The electrical conductivity and thermopower of Mn3O4 were measured in the temperature range 920° to 1530°C. Electrical conduction in cubic Mn3O4 is explained by the small polaron hopping of electron holes between Mn4+ and Mn3+ on octahedral sites. The concentrations of Mn4+ and Mn3+ are governed by the disproportionation equilibrium 2Mnoct3+⇄Mnoct4++ Mnoct2+. This model also explains the electrical behavior of NiMn2O4 and CuMn2O4.

Electrical Properties and Cation Valencies in Mn3O4

X-ray photoelectron spectroscopy study of the cation valencies in nickel manganite

Publisher Summary This chapter discusses the progress in spinel ferrite research. A large number of oxides with a metal–oxygen ratio of 0.75 as composition is known to crystallize into the spinel structure. Among these oxides, magnetite Fe 3 O 4 is an important compound, from which the spinel ferrites can be derived by partial substitution of the iron ions by other cations. Between most of the spinel oxides solid solutions can be formed, which means that a great variety of spinel oxides is possible. The simple spinel ferrites are those ferrites in which x = 0 and the bivalent metal ion is Mg or Cd or one of the bivalent transition metal elements Mn, Fe, Co, Ni, Cu, and Zn. Partial solid solutions of the simple ferrites are also possible with compounds that either do not exist or that do not have the spinel structure. A great variety in the chemical composition of spinel ferrites is possible. The diversity in composition results into a large range of physical properties that permits the tuning of the properties for specific applications and makes spinel ferrites of particular interest.

Chapter 3 Progress in spinel ferrite research

Since the determination of the crystal structure of the mineral spinel, MgAl2O4, by Bragg,1 and Nishikawa2 in 1915, many metal oxides and mixed metal oxides have been found to possess the same structure,3 and these are usually referred to as spinel compounds or spinels. Such materials, and in particular those containing iron, exhibit suitable combined electrical and magnetic properties and therefore soon became extremely important for many technological applications, such as high-frequency devices, memory cores, and magnetic recording. The rapid expansion of these technologies after World War II was a great stimulus for many researchers to focus their efforts on this kind of ferrite materials. Consequently, the fifties and the early sixties were characterized by intensive investigations of spinel systems with a large variety of compositions. Examples in this respect are the works of Gorter,4,5 followed by that of Blasse,6 reporting structural and magnetic properties of hundreds of new synthetic spinel compounds. The ability of the spinel structure to contain different kinds of cations, which can, moreover, gradually be substituted, made these materials also very suitable for experiments on certain magnetic and structural concepts, new at that time, such as Neel’s ferrimagnetism, ligand field stabilization, and the cooperative Jahn-Teller effect.

Mössbauer Effect Studies of Oxidic Spinels

The change in the activation energy of the electrical properties of NiMn2O4 at about 400 °C and the sudden increase of the thermal expansion are indicative of the cation exchange between the octahedral and tetrahedral sublattices. During this exchange, the manganese ions have to change their valencies according to the reaction Mn + Mn + Ni → 2 Mn + Ni — 0.6 eV. This implies that the cation valencies in NiMn2O4 have to be represented with the formula NiMn(NiMnMn)O4.

V.A.M. Brabers

Papers

Cation migration, cation valencies and the cubic-tetragonal transition in MnxFe3−xO4