碘酸盐的晶体结构与热力学性质——(Ⅰ)碘酸盐的晶体结构特点

本文根据作者等人的实验结果并结合文献报导的数据,综述了碘酸盐及其复盐的晶体结构特点和分类。对α-LiIO_3,Li_2H(IO_3)3,Mg(IO_3)_2,Mg(IO_3)_2-LiIO_3固溶体,Zn(IO_3)_2等晶体结构变迁的情况进行了分析。阐述了由于空位数量和分布的不同,以及阳离子性质(电价、离子极化能力和离子半径等)的不同,晶格场发生变化,引起了IO_3~-在不同方向和不同程度的移动或转动,因而有可能在不同方向上形成不同程度的各种畸变型结构和超结构。

The Crystal Structures and Thermodynamic Properties of Iodate Salts(Ⅰ) The Crystal Structures of Iodates

Based upon the experimental results obtained by the authors and the data reported in the literatures, the characteristics and classification of the crystal structures of iodates and their complex salts have been summarized (especially for α-LiIO3, Li2H(IO3)3, Mg(IO3)2, Mg(IO3)2-LiIO3 solid solutions, Zn(IO3)2 etc.).The crystal structures of average iodates exhibit lower symmetries and packing coefficients. However, according to the peculiarities of the coordination polyhedra made up of iodine and oxygen ions, iodates may be grossly divided into two groups. In the first group, the octahedron IO6 constitutes the skeleton of the whole crystal structure, with I+5 ion deviated from the center of the octahedron. This group may be further subdivided into 3 classes: (a) The crystal structure is of a distorted perovskite with no center of symmetry at all, in which the oxygen ions are situated at the distorted cubic close-packed sites. Iodates of K+, NH4+,Rb + , Cs+,……are examples of this kind, (b) The crystal structure belongs mainly to hexagonal system with no center of symmetry, in which 'he oxygen ions, are situated at the distorted HCP sites. α-LiIO3 is the best example, (c) The crystal structure has a center of symmetry, in which the octahedra IO6 are arranged in opposite orientations with each other. Li2K(IO3)3 etc. belong to this kind. In the second group, the number of oxygen ions around an iodine ion in a coordination polyhedron is larger than 6 , although the basic unit of the structure may still be IO3-. The IO3- may arrange itself in opposite orientations with each other, and there is a center of symmetry in this kind of crystal. β-LiIO3 is an example. .The mechanism of variation of structures in crystals of α-LiIO3, Li2H-(IO3)3, Mg(IO3)2, Zn(IO3)2 and solid solutions in binary system Mg(IO3)2-LiIO3 was also discussed. In these crystals, owing to the differences in distribution of vacancies and in properties of the cations (valency, polarizabi-lity and ionic radius etc), the crystal lattice field would be. varied accordingly and leads to a translation (and/or a rotation) of IO3 along various orientations and in various degrees. Consequently, various kinds of distorted structures (or superlattices) result.