Thermopower and electrical conductivity of Mn1.68−XCu0.6+X+Y+ZCo0.24−YNi0.48−Z thin film oxides obtained through metal organic decomposition processing

Abstract

Compositional variations of spinel oxides to a reference composition Mn1.68−XCu0.6+X+Y+ZCo0.24−YNi0.48−ZO4 (x and z=0.2, 0.4 and y=+∕−0.1) are made. Thermopower and electrical conductivity of these materials are measured in thin film forms obtained through metal organic decomposition. This low-temperature fabrication technique allows the concentration of metastable Mn4+ and Cu1+ ions, which are essential to charge transport in the oxide spinel, to be controlled. All compositions have n-type behavior, and the conduction occurs via polaron hopping between neighboring Mn3+oct and Mn4+oct sites. Compositional variations alter the Mn and Cu oxidation state concentrations, and modify the conductivity and thermopower. Charge balance models are used in some cases to interpret the influences that doping and fluctuating oxygen content have on the variable valence Mn and Cu. After accounting for changes in oxygen concentration, we found that replacing portions of Co with Cu raise both the conductivity and thermopower magnitude, and substituting small amounts of Cu for Ni reduces these thermoelectric factors. Surprisingly, the effects of doping show parallel rises or declines in conductivity and thermopower magnitude. This implies that conductivity is more strongly impacted by the activated carrier mobility than by the number of charge carriers in these oxides.