Analysis of nanostructuring in high figure-of-merit Ag-_1-x Pb_m SbTe_2+m thermoelectric materials

Thermoelectric materials based on quaternary compounds Ag-_1-x Pb_m SbTe_2+m exhibit high dimensionless figure-of-merit values, ranging from 1.5 to 1.7 at 700 K. The primary factor contributing to the high figure of merit is a low lattice thermal conductivity, achieved through nanostructuring during melt solidification. As a consequence of nucleation and growth of a second phase, coherent nanoscale inclusions form throughout the material, which are believed to result in scattering of acoustic phonons while causing only minimal scattering of charge carriers. Here, characterization of the nanosized inclusions in Ag₀₅₃ Pb₁₈ Sb₁₂ Te₂₀ that shows a strong tendency for crystallographic orientation along the {001} planes, with a high degree of lattice strain at the interface, consistent with a coherent interfacial boundary is reported. The inclusions are enriched in Ag relative to the matrix, and seem to adopt a cubic, 96 atom per unit cell Ag₂ Te phase based on the Ti₂ Ni type structure. In-situ high-temperature synchrotron radiation diffraction studies indicated that the inclusions remain thermally stable to at least 800 K.