Pseudoatoms and preferred skeletons in crystals

Abstract

The generalization of the Zintl–Klemm concept provides a universal formulation of a crystal structure in terms of universal building skeletons formed by Klemm's pseudoatoms: atoms that behave structurally according to their formal total electron charge. An important difference in this novel view is that charge is considered to be transferred, in the strict Zintl's sense, from the donor cations to the building skeleton as a whole, not specifically to a given atom or ion. Although application is restricted to (IV)–(IV) compounds (group 14 structures), the principle seems to be universal and can be applied to understand, to relate and to predict the structure of complex compounds on the basis of more simple structures, e.g. a given AB skeleton provides the building block for A 2 B, AB 2, ABX m etc. compounds of a very different nature. The application of such a principle only requires information on the constituent atoms and on the existing phases of the p-block elements (observed under ambient and high-pressure and/or high-temperature conditions). The ideas introduced here demonstrate, for the first time, that a generalization of the Zintl–Klemm concept is possible and that such a generalization helps to establish a univocal link between chemical composition (in terms of pseudoatoms) and the crystalline structures observed experimentally.

Compounds with a `stuffed' anti-bixbyite-type structure, analysed in terms of the Zintl–Klemm and coordination-defect concepts

“…3 (Niewa et al, 2003). This feature is also in agreement with the 'general principle' deduced in the recent work of Vegas & García-Baonza (2007), by which atoms try to form pseudo-structures characteristic of their group 14 isoelectronic counterparts.…”

“…The first impression might be that our assumption was not justified, and indeed this raises the possibility that any of the other atomic species could be lodged in these voids. However, this apparent contradiction can be resolved in the context of the extended Zintl-Klemm concept (Vegas & García-Baonza, 2007), as we shall demonstrate below. It is important to note that the application of the Zintl-Klemm concept to a compound invariably allows for several alternative structural interpretations.…”

“…This decision was based on two observed features. On the one hand, there was our previous experience of how the elemental structures were often preserved in the compounds (Vegas, 2000;Vegas & Jansen, 2002), and on the other because the presence of 'foreign' atoms can stabilize a given structure, as happens in the Zintl phases (Santamaría-Pé rez Santamaría-Pé rez et al, 2005;Vegas & García-Baonza, 2007). This feature occurs not only in alloys but also in oxides and can be considered as an extension of the Zintl-Klemm concept.…”

“…It could be said that such compounds highlight the unequivocal relationship existing between composition and structure which was postulated by Vegas & García-Baonza (2007 (a) The structure of BPO 4 (I " 4 4) at 6.1 GPa, projected on the ab plane. It is cristobalite-like and, under pressure, undergoes a continuous tilting of the B(P)O 4 tetrahedra up to collapse, at 50 GPa, into an almost regular f.c.c.…”

“…This concept and, especially, Klemm's pseudo-atom concept, correlate the stereochemical properties of an atom with the transfer of electrons from a donor to an acceptor atom. Both atoms are thereby transposed to pseudo-atoms, each with a new electronic configuration and stereochemical properties (Santamaría-Pé rez Santamaría-Pé rez et al, 2005;Vegas & García-Baonza, 2007). It is generally accepted that the pseudo-atoms are denoted by the prefix É (See Table 2).…”