Electropositive groups, definition

Zintl phases remarks on their definition. We have seen that the Zintl phases may be considered as a group of compounds formed by an electropositive (cationic) component (alkali, alkaline earth metal, lanthanide) and an anionic component (for instance a main group element of moderate electronegativity). The anionic part of the structure may be described in terms of normal valence combination. 

The term Zintl phase is applied to solids formed between either an alkali- or alkaline-earth metal and a main group p-block element from group 14, 15, or 16 in the periodic table. These phases are characterized by a network of homonuclear or heteronuclear polyatomic clusters (the Zintl ions), which carry a net negative charge, and that are neutralized by cations. Broader definitions of the Zintl phase are sometimes used. Group 13 elements have been included with the Zintl anions and an electropositive rare-earth element or transition element with a filled d shell (e.g. Cu) or empty d shell (e.g. Ti) has replaced the alkali- or alkaline-earth element in some reports. Although the bonding between the Zintl ions and the cations in the Zintl phases is markedly polar, by our earlier definition those compounds formed between the alkali- or alkaline-earth metals with the heavier anions (i.e. Sn, Pb, Bi) can be considered intermetallic phases. 

Components of Zintl phases are metals A (Li-Cs, Mg-Ba) and semimetals X (B-Tl, Si-Pb, P-Bi, Te). The definition of the components of Zintl phases is not quite sharp, neither for the metals nor for the semimetals, until today. While the alkali- and alkaline earth metals are undoubtedly metals in the classical sense, there is still the vast field of transition metals T, which in Zintl type compoimds, might also occur. Electropositive T atoms may replace the main group metal A and electronegative ones tike a semimetal X. One of the wellknown examples here is CsAu (a diamagnetic semiconductor). This phase shows the typical properties of a valence compound according to the electron transfer description Cs Au [4]. 

An accepted theory of ionic membranes is called the fixed-charge theory (Sollner, 1968). The pore walls of the membrane contain inherently a definite number of potentially dissociable groups. Electronegative membranes contain anionic (acid) groups and electropositive membranes... 

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