In situ characterisation of nanostructured multiphase thermoelectric materials at elevated temperatures.

Multiphase thermoelectric materials have recently attracted considerable attention due to the high thermoelectric efficiencies which can be achieved in these compounds compared to their single-phase counterparts. However, there is very little known on the structural evolution of these phases as a function of temperature. In this work we performed an in situ high temperature structural characterisation of recently reported high efficiency p-type multiphase (PbTe)0.65 (PbS)0.25 (PbSe)0.1 compounds by hot stage transmission electron microscopy and high-resolution neutron powder diffraction. We observed the microstructural evolution of precipitates and determined the lattice parameters of phases as a function of temperature for materials, which have been heavily and lightly doped with sodium. The role of the sodium is to optimize the concentration of charge carriers. It has been shown to distribute heterogeneously between the phases in multiphase compounds. The dissolution of secondary phases is found to occur at elevated temperatures. Although sodium concentration produces no significant differences between the lattice constants of the phases and the dissolution sequence of precipitates, it affects quite significantly the kinetics of precipitation. The heavily doped samples reach structural thermodynamic equilibrium more quickly than the lightly doped compound. These results are a step forward in designing high performance multiphase thermoelectric materials.