Transport and x-ray photoelectron spectroscopy properties of (Ni1−xCux)Mn2O4 and Ni(Mn2−yCuy)O4

Abstract

The temperature dependence of the electrical resistivity and the thermoelectric power in (Ni1−xCux)Mn2O4 (x = 0 to 0.5) and Ni(Mn2−yCuy)O4 (y = 0 to 0.2) are investigated. The electrical resistivity of samples with x < 0.1 appears to be hardly changed with increasing Cu content. In all of the samples, the electron transfer is thermally activated and shows semiconducting behavior. The electric conductivity is described by a small polaron hopping mechanism except for x ≥ 0.3. The thermoelectric power at 100 °C is found to change sign from negative to positive with Cu substitution both in (Ni1−xCux)Mn2O4 and Ni(Mn2−yCuy)O4. Furthermore, the thermoelectric powers of these samples with x, y < 0.1 are found to change sign from negative to positive with increasing temperature. The valence distribution of the Mn ions is estimated using x-ray photoelectron spectroscopy. The peak intensity ratio of Mn3+/Mn4+ is maximized whereas that of Ni3+/Ni2+ is minimized at about x, y = 0.05 to 0.07. These results suggest that the Mn4+ and Ni2+ ions change disproportionately into Mn3+ and Ni3+ ions with increasing Cu content up to x, y = 0.1. The valence states of Ni in the system are in accordance with those of Mn, which is necessary in order to maintain charge neutrality and oxygen stoichiometry. The normalized peak intensity of the charge transfer satellite peak of Mn 2p3/2 is rapidly increased by Cu substitution up to x, y = 0.1. With further Cu substitution when x, y > 0.1, the ratio of increase in the peak intensity of the charge transfer satellite is decreased. These facts suggest that the decrease of electrical resistivity when x, y ≥ 0.1 is caused by an increase of holes having O 2p character, and the changes of sign in the thermoelectric power might take care of the competition between the electron conduction term and the hole conduction term caused by the charge transfer from O 2p to transition metal 3d orbitals.