Phase separation and thermoelectric properties of Ag ₂Te-doped PbTe _0.9S _0.1

Spinodal decomposition is an ideal mechanism for producing bulk nanostructured materials with promising thermoelectric (TE) performance. In this contribution, the phase separation and TE properties of PbTe-PbS samples are investigated. Phase separation driven by spinodal decomposition is observed in PbTe _0.4S _0.6, PbTe _0.5S _0.5, PbTe _0.6S _0.4 and (PbTe _0.9S _0.1) _1-x(Ag ₂Te) _x with x = 0, 0.01 and 0.03. The addition of Ag ₂Te leads to a deterioration in electrical transport properties at low temperature but to a significantly enhanced higher-temperature power factor of the Ag ₂Te-doped PbTe _0.9S _0.1 sample. The very low thermal conductivity of the Ag ₂Te-doped sample is attributed to the doping effect of Ag ₂Te, the precipitated Ag ₂Te, and the nanoscale phase segregation driven by spinodal decomposition. In particular, the spinodal decomposition produces finely dispersed PbTe-rich and PbS-rich phases with solute atoms, coherent or semicoherent interfaces, lattice bending, and other lattice defects, which contribute to the phonon scattering and minimize the thermal conductivity. The highest TE figure of merit, ZT, is ∼1.2 at 773 K for the sample with x = 0.03, and even larger ZT values at higher temperature might be expected based on its tendency to increase with the temperature.