Structural evolution and thermoelectric performance in (GeTe)m(Sb2Te3)n compounds

Exploring the relationship between crystal structure and thermoelectric performance is a pivotal topic in the thermoelectric field. In this study, we have comprehensively investigated the correlation between the structural evolution of (GeTe)_m(Sb₂Te₃)_n pseudo-binary system and the thermoelectric properties. The proportion of van der Waals bonds increases with the rising Sb₂Te₃ content, resulting in an increase in the anisotropy of the electrical conductivity and a decrease in the average sound velocity. Additionally, the cation sites in the crystal lattice of these compounds exhibit a mixed occupancy of Ge/Sb atoms, although the cation sites adjacent to the van der Waals gaps are predominantly occupied by Sb atoms. The ultra-low lattice thermal conductivity of the GST124 and GST147 compounds is mainly attributed to the high concentration of van der Waals bonds and enhanced phonon scattering arising from Ge/Sb mixed cation occupancy and high density of defect structures. The high electrical conductivity combined with the low lattice thermal conductivity enables GST124 and GST147 compounds to achieve a maximum ZT value of 0.56 and 0.57, respectively. Higher thermoelectric performance can be achieved through optimization of the microstructure as well as the carrier concentration.