Résumé
We present a combined experimental and theoretical study of thermoelectric properties and lattice dynamics of the R4Sb3 (R = La, Yb) compounds with the anti-Th3P4 structure. Bi alloying of Yb4Sb3 was found to improve the thermoelectric properties but not as well as La or Sm alloying. From the band structure, we are able to explain the origin of the relatively large and positive thermopower in Yb4Sb3 and of the small and negative thermopower found in La4Sb3 at high temperatures. Since the contribution of the 4f level of Yb atoms must be negative, as in other intermediate valence Yb-based compounds, the positive thermopower in Yb4Sb3 is due to the conduction electrons. We conjecture that the origin of the low thermal conductivity is similar to that of skutterudites, i.e., due to hybridization between acoustic and low-energy optical modes of the same symmetry.