CHARACTERIZATION OF NONSTOICHIOMETRIC NICKEL MANGANITE SPINELS NIXMN3-X-SQUARE-O-3-DELTA 4(4+DELTA) BY TEMPERATURE-PROGRAMMED REDUCTION/

Cation deficient spinels Ni(x)Mn(3-x)square(3 delta/4)O(4+delta) (0 le ss than or equal to x less than or equal to 1) have been prepared by t hermal decomposition of mixed oxalates Nix/3Mn(3-x/3)(C2O4). nH(2)O in air at 623 K. They have been characterised by temperature programmed reduction (TPR) under H-2, the reaction being followed by gravimetric and powder X-ray diffraction measurements. It has been shown that TPR proceeds in several steps. The first steps correspond to the loss of n onstoichiometric oxygen leading to the formation of a stoichiometric o xide. During the following stages the manganese cations are reduced, c ausing the spinel structure to be destroyed, and the formation of soli d solution of NiO in a cubic MnO. Subsequently, Ni2+ cations undergo a reduction to metallic nickel, and, finally, a mixture of nonstoichiom etric MnO1-delta and metallic nickel is formed. These oxides contain a high level of vacancies which vary with the nickel content with a max imum of delta approximate to 1 near x = 0.6. This nonstoichiometry is ascribed both to the presence of Ni3+ and excess of Mn4+. (C) 1998 Els evier Science B.V. All rights reserved.