Transport in Mn spinel oxides alloyed with Zn–Ni: Polaron hopping in an inhomogeneous energy landscape

Abstract

Electronic transport in transition metal spinel oxides is associated with small polaron hopping, either nearest-neighbor, resulting in Arrhenius activated conductivity, or variable energy, leading to a sub-Arrhenius relationship, with the conductivity logarithm being a convex function of inverse temperature. For the case of manganese spinel oxides alloyed with zinc and nickel, instances of super-Arrhenius behavior are measured, with the conductivity logarithm functional dependence on temperature deviating quadratically. Here, we study the transport in Zn0.5NixMn2.5−xO4 ternary oxide pellets, as a function of Ni content in the range 0 ≤ x ≤ 1.25, in combination with structural characterization and theoretical investigations of their electronic and structural properties using density functional theory. The coexistence of cubic spinel and tetragonal Hausmannite structures is revealed along with the presence of various magnetic conformations that are metastable at room temperature. For systems where metastable structures exist, having similar formation energy but different electronic structures, conductivity is a non-trivial function of temperature. Considering nearest-neighbor polaron transfer in such an energetically inhomogeneous landscape, a new hopping mechanism model is proposed which consistently describes the temperature dependence of conductivity in this ternary alloy spinel oxide system. Understanding the underlying physical transport mechanism is vital for sensor, electrochemical, and catalytic applications.