Large thermoelectricity and Nernst effect at low temperature in quasi-one-dimensional <i>A</i>Mn6Bi5 (<i>A</i> = alkali metals)

Searching for promising materials that show large thermoelectricity or the Nernst effect at low temperatures is critical for cryogenic refrigeration applications. In this work, we investigated systematically the electrical and thermal transport properties of quasi-one-dimensional (Q1D) antiferromagnet AMn6Bi5 (A = Na, Rb, and Cs) and find some significant differences among them. All of them exhibit large thermopowers and considerable power factors at low temperature, but interestingly, the sign of thermopower changes from negative to positive with decreasing temperature for NaMn6Bi5. The low-temperature thermal conductivity of NaMn6Bi5 is also lower than those of RbMn6Bi5 and CsMn6Bi5, which is attributed to the high-density defects and stronger scattering. In addition, sufficiently large Nernst coefficients were observed in RbMn6Bi5 and CsMn6Bi5 but not in NaMn6Bi5. It was deduced that the dramatic change in thermoelectric characteristics observed in AMn6Bi5 was due to a large difference in their ionic sizes, leading to a complex interaction among the spin, charge, lattice, and one-dimensionality. Our work provides a unique example of the influences of alkali ions on the thermoelectric properties of Q1D-magnetic alkali metal compounds and can serve as a guide into the future direction for the improvement of the performance of the title compounds.