Pyroaurite-type nickel hydroxides

Allmann found that when suitable trivalent ions were introduced during the precipitation of the hydroxides of Mg, Zn, Mn, Fe, Co, and Ni, these were incorporated in the lattice and the structure changed from the brucite (Mg(OH)2 to the pyroaurite [Mg,Fe2(OH),J.[C03 4H2O]) type of structure [68], One of the nickel materials he prepared was an Ni/Al hydroxide. Axmann et al. [69-71] have given the nickel compounds the general formula 

Del mas and his co-workers have done extensive work on pyroaurite-type materials which has recently been reviewed [73]. In addition to precipitation methods, they have prepared the materials by mild oxidative hydrolysis of nickelates that were prepared by thermal methods similar to those used for the preparation of LiNiOj [74]. A cobalt-substituted material NaCo, (Ni, A ) was prepared by the reaction of Na20, CO3O4 and NiO at 800 °C under a stream of oxygen. The material was then treated with a 10 molL NaCIO -1-4 molL KOH solution for 15h to form the oxidized y -oxyhydroxide. The pyroau- 

Most of the work on pyroaurite materials has been done on materials with Fe [68-73, 77], Co [68, 76, 78], Mn [72, 79], or Al [68, 70, 72] substitutions. When at least 20% of the Ni atoms are replaced by the trivalent substituent, the materials are stable in concentrated KOH. In many ways, the pyroaurite phase is similar to 

Because of the possibiHty of applying Mossbauer spectroscopy, the sohd-state chemistry of the Fe-substituted material is the best understood [69, 72, 73]. Mossbauer spectroscopy confirms that the Fe in the pyroaurite type material is Fe(III). Glemser and co-workers have found that electrochemical oxidation of the material converts about 30% of the Fe(III) to Fe(IV) [69, 72]. The results were consistent with a high-spin configuration with the Fe(IV) in FeOe octahedra with Oh symmetry. The Oh symmetry can only occur if the surrounding NiOg octahedra also have an Oh symmetry. Hence, the Fe(IV) ions in the layer must be surrounded by six NiOfi octahedra with the Ni in the Ni(IV) state. Delmas and coworkers found evidence for Fe(IV) in both high- and low-spin states for oxidized materials prepared by the chemie douce method [73]. The difference in the results may be due to the effect of the platelet size on the pyroaurite structure. 

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