Precipitation of Ag2Te in the thermoelectric material AgSbTe2

The microstructure of AgSbTe2, prepared by solidification, is investigated using electron microscopy. During solidification and thermal treatment, the material separates into a two-phase mixture of a rocksalt phase, which is Ag22Sb28Te50, and silver telluride, Ag2Te. Ag2Te formation results either from eutectic solidification (large lamellar structures), or by solid-state precipitation (fine-scale particles). The crystal structure of the AgSbTe2 phase determined by electron diffraction is consistent with a rocksalt structure that has a disordered cation sublattice. A preferred crystallographic orientation relationship at the interface between the matrix and the low-temperature monoclinic Ag2Te phase is defined and discussed. This orientation relationship is observed for both second-phase morphologies. In both cases, the orientation relationship originates from a topotactic (cube-on-cube) alignment of the Te sublattices in the initially cubic Ag2Te and the matrix at elevated temperature. This Te sublattice alignment is retained as the Ag2Te undergoes a cubic-to-monoclinic transformation during cooling. This orientation relationship is observed for both second-phase morphologies.