A revised interpretation of the structure of (NH_4)_2Ge_7O_(15) in the light of the Extended Zintl– Klemm Concept

摘要

The structure of (NH_4)_2Ge_7O_(15) recently described as being a microporous material containing rings, in which GeO_6 octahedra coexist with GeO_4 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 NH_4~+ groups act as true cations, transferring their 6 valence electrons to the acceptor Ge_2O_5 moiety, so converting it into the [Ge_6O_(15)]~(6-)≡3(Ψ-As_2O_5) ion (where Ψ refers to a pseudo-lattice) and yielding threefold connectivity. The tetrahedral Ge network shows similarities with the Sb_2O_3 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 GeO_2 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 FeLiPO_4 [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 NH_3 and H_2O to give rise to the metastable structure Ge_7O_(14), which further collapses to the rutile-type GeO_2 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.