  The best p-type skutterudites so far are didymium filled, Fe/Co substituted,
Sb-based skutterudites. Substitution at the Sb-sites influences the electronic
structure, deforms the Sb4-rings, enhances the scattering of phonons on
electrons and impurities and this way reduces the lattice thermal conductivity.
In this paper we study structural and transport properties of p-type
skutterudites with the nominal composition DD0.7Fe2.7Co1.3Sb11.7{Ge/Sn}0.3,
which were prepared by a rather fast reaction-annealing-melting technique. The
Ge-doped sample showed impurities, which did not anneal out completely and even
with ZT > 1 the result was not satisfying. However, the single-phase Sn-doped
sample, DD0.7Fe2.7Co1.3Sb11.8Sn0.2, showed a lower thermal and lattice thermal
conductivity than the undoped skutterudite leading to a higher ZT=1.3, hitherto
the highest ZT for a p-type skutterudite. Annealing at 570 K for 3 days proved
the stability of the microstructure. After severe plastic deformation (SPD),
due to additionally introduced defects, an enhancement of the electrical
resistivity was compensated by a significantly lower thermal conductivity and
the net effect led to a record high figure of merit: ZT = 1.45 at 850 K for
DD0.7Fe2.7Co1.3Sb11.8Sn0.2.
