From detailed experimental and theoretical analysis of the high zT compositions Yb 14 MnSb 11 and Yb 14 MgSb 11 and related compounds it has been shown that the valence bands are essentially the same for the Sb compounds. The valence bands contain a simple, light valence band with a second heavier valence band close in energy that has high valley degeneracy which provides additional conduction paths, contributing to a high electrical conductivity, while maintaining a large thermopower .