Characterization of nonstoichiometric nickel manganite spinels Ni xMn 3− x□ 3 δ/4 O 4+ δ by temperature programmed reduction

Abstract

Cation deficient spinels Ni x Mn 3− x □ 3 δ/4 O 4+ δ (0≤ x≤1) have been prepared by thermal decomposition of mixed oxalates Ni x/3 Mn (3− x)/3 (C 2O 4)· nH 2O 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 nonstoichiometric oxygen leading to the formation of a stoichiometric oxide. During the following stages the manganese cations are reduced, causing the spinel structure to be destroyed, and the formation of solid solution of NiO in a cubic MnO. Subsequently, Ni 2+ cations undergo a reduction to metallic nickel, and, finally, a mixture of nonstoichiometric MnO 1− δ and metallic nickel is formed. These oxides contain a high level of vacancies which vary with the nickel content with a maximum of δ≈1 near x=0.6. This nonstoichiometry is ascribed both to the presence of Ni 3+ and excess of Mn 4+.