Zintl border

As underlined by Miller el al. (2002) in a comprehensive review about structure and bonding around the Zintl border of the Periodic Table, the Nesper criteria imply that Zintl phases (as valence compounds) are generally point compounds and may have a semi-metallic behaviour. 

The diversity in structure and bonding possible for phosphides is effectively demonstrated by the monophosphides. Monophosphides MP of the group 1 and 2 elements (El, E2) are polyphosphides with i(P ) chains and P2" dumbbells, respectively. Ell and E12 monophosphides are not known. The E3 and E13 monophosphides are the so-called normal compounds with 3x = (M) (see Section 2). With El3, they form the zinc blende structure with tetrahedral heteroatomic bonds. Ternary derivatives such as MgGeP2 and CuSi2P3 have a random distribution of the M atoms, whereas CdGeP2, crystallizes in the ordered chalcopyrite type with a TO[GeP4/2] tetrahedral net (see Section 6.4). The E3 monophosphides form the NaCl structure. CeP is remarkable because of its physical properties (metal-semiconductor transition heavy-fermion behavior). The E14 monophosphides show the break usually observed when passing the Zintl border. Binary lead phosphides are not known SiP and GeP... 

General characteristics of alloys such as those presented in Fig. 3.3 have been discussed by Fassler and Hoffmann (1999) in a paper dedicated to valence compounds at the border of intermetallics (alkali and alkaline earth metal stannides and plumbides) . Examples showing gradual transition from valence compounds to intermetallic phases and new possibilities for structural mechanisms and bonding for Sn and Pb have been discussed. Structural relationships with Zintl phases (see Chapter 4) containing discrete and linked polyhedra have been considered. See 3.12 for a few remarks on the relationships between liquid and amorphous glassy alloys. 

The strong influence of Zintl on the description of chemical bonding in compounds at the border of salts and intermetallics led to the nomenclature Zintl ion f ° for soluble polyanions (as part of a polyanionic salt ) and Zintl phase f for compounds with anionic substructures obeying the (8 — N) rule. Further development and the perception that the salt-metal transition is not abrupt led to a continuous extension of these terms. Soluble polycations, discrete units, and low-dimensional substructures in Zintl phases are called Zintl ions. These ions commonly consist of metal- or semi metal-atoms, or of atoms of semiconducting elements. Clearly, they must be distinguished from classical ions as elucidated by a comparison of SnTe4 and the iso(valence)electronic ion S04 . 

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