Intermediate Yb valence in the Zintl phases Yb14MSb11(M=Zn,Mn,Mg): XANES, magnetism, and heat capacity

Abstract

Yb14MnSb11 is a magnetic Zintl compound as well as being one of the best high temperature p-type thermoelectric materials. According to the Zintl formalism, which defines intermetallic phases where cations and anions are valence satisfied, this structure type is nominally made up of 14 Yb2+, 1 MnSb49-, 1 Sb37-, and 4 Sb3- atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 Yb2+, 1 Yb3+, 1 (Mg, Zn)Sb410-, 1 Sb37-, and 4 Sb3-. The predicted existence of Yb3+ based on simple electron counting rules of the Zintl formalism calls the Yb valence of these compounds into question. X-ray absorption near-edge structure, magnetic susceptibility, and specific heat measurements on single crystals of the three analogs show signatures of intermediate valence Yb behavior and in particular, reveal the heavy fermion nature of Yb14MgSb11. In these isostructural compounds, Yb can exhibit a variety of electronic configurations from intermediate (M=Zn), mostly 2+ (M=Mn), to 3+ (M=Mg). In all cases, there is a small amount of intermediate valency at the lowest temperatures. The amount of intermediate valency is constant for M=Mn, Mg and temperature dependent for M=Zn. The evolution of the Yb valence correlated to the transport properties of these phases is highlighted. The presence of Yb in this structure type allows for fine tuning of the carrier concentration and thereby the possibility of optimized thermoelectric properties along with unique magnetic phenomena.