A revised interpretation of the structure of (NH₄)₂Ge₇O₁₅ in the light of the Extended Zintl-Klemm Concept

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• 作者：Angel Vegas and Harry Donald Brooke Jenkins
• 关键词：germanates ; crystal chemistry ; Zintl polyanions ; ionic strength
• 刊名：Acta Crystallographica Section B
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：73
• 期：1
• 页码：94-100
• 全文大小：1127K
• ISSN：2052-5206

文摘

The structure of (NH₄)₂Ge₇O₁₅ recently described as being a microporous material containing rings, in which GeO₆ octahedra coexist with GeO₄ tetrahedra, is re-examined in the light of the Extended Zintl–Klemm Concept as applied to cations in oxides. The Ge^[6] atoms together with the NH₄⁺ groups act as true cations, transferring their 6 valence electrons to the acceptor Ge₂O₅ moiety, so converting it into the [Ge₆O₁₅]⁶⁻[triple-bond]3(Ψ-As₂O₅) ion (where Ψ refers to a pseudo-lattice) and yielding threefold connectivity. The tetrahedral Ge network shows similarities with the Sb₂O₃ analogue. At the same time, the Ge^[6] atoms are connected to other Ge^[4] atoms forming blocks that are part of a rutile-type GeO₂ structure. Such an analysis shows that both substructures (the Zintl polyanion and the rutile fragments) must be satisfied simultaneously as has already been illustrated in previous articles which considered stuffed-bixbyites [Vegas et al. (2009). Acta Cryst. B65, 11–21] as well as the compound FeLiPO₄ [Vegas (2011). Struct. Bond.138, 67–91]. This new insight conforms well to previous (differential thermal analysis) DTA–TGA (thermogravimetric analysis) experiments [Cascales et al. (1998). Angew. Chem. Int. Ed. 37, 129–131], which show endothermic loss of NH₃ and H₂O to give rise to the metastable structure Ge₇O₁₄, which further collapses to the rutile-type GeO₂ structure. We analyze the stability change in terms of ionic strength, I, and so provide a means of rationalizing the driving force behind this concept capable of explaining the atomic arrangements found in these types of crystal structures. Although the concept was formulated in 2003, later than the publication of the germanate structure, it was not used or else ignored by colleagues who solved this crystal structure.