Nickel hydroxide as a matrix for unusual valencies: The electrochemical behaviour of metal(III)-ion-substituted nickel hydroxides of the pyroaurite type

Electrochemical and chemical oxidation transforms the pyroaurite structure of metal(III)-ion-substituted nickel hydroxide into the layer structure of gamma-NiOOH. In the case of trivalent iron as a substituent, the unusual oxidation states nickel(IV) and iron(IV) are formed. As deduced from Mossbauer spectra, iron(IV) has a high spin configuration and, due to the absence of quadrupole splitting, a regular arrangement of its surrounding oxide ions. The matrix gamma-NiOOH is responsible for the undistorted (O-h) symmetry. Domains are produced containing one regular (FeO6)-O-IV octahedron surrounded by six regular (NiO6)-O-IV octahedrons (d(6) low spin configuration). This result requires the presence of exclusively di- and tetravalent nickel ions in gamma-NiOOH, i.e. the oxidation process results in the direct two-electron step Ni(II)-->Ni(IV)+2e(-). Manganese(III) as substituent also leads to the formation of the pyroaurite structure. Partial oxidation to tetravalent manganese occurs when the samples are exposed to air. During cycling structural changes are observed, mainly due to reduction of tri- to divalent manganese leading to beta-NiOOH- and beta-Ni(OH)(2)-type domains in the gamma-NiOOH, pyroaurite structure, respectively. No Mn3O4 is formed, indicating that the manganese ions are located on nickel sites and do not leave the lattice during prolonged cycling.