Zintl phase

In chemistry a Zintl phase is the product of a reaction between

• group 1 (alkali metals) or group 2 (alkaline earths) and
  
• post transition metals or metalloids from group 13, 14, 15 or 16.
  

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Zintl phases are named after the German chemist Eduard Zintl who discovered them in the 1930’s. Typically they were made from the elements or by reducing the post transition element in liquid ammonia.

Zintl phases are diamagnetic, poor conductors and brittle. As such they are quite unlike alloys. Zintl noted that there was an atomic volume contraction when these compounds were formed and realised this could indicate cation formation. He suggested that the structures of Zintl phases were ionic, where there was complete electron transfer from the more electropositive metal. The structure of the anion (nowadays called the Zintl ion) should then be considered on the basis of the resulting electronic state. These ideas were further developed to become the Zintl rule or Zintl Klemm concept where the polyanion structure should be similar to an isoelectronic element. Examples of Zintl phases:-

• NaTl, where it is now known that the structure consists of a polymeric anion (-Tl⁻-)_n with a covalent diamond structure with Na⁺ ions fitted into the anionic lattice.
• NaSn where the polyanion is tetrahedral (Sn₄)⁴⁻ similar to phosphorus molecule P₄.
• Na₂Tl which the polyanion is tetrahedral (Tl₄)⁸⁻ similar to phosphorus molecule P₄.

The Zintl line is a hypothetical boundary drawn between group 13 and group 14, to highlight the tendency for group 13 metals to form phases with a variety of stoichiometries, which contrasts to group 14 and above that tend to form salts with polymeric anions. It is now recognised that some Zintl phases contain Zintl clusters and that this accounts for the variable stoichiometries. The bonding in many of these clusters cannot be accounted for by classical octet rule involving covalent, 2 centre 2 electron bonds, as implied by the Zintl rule.

There are examples of a new class of compounds that on the basis of their chemical formulae would appear to be Zintl phases, e.g., K₈In₁₁. However, they are metallic and paramagnetic. Molecular orbital calculations have shown that the anion is (In₁₁)⁷⁻ and that the extra electron is distributed over the cations and, possibly, the anion antibonding orbitals.

General References

• Sevov, S.C. Zintl Phases in Intermetallic Compounds, Principles and Practice: Progress, Westbrook, J.H.; *Freisher, R.L.: Eds.; John Wiley & Sons. Ltd., Chichester, England, 2002, pp. 113-132
• Dong-Kyun Seo, Corbett J. D J. Am. Chem. Soc. 2001, 123, 4512-4518
• Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
• Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.