Ag and Cu doping and their effects on the thermoelectric properties of β -Zn₄Sb₃

The measurements of electrical resistivity and Seebeck coefficients of Ag- and Cu-doped compounds β- (Zn_1-xM_x) 4Sb ₃(M=Ag,Cu;x=0,0.0025,0.005,0.01) were carried out, and the results indicated that both their resistivity and Seebeck coefficients increase first (x≤0.0025 for Ag and x≤0.005 for Cu) and then decrease obviously with further increase in their doping content. Correspondingly, hole concentration behaviors in the opposite way. In terms of the one vacancy-two interstitial Zn atom model: A11 BCSb10 and two-vacancy-three-interstitial Zn atom model: A10 BCDSb10 (here A is normal Zn atom and B, C, and D interstitial Zn atoms) proposed by Cargnoni [Chem. Eur. J. 10, 3861 (2004)]10.1002/chem.200400327, first-principles calculations were performed on the occupation options of Ag and Cu atoms in disordered β -Zn₄Sb₃. The results indicated that both Ag and Cu atoms occupy preferentially the Zn vacancies in normal sites. Subsequently, Ag and Cu atoms will substitute for interstitial atoms D (for Ag) and B (for Cu). The calculations also showed that as Ag and Cu atoms fill Zn vacancies they play the role of donors, leading to a decrease in hole concentration; while Ag and Cu atoms replace the interstitial atoms they act as acceptors resulting in an increase in hole concentration, which are in good agreement with the nonmonotonous change behavior in the transport properties and Hall carrier concentrations observed experimentally. In addition, calculations revealed that Cu instead of Ag can also fill the Zn vacancies with smaller volumes and Cu doping nearly always has lower formation energies than Ag, which could give a reasonable explanation for the higher solubility of Cu than that of Ag in β -Zn₄Sb₃.