Atomic Ordering and Gap Formation in Ag-Sb-Based Ternary Chalcogenides

Novel semiconductors with tailored properties can be designed theoretically based on our understanding of the interplay of atomic and electronic structures and the nature of the electronic states near the band-gap region. We discuss here the realization of this idea in Ag-Sb-based ternary chalcogenides, which are important optical phase change and thermoelectric materials. Based on our studies we propose new systems for high-performance thermoelectrics.

Figure 1

Possible ordered structures of $\mathrm{AgSb}{Q}_2$ ($Q=\mathrm{Te}$, Se, S): (a) AF-I (space groups: $Pm\overline{3}m$, $P4/mmm$), (b) AF-IIb ($F\overline{3}dm$), and (c) AF-III ($I{4}_1/amd$). Large balls are for $\mathrm{Ag}/\mathrm{Sb}$; small balls are for $Q$.

Figure 2

Total DOS of $\mathrm{AgSb}{Q}_2$: (a) $Q=\mathrm{Te}$ (AF-I, AF-II, and AF-III), and (b) $Q=\mathrm{Te}$, Se, and S (AF-IIb). The Fermi energy ($E_F$) is at 0 eV; $\mathrm{f}.\mathrm{u}.=\mathrm{AgSb}{Q}_2$.

Figure 3

(a) Band structure (in AF-IIb) and (b) the logarithm of electrical conductivity ($\sigma$, in $\mathrm{S}{\mathrm{cm}}^{-1}$) of ${\mathrm{AgSbTe}}_2$ versus $T^{-1}$; $\sigma$ at low $T$ is also given (inset).