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

Abstract

Yb 14 MnSb 11 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 Yb 2+ , 1 MnSb 9− 4 , 1 Sb 7− 3 , and 4 Sb 3− atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 Yb 2+ , 1 Yb 3+ , 1 (Mg, Zn)Sb 10− 4 , 1 Sb 7− 3 , and 4 Sb 3− . The predicted existence of Yb 3+ 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 Yb 14 MgSb 11 .Inthese 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.

Symmetric Bloch oscillations of matter waves

“…Spectra collected at other temperatures for Yb 14 MnSb 11 and Yb 14 MgSb 11 (see Supplemental Material Fig. S2 [40]) are nearly identical and indicate no significant temperature dependence with regards to the Yb valence.…”

“…The crystals were approximately 2 × 2 × 2 mm 3 in dimensions with highly reflective facets (see Supplemental Material Fig. S1 [40]).…”

Discovery of multivalley Fermi surface responsible for the high thermoelectric performance in Yb 14 MnSb 11 and Yb 14 MgSb 11

“…Yb 14 ZnSb 11 , with the smallest unit cell reported to date and high carrier concentration, shows intermediate valent Yb ( 35 , 36 ). In the case of Yb 14 Mg 1− x Al x Sb 11 , the smaller unit cell and disorder added with the solid solution may initially increase the tendency for intermediate valency until the carrier concentration is diminished ( 25 , 36 , 37 ). The addition of an extra electron from Al may be adjusting the amount of Yb 3+ in the case of low x , buffering the system with intermediate valent Yb until x ~ 0.5, thereby reflecting a lower than expected carrier concentration at low x .…”

“…The lower than expected carrier concentration by about 63% could be due to the experimental uncertainty of the Hall measurement or indicate the presence of a compensating Yb valence or intrinsic defect manifestation. Yb 14 MgSb 11 has been shown to contain Yb 3+ and intermediate valent Yb by x-ray absorption near-edge structure (XANES), magnetic, and heat capacity studies ( 35 , 36 ). Yb 14 ZnSb 11 , with the smallest unit cell reported to date and high carrier concentration, shows intermediate valent Yb ( 35 , 36 ).…”

“…Yb 14 MgSb 11 has been shown to contain Yb 3+ and intermediate valent Yb by x-ray absorption near-edge structure (XANES), magnetic, and heat capacity studies ( 35 , 36 ). Yb 14 ZnSb 11 , with the smallest unit cell reported to date and high carrier concentration, shows intermediate valent Yb ( 35 , 36 ). In the case of Yb 14 Mg 1− x Al x Sb 11 , the smaller unit cell and disorder added with the solid solution may initially increase the tendency for intermediate valency until the carrier concentration is diminished ( 25 , 36 , 37 ).…”