X-ray photoelectron spectroscopy of nickel manganese oxide thermistors
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The major problem in the NiMn 2 O 4 system has been the determination of the valencies and the cation distribution between the two sublattices of the inverse spinel structure, in which both manganese and nickel can adopt more than one valence state. X-ray photoelectron spectroscopy (XPS) was used to elucidate the valence distribution of the manganese and nickel ions. The results showed that identification of the three Mn and two Ni species in the nickel manganite phase is possible, enabling the validity of crystal structure configurations, proposed in the literature, to be assessed. A small amount of copper, added as CuO to the system, was found to be present in both monovalent and divalent states, the Cu+ ions inducing the creation of the large amount of tetravalent Mn observed. This led to a larger number of sites for electron hopping, the mechanism of conduction, and consequently the electrical conductivity of the Cu doped material is much higher than the undoped NiMn 2 O 4 .
I. INTRODUCTION Semiconducting metal oxide thermistors with a negative temperature coefficient of resistance (NTC) have been used in a number of electronic and communications applications for sensing temperature. Nickel manganite, NiMn2O4, is a commonly used material for these thermistors despite its poor stability at high temperatures. A number of investigations have reported measurements of electrical conductivity,1"5 Seebeck coefficient,2"5 magnetic properties,2'4-6 neutron diffraction,7 and x-ray, Auger, and microstructural studies8 of NiMn 2 O 4 . A major problem associated with the complete understanding of this system, however, has been the determination of the valencies and cation distribution between the two sublattices of its inverse spinel structure, in which both manganese and nickel can adopt more than one valence state. It is well established14'10 that the semiconducting properties of NiMn 2 O 4 are satisfactorily described by a hopping mechanism of charge carriers (electrons) via localized states, as often encountered in compounds having the same cation in two or more different valence states on equivalent lattice sites, as in the octahedral sublattice in the spinel structure.9'10 A knowledge of the cation valencies is, therefore, of great importance in the interpretation of the electrical properties of this material. Considerable controversy exists in the conclusions that have been drawn from the different experimental studies reported in the literature. Boucher et al.1 and O'Keeffe11 suggested that only trivalent manganese is present on two different sites. In Bhandage and Keer's model,4 on the other hand, a deficiency in oxygen is also proposed to preserve charge neutrality in a cation distribution devoid of Mn4+. 1278 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 5, May 1992 Downloaded: 10 Feb 2015
X-ray photoelectron spectroscopy (XPS) with its capability for obtaining information on the chemical nature of the materials is one technique that could be used to elucidate the valence distribution problem in su
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