Structural and thermoelectric properties of ${\text{AgSbTe}}_2{\text{-AgSbSe}}_2$ pseudobinary system

Structural properties of the ${\text{AgSbTe}}_2{\text{-AgSbSe}}_2$ pseudobinary system were examined using thermal analysis, scanning electron microscopy, and x-ray powder diffractometry. It was found that partial substitution of Te by Se atoms leads to stabilization of the cubic crystal structure of alloys. The electronic-transport properties of materials were measured in order to investigate carrier conduction, band-gap features, and thermoelectric properties. The undoped homogeneous solid solution exhibits extremely low thermal conductivity of $0.5\text{W}{\text{m}}^{-1}{\text{K}}^{-1}$, a very large positive Seebeck coefficient of about $400–600\mu \text{V}{\text{K}}^{-1}$ at room temperature, low carrier densities of ${10}^{16}–{10}^{18}{\text{cm}}^{-3}$, and thermally activated conduction. The influence of alloying on thermal-conductivity mechanisms and electron properties was discussed. The highest experimental dimensionless figure of merit $ZT$ of the undoped ${\text{AgSbSe}}_{0.25}{\text{Te}}_{1.75}$ sample is about 0.65 at a temperature of 520 K. The influence of doping on enhancement of thermoelectric properties of these materials was analyzed and optimal values of transport parameters were estimated.

Figure 1

(Color online) XRD patterns of ${\text{AgSbSe}}_x{\text{Te}}_{2-x}$ samples (series 1). For comparison, XRD patterns of slowly and rapidly (marked with ${}^{\ast }$) cooled ${\text{AgSbTe}}_2$ samples are presented.

Figure 2

(Color online) DSC curves of ${\text{AgSbSe}}_x{\text{Te}}_{2-x}$ samples (heating rate $5\text{deg}{\text{min}}^{-1}$ and sample mass 20/60 mg).

Figure 3

(Color online) Lattice parameter $a$ vs composition $x$ of ${\text{AgSbSe}}_x{\text{Te}}_{2-x}$ samples (estimated errors of regression-line coefficients are calculated at the significance level $p=0.05$).

Figure 4

(Color online) Variation in electrical conductivity $\sigma$ with composition $x$ of ${\text{AgSbSe}}_x{\text{Te}}_{2-x}$ materials at temperatures of 336, 407, and 507 K.

Figure 5

Temperature dependence of the Seebeck coefficient of ${\text{AgSbSe}}_x{\text{Te}}_{2-x}$ (filled markers: heterogeneous materials; hollow markers: homogeneous materials).

Figure 6

(Color online) Temperature dependence of the electrical conductivity of ${\text{AgSbSe}}_x{\text{Te}}_{2-x}$ (filled markers: heterogeneous materials; hollow markers: homogeneous materials).

Figure 7

(Color online) Variation in thermal conductivities $\lambda$, ${\lambda }_{\text{latt.}}$, and ${\lambda }_{\text{el}}$ with composition $x$ of ${\text{AgSbSe}}_x{\text{Te}}_{2-x}$ materials at RT (experimental error 5%).

Figure 8

Temperature dependence of the thermoelectric figure of merit $ZT$ of selected samples (filled markers: heterogeneous materials; hollow markers: homogeneous materials).

Figure 9

(Color online) Calculated theoretical dependences (lines) and experimental values (markers) of $ZT$ parameter on reduced Fermi energy $\eta$ for selected compositions $x$ of ${\text{AgSbSe}}_x{\text{Te}}_{2-x}$ materials $\left(T=300\text{K}\right)$.