Valence band study of thermoelectric Zintl-phase ${\text{SrZn}}_2{\text{Sb}}_2$ and ${\text{YbZn}}_2{\text{Sb}}_2$: X-ray photoelectron spectroscopy and density functional theory

The electronic structure of ${\text{SrZn}}_2{\text{Sb}}_2$ and ${\text{YbZn}}_2{\text{Sb}}_2$ is investigated using density functional theory and high-resolution x-ray photoemission spectroscopy. Both traditional Perdew-Burke-Ernzerhof and state-of-the-art hybrid Heyd-Scuseria-Ernzerhof functionals have been employed to highlight the importance of proper treatment of exchange-dependent $\text{Zn}3d$ states, $\text{Yb}4f$ states, and band gaps. The role of spin-orbit corrections in light of first-principles transport calculations are discussed and previous claims of ${\text{Yb}}^{3+}$ valence are investigated with the assistance of photoelectron as well as scanning and transmission electron microscopy.

Figure 1

(Color online) Zintl $AB{\mathrm{Sb}}_2$ compound in the $P\overline{3}m1$ space group. The $A$ and $B$ sites are green (darkest) and blue (dark), respectively. The Sb site is orange (light). The tetrahedral-coordinated $B$ site and the tilted rectangular $B\text{-Sb}$ network are illustrated.

Figure 2

(Color online) ${\text{SrZn}}_2\text{Sb}$ DFT projected density of states using the HSE03 functional.

Figure 3

(Color online) (a) ${\text{SrZn}}_2\text{Sb}$ and (b) ${\text{YbZn}}_2{\text{Sb}}_2$ DFT density of states for the HSE03 and the PBE functional and the $\text{PBE}+U$ approach used in Ref. 4. The XPS spectra are also given.

Figure 4

(Color online) (a) ${\text{SrZn}}_2\text{Sb}$ and (b) ${\text{YbZn}}_2{\text{Sb}}_2$ DFT density of states close to the Fermi level. Solid and dashed lines are calculations with HSE03 and PBE functionals, respectively.

Figure 5

(Color online) (a) Top valence XPS spectra of ${\text{YbZn}}_2{\text{Sb}}_2$ for three different ${\text{Ar}}^{+}$ sputtering events. (b) Calculated density of states for the ${\text{Yb}}_2{\text{O}}_3$ oxide using the PBE functional.

Figure 6

(Color online) (a) Peak fitting of $\text{Yb}4d$ spectrum. Three different XPS spectra are included, where AR, 10M, and 1H is sample as received, sputtered for 10 min and 1 h, respectively. Vertical lines highlight the contribution of ${\text{Yb}}^{2+}$. All other peaks are due to the ${\text{Yb}}^{3+}$ contribution. Peak fitting was performed according to previous work (Ref. 34). The background was subtracted according to the Shirley method (Ref. 36). (b) The $\text{Sb}3d$ and $\text{O}1s$ peaks of ${\text{YbZn}}_2{\text{Sb}}_2$ and ${\text{SrZn}}_2{\text{Sb}}_2$.

Figure 7

Bright-field TEM micrograph of an oxygen-rich particle embedded in the matrix. The insets are EDS spectra confirming the increased amount of oxygen in the particle.