Electropositive cations

Fig. 16.7 The coordination around chlorine (drawn as light grey solid circles) in (a) NaCI, (b) CU-9 and CU-11, and (c) CU-13. The electropositive cations are shown as solid black circles and transition metal...

Zintl phases remarks on their definition. We have seen that the Zintl phases may be considered as a group of compounds formed by an electropositive (cationic) component (alkali, alkaline earth metal, lanthanide) and an anionic component (for instance a main group element of moderate electronegativity). The anionic part of the structure may be described in terms of normal valence combination. 

In the case of oxide catalysts or alkali metal-doped oxide catalysts, basic surface sites can be generated by decarboxylation of a surface metal carbonate exchange of hydroxyl hydrogen ions by electropositive cations thermal dehydroxylation of the catalyst surface condensation of alkali metal particles on the surface and reaction of an alkali metal with an anion vacancy (AV) to give centers (e.g., Na + AV — Na + e ). 

The electropositive cation induces the H2O ligands to act as adds and, except at very low pH, hydrolysis, i. e. deprotonation of these ligands, takes place. The process is stepwise with ultimately all six ligands being deprotonated. The rates of water exchange, k x"° for Fe(H20) Fe0H(H20)5" and Fe(0H)2(H20)J were estimated to be 1.6 10 1.4 10 and 10 s respectively (Grant Jordan, 1981 Schneider Schwyn, 1987). Complete hydrolysis corresponds to formation of an Fe " oxide or oxide hydroxide, i.e. 

The molecule Ceo was first observed in the mass spectrum of laser evaporated carbon by Kroto et al., who proposed the now familiar soccer ball stmcture. Subsequently, macroscopic quantities were synthesized. In the solid state, Ceo adopts a face-centered cubic (fee) stmcture, which can be considered to be based on the close packing of spheres with a radius of 5 A. Two tetrahedral sites and one octahedral site with radii 1.12 and 2.06 A are present in the stmcture. The availability of these empty sites and the electronegativity of Ceo make it a suitable host lattice for reductive intercalation by electropositive cations such as the alkah metals. The octahedral site is larger than any of the alkah cations, but the tetrahedral site is about the same size as Na+. The discovery of Superconductivity in K Ceo (Tc = 18K) has focused considerable interest on these materials. Intercalation compounds can be prepared by direct reaction of the alkali metal with Ceo to form compositions Aj,C6o (x = 2, 3, 4, or 6) depending on the specific A cation. Direct reaction of Ceo with the alkali metals is the most conveiuent route to the AeCeo phases other compositions can be prepared by use of the appropriate stoichiometry or by reaction of AeCeo with Ceo. Other alkali metal reagents such as NasHg2 and NaH have also been used. ... 

The maximum value of n in a stepwise complex is constrained by the valences of the addends and maximum possible coordination number of the ligand around the cation (usually 6). Monovalent and divalent cations rarely associate with more than 2 ligands, whereas trivalent and more electropositive cations have maximum coordination numbers up to 6 with spherical ligands such as fluoride. Maximum coordination numbers of 3 or less are usual for... 

Reactions of carboxylates containing the more electropositive cations yield product carbonates, or sometimes the basic carbonates. Some of these salts, e.g., those of the alkali metals, melt before decomposition. The oxide products from decomposition of the lanthanide compounds may contain carbon deposited as a result of carbon monoxide disproportionation. Kinetic measurements must include due consideration of the possible retention of carbon dioxide by the product (as COj ) and the secondary reactions involved in carbon deposition. 

Calcium salts. The thermal decompositions of calcium oxalate, malonate, maleate and fumarate were studied in significantly higher temperature ranges (above 720, 612 to 653, 733 to 763 and 733 to 803 K, respectively) than those of the same salts of the transition metals. This is evidence of a stabilizing influence on these anions of the strong bond formed with this strongly electropositive cation. 

The covalency of Cu11- bonds can be controlled inductively. In series such as A2CuO, the covalency of the Cu-0 bonds increases as the A cation becomes more electropositive. Thus for a A-O-Cu linkage, Cu-0 bonds are forced more covalent as the A-0 bond becomes more ionic. This can be thought of as an issue of competition between A and Cu for overlap of the oxygen orbitals. Perhaps a better view is that when a highly electropositive cation ionizes, the electron density shifts to the rest of the system, e.g. oxygen and copper. 

The calculations also suggest that the electropositive cations (alkaline earth or rare earth metals) have little influence on the electronic structure near the Fermi level. I believe that their role is two fold. First, they help "enforce" a particular structure that is, the large cations are responsible for the compounds adopting the perovskite structure. Second, the electropositive metals effectively increase the oxidizing power of... 

Some attention has been drawn to the low dimensional aspects of these materials. While the high Tc materials have two dimensional sheets of Cu-0, and the 90K superconductor has in addition one dimensional Cu-0 chains, Ba(Pb/Bi)03 has an almost cubic structure. Consequently, low dimensionality seems not to be a necessary condition for materials in this class to be superconducting. In any case low dimensional structures will likely result in other materials from low coordination number of the transition metal and by including large electropositive cations. 

Now we can try to put this all together to try to predict where else to look for new superconducting phases. The main themes that arise from the preceding discussion can be condensed into four general characteristics 1) A large cation-anion mixing of the wavefunctions near the Fermi level, 2) metallic conductor, but close to a Mott transition, 3)fast anion conductor, 4) the electropositive cations do not play an essential role in the electronic properties. For the sake of discussion, we can arbitrarily break these materials into oxides and other anion compounds. 

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