Crystal structure and XPS analysis of in-doped β-Zn₄Sb₃

β-Zn₄Sb₃ is a promising candidate thermoelectric material with extraordinary low lattice thermal conductivity and high dimensionless figure of merit. In the paper, X-ray diffraction (XRD) patterns and X-ray photoelectron spectra (XPS) of In-doped β-Zn₄Sb₃ compounds with nominal composition Zn₄Sb_3-mIn_m (0≤m≤0.18) were investigated to reveal the effect of indium doping on crystal structure and binding characteristics. The Rietveld refinement based on the XRD data indicates that the indium impurity preferentially occupies the 12c Sb(2) site in Zn₄Sb_3-mIn_m. The lattice parameter a increases and c decreases with the increasing m, accompanying with the lattice distortion and the bond length and angle modification. Indium doping enhances the occupancies of Zn atoms at the 36f Zn(1) and interstitial Zn sites, whereas the total occupancy of Sb and In atoms maintains constant. XPS analysis shows that the valence of Sb decreases and that of Zn increases with increasing the indium doping in Zn₄Sb_3-mIn_m. This implies that the negative charge might transfer from In and Zn to Sb atoms. Curve-fittings of Sb 3d photoelectron peaks prove that only the Sb atom at the 12c Sb(2) site is charged to more negative with the increasing m. Thus more Zn²⁺ ions near the 12c Sb(2) site are needed to maintain the charge balance, which is why the occupancy of electropositive Zn increases in the compound. The asymmetric Sb-In ionic bond forms by the In substituting Sb at the 12c Sb(2) site, which changes the vibration behavior of Sb-Sb dimer and leads to more dynamical disorders of the localized dumbbell vibrations in β-Zn₄Sb₃. These new lattice disorders in β-Zn₄Sb₃, introduced by the substitution of In for Sb, result in a much low lattice thermal conductivity of 0.49 W·m⁻¹·K⁻¹ of Zn₄Sb_2.96In_0.04.