A simple model for vacancy order and disorder in defective half-Heusler systems.

Nikolaj Roth,Tiejun Zhu,Bo B Iversen

doi:10.1107/s2052252520005977

Abstract

Defective half-Heusler systems X 1-x YZ with large amounts of intrinsic vacancies, such as Nb1-x CoSb, Ti1-x NiSb and V1-x CoSb, are a group of promising thermoelectric materials. Even with high vacancy concentrations they maintain the average half-Heusler crystal structure. These systems show high electrical conductivity but low thermal conductivity arising from an ordered YZ substructure, which conducts electrons, while the large amounts of vacancies in the X substructure effectively scatters phonons. Using electron scattering, it was recently observed that, in addition to Bragg diffraction from the average cubic half-Heusler structure, some of these samples show broad diffuse scattering indicating short-range vacancy order, while other samples show sharp additional peaks indicating long-range vacancy ordering. Here it is shown that both the short- and long-range ordering can be explained using the same simple model, which assumes that vacancies in the X substructure avoid each other. The samples showing long-range vacancy order are in agreement with the predicted ground state of the model, while short-range order samples are quenched high-temperature states of the system. A previous study showed that changes in sample stoichiometry affect whether the short- or long-range vacancy structure is obtained, but the present model suggests that thermal treatment of samples should allow controlling the degree of vacancy order, and thereby the thermal conductivity, without changes in composition. This is important as the composition also dictates the amount of electrical carriers. Independent control of electrical carrier concentration and degree of vacancy order should allow further improvements in the thermoelectric properties of these systems.

Highlights

To combat climate change while supplying a growing world population with affordable and clean energy, improvements in sustainable energy production are needed (UN General Assembly, 2015)
The 0KL plane shows additional peaks corresponding to those Xia et al observed for their Nb0.81CoSb sample (Xia et al, 2019), without the additional diffuse scattering they observed
Scattering from samples with nominal stoichiometry Nb0.83CoSb and Nb0.84CoSb [measured to Nb0.81CoSb and Nb0.82CoSb (Xia et al, 2018)] match the model scattering in the region 1/4 > x > 1/6 with multiple domains of the same type oriented along different directions, which are equivalent in the average cubic structure, as shown in the second column of Fig. 5

Summary

Introduction

To combat climate change while supplying a growing world population with affordable and clean energy, improvements in sustainable energy production are needed (UN General Assembly, 2015). Thermoelectric materials must have a high electrical conductivity together with a low thermal conductivity (Snyder & Toberer, 2008). High electrical conductivities are usually found in highly ordered crystalline materials, while low thermal conductivities are usually associated with disordered or amorphous materials. One way to overcome this apparent paradox is through materials that have a long-range crystalline-ordered substructure with a disordered substructure. The ordered substructure should be responsible for the conduction of electrons while the disordered substructure will efficiently scatter phonons. Such materials are typically understood using the phonon-glass electron crystal concept

Methods

Results

Discussion

Conclusion