ICT2014 Abstracts

Zintl phases, a subset of intermetallic compounds characterized by covalently bonded substructures surrounded by highly electropositive cations, exhibit many of the characteristics desired for thermoelectric applications. The requirement that Zintl compounds satisfy the valence of anions through the formation of covalent substructures leads to many unique, complex crystal structures, and thus to low lattice thermal conductivity.  As valence precise compounds, traditional Zintl phases are also expected to behave as intrinsic semiconductors.  AZn₂Sb₂ phases are an exception (where A = Sr, Ca, Yb, Eu), forming with high carrier concentrations (10¹⁹-10²⁰ holes/cm³) and extrinsic semiconducting behavior, without the addition of dopants.  Here we use a combination of density functional theory and transport measurements to demonstrate that this extrinsic behavior can be explained by large concentrations of thermodynamically stable cation vacancies.  We show that the calculated vacancy concentration is highly dependent on the choice of cation (A), consistent with the observed trends in experimental carrier concentration in AZn₂Sb₂ compounds.  To investigate the phase width of YbZn₂Sb₂ experimentally, samples with nominal compositions of Yb_1-xZn₂Sb₂ (x = 0.98, 0.99, 1.00, 1.025, 1.05) were synthesized. By varying x within the predicted stable phase width (x = 0.97-1.00 at 800 K), the vacancy concentration, and thus the carrier concentration can be controlled. However, intrinsic semiconducting behavior was not obtained, even with the addition of extra Yb (x > 1.00), suggesting that a Yb-deficient composition (x < 1) is thermodynamically preferable to the “Zintl” stoichiometry (x = 1).  These results provide a new route to controlling the electronic properties of AM₂Sb₂ compounds, and may shed light of similar trends observed in other Zintl systems.