Structure of NaTl

Therefore, the chemical bonding in NaTl is expected to be a mixture of covalent, ionic, and metallic interactions. 

The NaTl-type structure is the prototype for Zintl phases, which are inter-metallic compounds which crystallize in typical non-metal crystal structures. Binary AB compounds LiAl, LiGa, Liln and Naln are both isoelectronic (isovalent) and isostructural with NaTl. In the Li2AlSi ternary compound, A1 and Si form a diamond-like framework, in which the octahedral vacant sites of the A1 sublattice are filled by Li atoms, as shown in Fig. 13.7.2(b). 

From the crystal structure, physical measurements, and theoretical calculations, the nature of the chemical bond in the NaTl-type compound AB can be understood in the following terms  

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The structure of sodium thallide NaTl can be understood as a diamond-like framework of T1 atoms, whose vacant sites are completely filled with Na atoms. Figure 13.7.2(a) shows the structure of NaTl, in which the Tl-Tl covalent bonds are represented by solid lines. The T1 atom has three valence electrons, which are insufficient for the construction of a stable diamond framework. The deficit can be partially compensated by the introduction of Na atoms. The effective radius of the Na atom is considerably smaller than that in pure metallic sodium. 

Around 1928, Zintl had begun to investigate binary intermetallic compounds, in which one component is a rather electropositive element, e.g., an alkali- or an alkaline earth metal [1,2]. Zintl discovered that in cases for which the Hume-Rothery rules for metals do not hold, significant volume contractions are observed on compound formation, which can be traced back to contractions of the electropositive atoms [2]. He explained this by an electron transfer from the electropositive to the electronegative atoms. For example, the structure of NaTl [3] can easily be understood using the ionic formulation Na Tl" where the poly- or Zintl anion [TF] forms a diamond-like partial structure - one of the preferred structures, for a four electron species [1,2], Zintl has defined a class of compounds, which, in the beginning, was a somewhat curious link between well-known valence compounds and somehow odd intermetallic phases. 

Fig. 20. Knight shift K, of the Na-, Hg-and Tl-NMR in the system NaHg., Tl, as a function of x. K (A) are the experimental results and K (A) and K5 (AB) are the direct contribution for the A nuclei obtained from the band structures of NaTl and NaHg(B32) using the rigid band model (Schmidt et al. )...

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The Zintl-Klemm concept evolved from the seminal ideas of E. ZintI that explained the structural behavior of main-group (s-p) binary intermetaUics in terms of the presence of both ionic and covalent parts in their bonding description [31, 37]. Instead of using Hume-Rother/s idea of a valence electron concentration, ZintI proposed an electron transfer from the electropositive to the electronegative partner (ionic part) and related the anionic substructure to known isoelectronic elemental structures (covalent part), e.g., TK in NaTl is isoelectro-nic with C, Si and Ge, and consequenUy a diamond substructure is formed. ZintI hypothesized that the structures of this class of intermetallics would be salt-like [16b, 31 f, 37e]. 

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Figure 3-4. Dimensions of a fully extended polypeptide chain. The four atoms of the peptide bond (colored blue) are coplanar. The unshaded atoms are the a-carbon atom, the a-hydrogen atom, and the a-R group of the particular amino acid. Free rotation can occur about the bonds that connect the a-carbon with the a-nitrogen and with the a-carbonyl carbon (blue arrows). The extended polypeptide chain is thus a semirigid structure with two-thirds of the atoms of the backbone held in a fixed planar relationship one to another. The distance between adjacent a-carbon atoms is 0.36 nm (3.6 A). The interatomic distances and bond angles, which are not equivalent, are also shown. (Redrawn and reproduced, with permission, from Pauling L, Corey LP, Branson PIR The structure of proteins Two hydrogen-bonded helical configurations of the polypeptide chain. Proc Natl Acad Sci U S A 1951 37 205.)...

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Left unit cell of NaTl. The plotted bonds of the thallium partial structure correspond to the C-C bonds in diamond. Right section of the structure of SrGa2 and MgB2 (A1B2 type)... 

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