Speaker
Description
The group IV-VI chalcogenides have important thermoelectric applications. GeTe has emerged as a promising non-toxic candidate, especially when the high-temperature cubic phase is suppressed to room temperature. However, even the mechanism of phase transition is disputed, as is the presence of disorder. Here we apply ab-initio MD, synchrotron X-ray and dynamic neutron pair distribution function (PDF) analysis to GeTe. We show that previous reports of disorder and symmetry breaking are entirely due to highly damped and anharmonic phonons. As predicted by metavalent bonding theories, this arises due to a softening in local bonding on heating, which strengthens long-range <100>c correlations. This picture is consistent with reported changes in resistivity and dielectric constant, and shown to be ubiquitously present in other polarisable binary chalcogenides and hR6 structured elements. Our results unify the results of local probes and spectroscopy as applied to binary chalcogenides, and should inspire a re-examination of other highly anharmonic energy materials such as hybrid perovskites and ’rattling’ thermoelectrics.
