Richardson Ian G
School of Civil Engineering, University of Leeds, Leeds LS2 9JT, England.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2013 Apr;69(Pt 2):150-62. doi: 10.1107/S205251921300376X. Epub 2013 Feb 26.
Atomistic modelling techniques and Rietveld refinement of X-ray powder diffraction data are widely used but often result in crystal structures that are not realistic, presumably because the authors neglect to check the crystal-chemical plausibility of their structure. The purpose of this paper is to reinforce the importance and utility of proper crystal-chemical and geometrical reasoning in structural studies. It is achieved by using such reasoning to generate new yet fundamental information about layered double hydroxides (LDH), a large, much-studied family of compounds. LDH phases are derived from layered single hydroxides by the substitution of a fraction (x) of the divalent cations by trivalent. Equations are derived that enable calculation of x from the a parameter of the unit cell and vice versa, which can be expected to be of widespread utility as a sanity test for extant and future structure determinations and computer simulation studies. The phase at x = 0 is shown to be an α form of divalent metal hydroxide rather than the β polymorph. Crystal-chemically sensible model structures are provided for β-Zn(OH)2 and Ni- and Mg-based carbonate LDH phases that have any trivalent cation and any value of x, including x = 0 [i.e. for α-M(OH)2·mH2O phases].
原子模型技术和X射线粉末衍射数据的Rietveld精修被广泛使用，但常常得出不切实际的晶体结构，大概是因为作者忽略了检查其结构的晶体化学合理性。本文的目的是强化适当的晶体化学和几何推理在结构研究中的重要性和实用性。这是通过运用此类推理来生成关于层状双氢氧化物（LDH）的新的基础信息得以实现的，LDH是一个庞大且被广泛研究的化合物家族。LDH相是由层状单氢氧化物通过用三价阳离子取代一部分（x）二价阳离子而衍生而来的。推导了一些方程，可根据晶胞的a参数计算x，反之亦然，预计这作为对现有和未来结构测定及计算机模拟研究的合理性检验将具有广泛用途。结果表明，x = 0时的相是二价金属氢氧化物的α型而非β多晶型。为具有任何三价阳离子和任何x值（包括x = 0，即对于α-M(OH)₂·mH₂O相）的β-Zn(OH)₂以及镍基和镁基碳酸盐LDH相提供了晶体化学上合理的模型结构。
