Transport anomalies in the high-temperature hopping conductivity and thermopower of Sr-doped La(Cr,Mn)O3.

Abstract

A minimum exists in the electrical conductivity of the perovskite-type ceramic ${\mathrm{LaCr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{O}}_{3}$ as a function of Mn content near x=0.05. This minimum has been explained in terms of a crossover from multiple trapping to percolation among energetically lower Mn sites. In this paper electrical conductivity and Seebeck measurements are presented on a similar series in which 10 mol % Sr was substituted for La in order to increase the small polaron concentration through the compensation of Sr ions according to the Verway mechanism. The data suggests that there is an apparent suppression of the Verway compensation mechanism in all Mn-doped samples. The hopping crossover observed in the Sr-free series is retained with Sr doping, although the position and depth of the electrical-conductivity minimum are altered. Difficulties in the present understanding and interpretation of the electrical conductivity and Seebeck measurements as a function of Mn and Sr content in these materials are discussed. An electronic structure is suggested, which seems to resolve many of these problems.