Solid-state precipitation of stable and metastable layered compounds in thermoelectric AgSbTe2

The precipitation of Sb2Te3 in Sb-rich AgSbTe2 is studied by X-ray diffraction and electron microscopy. The results indicate that Sb2Te3 does not form directly, but rather through the precipitation of an intermediate metastable phase. Diffraction, energy-dispersive spectroscopy, and high-resolution transmission electron microscopy indicate that this intermediate phase has a nominal composition (Ag, Sb)(3)Te-4 and a structure with a seven-layer stacking sequence rather than a five-layer one as in Sb2Te3. Two mechanisms based on experimental observations are proposed for the conversion of (Sb, Ag)(3)Te-4 to Sb2Te3: evaporation-condensation and individual step motion. The microstructural evolution and mechanisms of the transformation are discussed in detail.