黄河矿晶体结构的重新测定及空间群对称的讨论

黄河矿 BaCe(CO₃)₂F(a=0.5072(1)nm,c=3.846(1)nm;V=856.8×10^-3nm³;Z=6)晶体结构的重新测定,采用中心对称空间群 R3m-D_3d⁵,基于337个可观察的单晶 X 射线数据(I>2σ₁)和30个可变参数,R=0.031。原子坐标和均方根振幅(据各向异性位移参数计算,O.013-0.025nm)表明,没有必要降低对称性,即没有必要采用相应的非中心对称空间群或其它次群。黄河矿的结构中,原子形成似层状排列,且垂直于 c 轴;碳酸根通过 Ba 与 Ce 的联结形成三维结构。C(1)O₃,和 C(2)O₃,中的 C 原子偏离三个氧组成的平面,其偏离的距离分别为0.0066(15)nm 和 0.0029(15)nm。

HUANGHOITE-(Ce), BaCe(CO_3)_2F, FROM KHIBINA,KOLA PENINSULA, RUSSIA: REDETERMINATION OF THE CRYSTAL STRUCTURE WITH A DISCUSSION ON SPACE GROUP SY MMETRY

A redetermination of the crystal structure of huanghoite-(Ce), BaCe(CO₃)₂F,(α-0.5072(Ⅰ)nm, c-3.846(1)nm, v =856.8×10^-3nm³ and z =6)in the centro-symmetric space group R3m-D3d5 yielded R = 0.031 for 337 observed single crystal x-ray data and 30 variable parameters(I > 2σ₁). The atomic coordinates and the root-mean-square amplitudes(from 0.013 to 0.025nm, recalculated from the anisotropic displacement parametefs)gave no evidence for a lower symmetry(neither the corresponding noncentrosymmetric space group nor any other subgroup). The structure of huanghoite-(Ce)has a layer-like atomic arrangement normal to C-axis. The carbonate groups are linked by the cations Ba and Ce to a framework with a moderate cleavage parallel to(001). The values of distance of the C atom from the oxygen plane are 0.0066(15)nm for C(1)O₃ and 0.0029(15)nm for C(2)O₃, respectively.