Different site occupancies in substitution variants of Mo3Sb7

•Various substitution variants of Mo3Sb7 with Fe and Ni were characterized.•While the Fe atoms replace the Mo atoms, the Ni atoms also fill the cubic voids.•Higher Fe/Ni amount on the Mo sites causes a decrease of the charge carriers.•The Ni atoms on the cubic sites do not decrease the charge carriers.•The lower the carrier concentration, the better the thermoelectric properties.

A series of samples with the initial formula Mo3–xFexSb7 and Mo3–xNixSb7 have been prepared by solid state reaction at 873 K, starting from the elements in the stoichiometric ratios. The materials were compressed via hot-pressing, and their thermoelectric properties determined. Single crystal data show that Ni atoms in part occupy the cubic voids of the Mo3Sb7 structure, as well as replace some Mo atoms, in contrast to the Fe analogues, where we found no evidence for Fe in the cubic voids. Supported by electronic structure calculations, replacing Mo with Fe or Ni increases the valence-electron count, thus decreases the numbers of p-type carriers of Mo3Sb7. On the other hand, filling the holes with Ni atoms appears to have no such effect. As a consequence, the Ni containing samples exhibit higher electrical conductivity than the Fe analogues, and ultimately lower thermoelectric figure-of-merit, as the p-type carrier concentration remains too high.

Graphical abstractTrying to substitute Ni atoms for Mo in Mo3Sb7 resulted in partial filling of the cubic voids, in contrast to Mo3–xFexSb7. Since the Ni atoms in the Sb8 cubes do not act as electron donors, the Ni-containing materials exhibit very high carrier concentration, and thus high electric conductivity and low Seebeck coefficient.Figure optionsDownload full-size imageDownload as PowerPoint slide