The Zintl–Klemm concept applied to cations in oxides. I. The structures of ternary aluminates

Abstract

The structures of 94 ternary aluminates are reinterpreted on the basis of the Zintl±Klemm concept and Pearson's generalized octet rule. In aluminates of highly electropositive metals such as alkali, alkaline-earth and rare-earth metals, the Al atoms form three-dimensional skeleta which can be interpreted as if the Al atoms were behaving as Zintl polyanions, adopting the structure of either main-group elements or Zintl polyanions showing the same connectivity. The O atoms are then located close to both the hypothetical two-electron bonds and the lone pairs, giving rise to a tetrahedral coordination. When more electronegative elements, such as W or Si, are present in the compound, the electron transfer towards the Al atoms does not take place. In this case, aluminium behaves as a base, transferring its electrons to the more electronegative atoms and the coordination sphere of aluminium becomes octahedral. In some compounds the Al atoms clearly show amphoteric character so that some Al atoms act as donors (bases) and hence are octahedrally coordinated, whereas others behave as acceptors (acids), adopting a tetrahedral coordination. From this it is concluded that the coordination sphere of aluminium is not a function of the ionic radius of the Al 3+ cations, but it depends on the nature of the other cations accompanying them in the structure. The networks formed by these aluminates are, in many instances, similar to those of the binary oxides of the main-group elements. For this reason, a systematic survey of these oxides is also reported. Compounds such as stuffed cristobalites and trydimites and also perovskites are examples of this new interpretation. Perovskites are then reinterpreted as a stuffed pseudo-TeO 3 structure. Other families of compounds such as silicates and phosphates are susceptible to a similar interpretation. This study provides additional examples of how cations recognize themselves in spite of being embedded in an oxygen matrix.

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

“…As explained above, in other related compounds the tetrahedral [TO 4 ] (T = Al, Si, P) groups are found to coexist with [AO 6 ] octahedra (A = Al, Si). However, their coexistence was puzzling in light of the radius ratio rules and it could only be explained when the approach by Santamaría-Pé rez & Vegas (2003) and Santamaría-Pé rez et al…”

“…One of the most important achievements which this new approach of ours brings to the table (Santamaría-Pé rez & Vegas, 2003;Santamaría-Pé rez et al, 2005) is the rationalization of the coordination number CN adopted by silicon in its varying compounds. As explained above, in other related compounds the tetrahedral [TO 4 ] (T = Al, Si, P) groups are found to coexist with [AO 6 ] octahedra (A = Al, Si).…”

“…Such a model arises from our extension of the Zintl-Klemm concept and has more than proved its usefulness already in rationalizing multitudinous structures of aluminates (Santamaría-Pé rez & Vegas, 2003), silicates (Santamaría-Pé rez et al, 2005) and many other structures (Vegas, 2011(Vegas, , 2017. In the present article we apply this concept to the structure of (NH 4 ) 2 Ge [6] [Ge [4] 6 O 15 ] in order to provide a rational explanation of its tetrahedral Ge-skeleton.…”

In-situlaser synthesis of Nd–Al–O coatings: the role of sublattice cations in eutectic formation

“…As will be shown below, the concept developed by Vegas and co-workers (Martinez-Cruz et al, 1994;Ramos-Gallardo & Vegas, 1997;Vegas, 2000;Vegas & Jansen, 2002;Santamaría-Pé rez & Vegas, 2003;Santamaría-Pé rez et al, 2005;Vegas & García-Baonza, 2007;Errandonea et al, 2008; has demonstrated that the preservation of the structure of the elements/alloy in their corresponding binary/ternary/ quaternary oxides is more general than it was believed. Reports published by Blatov and co-workers, concerning the topological comparison of the tetrahedral structures M y (TO 4 ) z with those of the binary compounds A y X z (Ilyushin et al, 2004), the analysis of the borates, carbonates and nitrates (Blatov & Zakutkin, 2002) and the crystallochemical analysis of binary compounds and simple salts M y (LO 3 ) z (L = S, Se, Te, Cl, Br, I; Blatov & Peskov, 2006) all deserve to be considered in this context.…”

“…According to the concept explained by Vegas in previous work (Errandonea et al, 2008;Martinez-Cruz et al, 1994;Ramos-Gallardo & Vegas, 1997;Vegas & Jansen, 2002;Santamaría-Pé rez & Vegas, 2003;Santamaría-Pé rez et al, 2005;Vegas & García-Baonza, 2007;, the preservation of the corresponding alloy structures in oxides affords an explanation for the phase relations observed in these experiments and may contribute to the advancement of the understanding of eutectic systems empirically identified in the corresponding phase diagram (Wu & Pelton, 1992).…”