Correlated local dipoles in PbTe

TL;DR

This study combines experimental diffuse scattering and ab-initio simulations to reveal correlated local dipoles and symmetry breaking in PbTe, clarifying previous controversies and linking local structural dynamics to its thermoelectric properties.

Contribution

The paper demonstrates a detailed analysis of local dipole correlations in PbTe using combined experimental and computational methods, resolving debates on off-centering phenomena.

Findings

Discovery of correlated local dipoles extending over several unit cells

Clarification that average ion positions remain centered despite local symmetry breaking

Linking local structural dynamics to PbTe's thermoelectric performance

Abstract

We present a combined single-crystal x-ray diffuse scattering and ab-initio molecular dynamics study of lead telluride, PbTe. Well-known for its thermoelectric and narrow-gap semiconducting properties, PbTe recently achieved further notoriety following the report of an unusual off-centering of the lead atoms, accompanied by a local symmetry breaking, on heating. This observation, which was named emphanisis, ignited considerable controversy regarding the details of the underlying local structure and the appropriate interpretation of the total scattering experiments. In this study, we demonstrate close agreement between our diffuse scattering measurements and our calculations, which allows us to analyze features such as higher-order correlations that are accessible in the simulations but not experimentally. This allowed us to discover an unusual correlated local dipole formation extending over several unit cells with an associated local reduction of the cubic symmetry in both our x-ray diffuse scattering measurements and our molecular dynamics simulations. Importantly, when averaged spatially or temporally, the most probable positions for the ions are at the centers of their coordination polyhedra. Our results therefore clarify the nature of the local symmetry breaking, and reveal the source of the earlier controversy regarding the existence or absence of off-centering. Finally, we provide an interpretation of the behavior in terms of coupled soft optical and acoustic modes, which is linked also to the high thermoelectric performance of PbTe.