Formation of metastable π phase in mechanically alloyed tellurium-rich Ag–Te alloys

This paper reports on the formation of metastable $\pi$ phase on mechanical alloying of elemental Ag and Te powders in the composition range of 50 to 75 at.% Te. Contrary to the reported results in vapor-deposited thin films, no amorphous phase could be detected during mechanical alloying. The extent to which the $\pi$ phase forms on milling is restricted, compared to rapid solidification. Formation of the metastable $\pi$ phase coincides with the achievement of nanometric grain size and is preceded by the formation of intermetallic compound $Ag_2$Te. An approximate estimation of the free energies of the competing phases has been attempted to provide insight into the phase selection process.It is suggested that tellurium diffusion through the nanoscale grain boundaries plays an important role in the formation of the metastable $\pi$ phase.