Zintl phase

Zintl phases are a subgroup of brittle, high-melting intermetallic compounds that are diamagnetic or exhibit temperature-independent paramagnetism and are poor conductors or semiconductors.

He suggested that the structures of these phases were ionic, with complete electron transfer from the more electropositive metal to the more electronegative main group element.

Further, the anionic sublattice can be isolated as polyanions (Zintl ions) in solution and are the basis of a rich subfield of main group inorganic chemistry.

A "Zintl Phase" was first observed in 1891 by M. Joannis, who noted an unexpected green colored solution after dissolving lead and sodium in liquid ammonia, indicating the formation of a new product.

[3] It was not until many years later, in 1930, that the stoichiometry of the new product was identified as Na4Pb94− by titrations performed by Zintl et al.;[4] and it was not until 1970 that the structure was confirmed by crystallization with ethylenediamine (en) by Kummer.

[6] Corbett has contributed improvements to the crystallization of Zintl ions by demonstrating the use of chelating ligands, such as cryptands, as cation sequestering agents.

[7] More recently, Zintl phase and ion reactivity in more complex systems, with organic ligands or transition metals, have been investigated, as well as their use in practical applications, such as for catalytic purposes or in materials science.

Thus, the valence electron concentration (VEC) of the anionic element is increased, and it formally moves to the right in its row of the periodic table.

The structure can be explained by the 8-N rule (replacing the number of valence electrons, N, by VEC), making it comparable to an isovalent element.

[8] Zintl phases can be prepared in regular solid state reactions, usually performed under an inert atmosphere or in a molten salt solution.

Typical solid state methods include direct reduction of corresponding oxides in solution phase reactions in liquid ammonia or mercury.

The product can be purified in some cases via zone refining, though often careful annealing will result in large single crystals of a desired phase.

An illustrative example: There are two types of Zintl ions in K12Si17; 2x Si4−4 (pseudo P4, or according to Wade's rules, 12 = 2n + 4 skeletal-electrons corresponding to a nido-form of a trigonal-bipyramid) and 1x Si4−9 (according to Wade's rules, 22 = 2n + 4 skeletal-electrons corresponding to a nido-form of a bicapped square antiprism) Examples from Müller's 1973 review paper with known structures are listed in the table below.

[8] Li9Al4 LiGa LiIn LiTl Li7Si2 Li22Ge5 Li9Ge4 Li22Sn5 Li2Sn5 Li3As LiAs Li3Sb Li3Bi LiBi Li2Se Li2Te LiBr LiI NaIn Na2Tl (the polyanion is tetrahedral (Tl4)8- Concept Tl2- ~ P) NaTl (See Figure) NaxSi136 (x ≤ 11) Na8Si NaGe Na15Pb4 Na13Pb5 Na9Pb4 NaPb Na3As Na3Sb NaSb Na3Bi NaBi Na2Se Na2Se2 Na2Te NaBr NaI K8Si46 K8Ge46 K8Sn46 KPb KPb2 K3As K3Sb KBi2 K2S2 K2Se K2Se2 K2Te KBr KI RbGe RbSn RbPb Rb3As Rb3Bi RbBi2 RbBr RbI CsGe CsSn CsPb Cs3Sb Cs3Bi CsBi2 Cs2NaAs7 (See Figure) CsBr CsI Mg5Ga2 Mg2Ga MgGa MgGa2 Mg2Ga5 Mg3In Mg5In2 Mg2In MgIn MgIn5 Mg2Tl MgTl Mg2Ge Mg2Sn Mg2Pb Mg3As2 Mg3Sb2 Mg3Bi2 MgSe MgTe MgTe2 MgBr2 MgI2 CaAl4 CaGa CaGa2 CaGa4 CaIn CaIn2 CaTl CaTl3 CaSi2 Ca7Ge Ca2Ge CaGe2 Ca3Sn Ca2Sn CaSn CaSn3 Ca3Pb Ca2Pb Ca5Pb3 CaSe CaTe CaBr2 CaI2 SrGa2 SrGa4 SrIn2 SrTl SrTl2 SrTl3 SrSi Sr4Si7 SrSi2 Sr3Ge4 SrGe2 SrSn SrPb3 Sr3P14 SrBi3 SrSe SrTe SrBr2 SrI2 BaGa2 BaGa4 BaIn2 BaIn4 BaTl2 BaSi Ba3Si4 BaSi2 Ba2Ge BaGe BaGe2 BaSn Ba5Pb3 BaPb BaPb3 BaP3 BaBi3 BaS3 BaSe BaTe BaBr2 BaI2 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, which is metallic and paramagnetic.

[14] Zintl phases that contain molecule-like polyanions will often separate into its constituent anions and cations in liquid ammonia, ethylenediamene, crown ethers, or cryptand solutions.

[15] Beyond the "aesthetic simplicity and beauty of their structures" and distinctive electronic properties, Zintl ions are also of interest in synthesis because of their unique and unpredictable behavior in solution.

[15][16] Many examples similarly exist for heteroatomic clusters where the polyanion is composed of greater than one main group element.

The second is method, performed at higher temperatures, is to dissolve a Zintl phase in liquid ammonia or other polar aprotic solvent like ethylenediamine (on rare occasions DMF or pyridine is used).

Corbett has also improved the crystallization of Zintl ions by demonstrating the use of chelating ligands such as cryptands, as cation sequestering agents.

For example, differently charged species can be present in solution because the polyanions are highly reduced and may be oxidized by solvent molecules.

[17] NMR is also useful for gaining information about the coupling between individual atoms of the polyanion and with the counter-ion, a coordinated transition metal, or ligand.

[19] As highly reduced species in solution, Zintl ions offer many and often unexpected, reaction possibilities, and their discrete nature positions them as potentially important starting materials in inorganic synthesis.

The rules were developed to predict the geometries of boranes from the number of electrons and can be applied to these polyanions by replacing the BH unit with a lone pair.

The Zintl-Klemm-Busmann concept describes how in an anionic cluster, the atoms arrange in typical geometries found for the element to the right of it on the periodic table.

Other 'spherical shell models' with spherical harmonic wave functions for molecular orbitals—analogous to atomic orbitals—that describe the clusters as pseduo elements.

[22][23] DFT or ab initio molecular orbital calculations similarly treat the clusters with atomic, and correspondingly label them S, P, D etc.

An indicator of this phenomenon is a negative Nucleus Independent Chemical Shift (NICS) values of the center of the cluster or of certain additional high symmetry points.

[26] The discrete nature of Zintl ions opens the possibility for the bottom up synthesis of nanostructured semiconductors and the surface modification of solids.