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

Abstract

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.

Rationalization of the crystal structure of eudidymite Na2Be2[Si[4]6O15]·H2O in light of the extended Zintl–Klemm concept

“…This approach offers an alternative to the classical description used to interpret inorganic crystal structures. In addition, this supports the thermodynamic concept of lattice ionic strength, I, devised by Vegas & Jenkins (2017) which can be used to rationalize the energetics taking place during the structural changes suggested in these newest interpretations.…”

“…Thus, the octahedrally coordinated Ge [6] and Si [6] atoms would act as donors, transferring their four valence electrons to the acceptor T atoms (T = Ge [4] , Si [4] ), so forming the tetrahedrally coordinated skeleton. Like in the Zintl polyanions, the skeleton's connectivity obeys the (8 À N) rule (Santamaría-Pé rez & Vegas, 2003;Santamaría-Pé rez et al, 2005;Vegas & García-Baonza, 2007;Vegas, 2012;Vegas & Jenkins, 2017), where N is the total number of valence electrons associated with the T atoms (i.e. their own four valence electrons plus the one transferred from the donors).…”

“…It should be also noted that the [Si [4] 6 O 15 ]-skeleton in eudidymite [ Fig. 3(b)] can be derived from the condensation of two parallel ladder-like anions [Al [4] 2 Si [4] 2 O 10 ] 6À , forming the structure of sillimanite Al [6] 2 [Al [4] 2 Si [4] 2 O 10 ], as was illustrated for the [Ge [4] 6 O 15 ] 6À anion (Vegas & Jenkins, 2017).…”

“…Altogether, the Ge atoms form a 3D anionic skeleton [Ge 7 O 15 ] 2À with tunnels, parallel to the c axis, where the NH 4 + cations are lodged. The structure was further described by Vegas & Jenkins (2017) on the basis of the extended Zintl-Klemm concept (hereafter EZKC) (Vegas & García-Baonza, 2007), a concept that has been successfully applied to rationalize the structures of hundreds of aluminates and silicates (Santamaría-Pé rez & Vegas, 2003;Santamaría-Pé rez et al, 2005).…”

Absolute ion hydration enthalpies and the role of volume within hydration thermodynamics

“…We have found it possible to make thermodynamic predictions regarding the formation of such new materials. VBT 37,38 has been used to predict the likely stability of certain alkali sulfoxalates and selenoxylates in the solid state. Since this publication 37 appeared we were contacted by Makarov to report that he had already detected the SO 2 2À moiety not in the solid state however but in aqueous solution.…”