胞参数为a=b=12.532?,c=12.717?,z=6。在PhilipsPW-1100四圆衍射仪上收集到独立衍射693个。采用重原子法通过三维帕特逊和三维傅里叶合成测定结构,并用对角矩阵最小二乘法修正至R=0.046。该晶体是一个由Ba²⁺和(B₃O₆)^3-环交错组成的层状阶梯式结构。其阴离子(B₃O₆)^3-环基本上是平面状的。基团平面垂直于三次轴;Ba²⁺近邻有7个氧原子。将此结构与偏硼酸钡高温相比较表明,由于钡阳离子的非中心对称分布改变了阴离子硼氧环共轭基团的电子云密度,这一结构上的变化对该晶体的倍频效应做了主要贡献。 关键词：

CRYSTAL STRUCTURE OF THE LOW TEMPERATURE FORM OF BARIUM BARATE Ba3(B3O6)2

In this paper, we report the crystal structure of the low temperature form of barium borate-a new type of SHG material. Ba₃(B₃O₆)₂ crystal belongs to trigonal system with space group C₃⁴-R3. There are 6 formula units in one hexagonal cell. The unit cell dimensions referring to hexagonal axes are a = b = 12.532?, c = 12.717?. Diffraction data were collected on Philips PW-1100 four-circle diffractometer. The number of independent reflections amounts to 693. The structure was determined by conventional three-dimensional Patterson and Fourier syntheses and refined by "block diagonal" matrix least-squres procedures to R = 0.046. This structure can be considered as a layer step type of lattice built up by Ba²⁺ and (B₃O₆)^3- rings atterna-tively. The anion (B₃O₆)^3- group is nearly planar with its plane perpendicular to the threefold axis which passes through the center of the ring. Each barium atom has seven oxygen neighbors. In comparison with the high temperature form of barium borate, it is shown that the acentric symmetry of the barium cations in this structure changes the electron density distribution of the inorganic conjugated boron-oxygen rings, thereby making principal contributions to the SHG effect.
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