Tuneable local order in thermoelectric crystals.

Nikolaj Roth,Kaiyang Xia,Jonas Beyer,Tiejun Zhu,Bo B Iversen,Karl F F Fischer

doi:10.1107/s2052252521005479

Abstract

Although crystalline solids are characterized by their periodic structures, some are only periodic on average and deviate on a local scale. Such disordered crystals with distinct local structures have unique properties arising from both collective and localized behaviour. Different local orderings can exist with identical average structures, making their differences hidden to Bragg diffraction methods. Using high-quality single-crystal X-ray diffuse scattering the local order in thermoelectric half-Heusler Nb1-x CoSb is investigated, for which different local orderings are observed. It is shown that the vacancy distribution follows a vacancy repulsion model and the crystal composition is found always to be close to x = 1/6 irrespective of nominal sample composition. However, the specific synthesis method controls the local order and thereby the thermoelectric properties thus providing a new frontier for tuning material properties.

Highlights

Crystalline solids are typically understood as being periodic on the atomic scale, and are described by a repeating unit cell in three dimensions
Using diffuse synchrotron X-ray scattering data measured on single crystals, we first validate the theoretical vacancy repulsion model, and we model the structural relaxation around the vacancies to provide direct experimental quantification of the local structure in these systems
The optimal ordering of vacancies essentially fixes the stoichiometry of the samples irrespective of the nominal starting composition or synthesis method

Summary

Introduction

Crystalline solids are typically understood as being periodic on the atomic scale, and are described by a repeating unit cell in three dimensions. With such an ordered arrangement of atoms, knowledge of the atomic configuration in one part of the crystal will perfectly predict the positions of all other atoms. Between the two extremes we find disordered crystals, which on average can be described by an ordered periodic structure, but with deviations on a local scale. These can have more complex properties arising from both collective and localized behaviour. The local deviations give rise to weak diffuse scattering, which is more difficult to measure and especially to interpret on a structural basis (Keen & Goodwin, 2015; Krogstad et al, 2020; Simonov et al, 2020)

Results

Discussion

Conclusion