Semi-metals introduction

A review about the Zintl phases has been published by Sevov (2002) from the introduction of this publication we quote a few remarks. It was preliminary observed that the number of Zintl phases has increased many-fold since Zintl s time and that the definition of a Zintl phase has never been very exact often compounds that include non-metals have been considered in this family. The paper by Sevov is mainly dedicated to clearly intermetallic Zintl phases (that is phases containing main group metals, semi-metals or semiconductors only). Attention has therefore been dedicated to compounds of alkali metals with the elements of the 13th, 14th and 15th groups (without B, Al, C, N and P). To this end the following definitions and statements have been considered. 

Electronegativity, Mendeleev number, Miedema parameters. A few semi-empirical parameters and scales which are useful as reference data in the systematic description (or even prediction) of the alloying behaviour of the different metals will be presented here also as an introduction to the following paragraphs. The closely related basic concepts of chemical periodicity and electron configurations will be reminded in Chapter 4. 

More simply, this model allows the introduction of two parameters representing on the one hand the electronegativity of the metal, and on the other hand the distance of the metal-carbon bond. These parameters may be considered as being semi-empirical and only of interest for comparing metals to each other. 

Volume 19 is devoted to considering simple processes occurring at the gas-solid interface. Chapter 1 serves as an introduction and deals with the methodology of experimental surface science. Experimental results for metal surfaces on both adsorption and desorption kinetics and surface diffusion are discussed in terms of the current theories of these processes. Chapter 2 deals in the same way with these processes on semi-conductor surfaces. Finally, Chapter 3 is concerned with radiation and photoeffects at gas—solid interfaces. 

The introduction of metal atoms into a pdymer, or the formaticm of a polymer through metal centres fequently provides semi-conducting materials. Unfortunately, and equally frequently, the polymers are insduble, infusable and thus intractable materuls. It is beyond the limited space available to discuss the conductance of the large number of known n tallo-organic pdymers. For this reason a selective approach is adopted here mth the aim to illustrate, and whenever possible, to present weU characterized system or ernes vriiich have outstanding electrical properties. 

The Hiickel method is a very primitive example of a semi-empirical method in which various integrals are set equal to either a or p and treated as empirical parameters overlap integrals are ignored. The removal of the restriction of the Hiickel method to planar hydrocarbon systems was achieved with the introduction of the extended Hiickel theory (EHT) in about 1963. In heteroatomic non-planar systems (such as d-metal complexes) the separation of orbitals into k and a is no longer appropriate and each type of atom has a different value of Hu (which in Hiickel theory is set equal to a for all atoms). In this approximation, the overlap integrals are not set equal to zero but ue cdculated expKdtly. Furthermore, the Hjk, which in Hiickel theory are set equal to p, in EHT are made proportional to the overlap integral between the orbitals J and K. 

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