Doping CoSb3 p-type with Al substitution for Sb

Skutterudites such as CoSb$_{3}$ are compounds composed of group IX-B atoms (Co, Rh, and Ir) forming a simple cubic structure, and group V-A$_{3}$ pnictide atoms (primarily Sb and As) forming rings inside 6 of every 8 cubes. The remaining cubes remain empty. A common method for reducing thermal conductivity is to introduce impurity atoms such as rare-earths in the cubes that act as rattlers. P-type doping of CoSb$_{3}$ has led to some advances in zT, but the p-type material remains less performing than the n-type material due to the fact that the valence band, dominated by Sb levels, has a low effective mass. A promising method for improving p-type properties is to introduce an effective resonant level into the energy levels occupied by the light hole band, thereby increasing the Seebeck coefficient without strongly effecting other transport properties. A first attempt using Sn substitution was not successful. Here we try various concentrations of Al substituted at Sb sites to generate a resonant level. Material properties are measured and compared with a calculated Pisarenko relation, where thermopower is plotted as a function of hole concentration.