A partially cobalt-substituted solid solution of Nowotny chimney-ladder phase,
(Mn1-xCox)Si, has been prepared using a tetra-arc-type furnace and a subsequent annealing process.
The compounds consist of two tetragonal subsystems of [Mn1-xCox] and [Si], with an irrational
c-axis ratio = cMn/cSi ~ 1.7. The crystal structure and thermoelectric properties of (Mn1-xCox)Si
solid solution were compared with those of the Fe-substituted solid solution, (Mn1-xFex)Si. In the
case of Co-series, extra valence electrons are introduced relative to Fe-series, since the valence
electron count is 3d74s2 for Co but 3d64s2 for Fe, respectively. It was naturally expected that the Feand
Co-substituted MnSi becomes n-type conductor from the p-type one at x > 0.23(5) and x >
0.06(1), respectively. Experimentally, the Fe-substituted samples become n-type at x > 0.28 but it
is not the case for the Co-substituted ones. It is thus evident that there is an unknown factor which
controls the thermoelectric properties of Co-substituted samples.
