Tuneable local order in thermoelectric crystals

TL;DR

This study reveals how local atomic arrangements in thermoelectric crystals can be tuned through synthesis methods, affecting material properties despite similar average structures.

Contribution

It demonstrates that local order in Nb1-xCoSb can be controlled via synthesis, with vacancy distribution following a repulsion model, offering new ways to tune thermoelectric properties.

Findings

Composition remains near x=1/6 regardless of nominal stoichiometry.

Synthesis method influences local vacancy arrangements.

Vacancy distribution follows a repulsion model.

Abstract

Although crystalline solids are characterized by their periodic structures, some are only periodic on average and deviate on a local scale. Distinct local orderings can exist with identical periodic structures making their differences hidden to normal diffraction methods. Using x-ray scattering we investigate the thermoelectric half-Heusler, Nb1-xCoSb, with high vacancy concentrations, where signs of local order have been attributed to differences in stoichiometry. We show that the composition is actually always very close to x = 1/6 irrespective of nominal sample composition, and that the synthesis method controls the local order. The vacancy distribution is shown to follow a vacancy repulsion model, and local structural relaxations around the vacancies are characterized. Control over the local structure provides a new frontier for tuning material properties.