ON THE COMPOSITION OF HIGHER SOLID NICKEL OXOCOMPOUNDS

Critically selected equilibrium or rest potentials of solid nickel oxo compounds with 2 < z(Ni) less-than-or-equal-to 3.6 measured at ambient temperatures by various authors were reevaluated. Comparing the measu red data converted to uniform conditions (25-degrees-C, a(KOH) = a(H2O ) = 1.0, E(r)' vs. RHE) with the theoretical course of E(r) vs. z(Ni), derived for the case that the NiO(x) system can be formed either by N i(II)+ Ni(III) + Ni(IV) or by Ni(II)+ Ni(IV) components, it was found that the so called ''betaNi(OH)2/beta-NiOOH'' systems with z(Ni) < 3.0 can be regarded as thermodynamically unstable or metastable, as they exhibit an unusual decrease of E(r)' with increasing z(Ni) in the rang e of 2.6 < z(Ni) < 3.0. Experimental results corresponding to a sequen ce of stable valence stages Ni(II) --> Ni(III) --> Ni(IV) were not obs erved. In contrary, the so called ''a-Ni(OH)2/gamma-NiOOH'' systems ex hibit a monotonous S-shaped course of E(r)' vs. z(Ni) in the whole inv estigated range of 2.0 < z(Ni) less-than-or-equal-to 3.6, with the low est E(r)' values at the given Z(Ni), so that they can be regarded as t hermodynamically stable systems representing solid solutions of Ni(II) + Ni(IV) components only.