Polytypism in the Lithium鈥揂luminum Layered Double Hydroxides: The [LiAl2(OH)6]+ Layer as a Structural Synthon

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• 作者：Sylvia Britto ; P. Vishnu Kamath
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：June 20, 2011
• 年：2011
• 卷：50
• 期：12
• 页码：5619-5627
• 全文大小：978K
• 年卷期：v.50,no.12(June 20, 2011)
• ISSN：1520-510X

文摘

The [LiAl₂(OH)₆]p>+p> layer obtained from gibbsite鈥揂l(OH)₃ belongs to the layer group symmetry P-312/m. This layer satisfies the defining characteristics of a synthon in that it predicts all the polymorphic modifications of the layered double hydroxides of Li and Al. The various possible ways of stacking these layers can be derived by the systematic elimination of the principal symmetry elements comprising the layer group. This approach yields the complete universe of possible structures. When the 3 axis of the layer is conserved in the stacking, the resultant crystal adopts the structure of the 1H, 2H, or 3R polytypes (H, hexagonal; R, rhombohedral). When the 3 axis is destroyed and the 2/m axis is retained, the crystal adopts monoclinic symmetry and crystallizes in the structures of the 1M₁ or 1M₂ (M, monoclinic) polytypes; the two polytypes differ only in their translational component. Experimentally, gibbsite-based precursors yield the 2H polytype, and bayerite-based precursors yield the 1M polytype. Faulted structures incorporating differently oriented 1M₁ motifs or a mixture of 1M₁ and 1M₂ motifs are also obtained. These stacking faults result in cation disorder along the c axis and produce signature effects on the line shapes of select reflections in the powder X-ray diffraction patterns. This symmetry-guided approach is general and can be extended to other classes of layered solids.