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Band or Polaron: The Hole Conduction Mechanism in the p-Type Spinel Rh2ZnO4


  • This work was supported by the Basic Energy Science Division, U.S. Department of Energy, under Grant No. DE-AC36-08GO28308 to NREL. The “Center for Inverse Design” is a DOE Energy Frontier Research Center. The high magnetic field work and use of the J. B. Cohen X-Ray Diffraction Facility were supported by the National Science Foundation's MRSEC Program (DMR-0520513) at the Materials Research Center of Northwestern University.

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Given the emerging role of oxide spinels as hole conductors, we discuss in this article the traditional vs. new methodologies of determining the type of conduction mechanism at play––localized polaronic vs. band-like transport. Applying (i) traditional small polaron analysis to our in-situ high temperature four-point conductivity and thermopower measurements, we previously found an activated mobility, which is indicative of the small polaron mechanism. However, (ii) employing the recent developments in correcting density functional methodologies for hole localization, we predict that the self-trapped hole is unstable and that Rh2ZnO4 is instead a band conductor with a large effective mass. The hole mobility measured by high-field room temperature Hall effect also suggests band rather than polaron conduction. The apparent contradiction between the conclusion of the traditional procedure (i) and first-principles theory (ii) is resolved by taking into account in the previous transport analysis the temperature dependence of the effective density of states, which leads to the result that the mobility is actually temperature-independent in Rh2ZnO4. Our case study on Rh2ZnO4 illustrates the range of experimental and theoretical approaches at hand to determine whether the transport mechanism of a semiconductor is band or small polaron conduction.