A simple model for vacancy order and disorder in defective half-Heusler systems
Defective half-Heusler systems X1−xYZ with large amounts of intrinsic vacancies, such as Nb1−xCoSb, Ti1−xNiSb and V1−xCoSb, are a group of promising thermoelectric materials. Even with high vacancy concentrations they maintain the average half-Heusler crystal structure. These systems show high elect...
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International Union of Crystallography
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doaj-e6a7c03b85c149b3b12e4440d9c0e6682020-11-25T03:36:42ZengInternational Union of CrystallographyIUCrJ2052-25252020-07-017467368010.1107/S2052252520005977fc5045A simple model for vacancy order and disorder in defective half-Heusler systemsNikolaj Roth0Tiejun Zhu1Bo B. Iversen2Center for Materials Crystallography, Department of Chemistry, Aarhus University, Aarhus 8000, DenmarkState Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of ChinaCenter for Materials Crystallography, Department of Chemistry, Aarhus University, Aarhus 8000, DenmarkDefective half-Heusler systems X1−xYZ with large amounts of intrinsic vacancies, such as Nb1−xCoSb, Ti1−xNiSb and V1−xCoSb, 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.http://scripts.iucr.org/cgi-bin/paper?S2052252520005977defective half-heuslersdiffuse scatteringshort-range ordercorrelated disorderthermoelectricsinorganic materialsmaterials modelingproperties of solids |
collection |
DOAJ |
language |
English |
format |
Article |
sources |
DOAJ |
author |
Nikolaj Roth Tiejun Zhu Bo B. Iversen |
spellingShingle |
Nikolaj Roth Tiejun Zhu Bo B. Iversen A simple model for vacancy order and disorder in defective half-Heusler systems IUCrJ defective half-heuslers diffuse scattering short-range order correlated disorder thermoelectrics inorganic materials materials modeling properties of solids |
author_facet |
Nikolaj Roth Tiejun Zhu Bo B. Iversen |
author_sort |
Nikolaj Roth |
title |
A simple model for vacancy order and disorder in defective half-Heusler systems |
title_short |
A simple model for vacancy order and disorder in defective half-Heusler systems |
title_full |
A simple model for vacancy order and disorder in defective half-Heusler systems |
title_fullStr |
A simple model for vacancy order and disorder in defective half-Heusler systems |
title_full_unstemmed |
A simple model for vacancy order and disorder in defective half-Heusler systems |
title_sort |
simple model for vacancy order and disorder in defective half-heusler systems |
publisher |
International Union of Crystallography |
series |
IUCrJ |
issn |
2052-2525 |
publishDate |
2020-07-01 |
description |
Defective half-Heusler systems X1−xYZ with large amounts of intrinsic vacancies, such as Nb1−xCoSb, Ti1−xNiSb and V1−xCoSb, 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. |
topic |
defective half-heuslers diffuse scattering short-range order correlated disorder thermoelectrics inorganic materials materials modeling properties of solids |
url |
http://scripts.iucr.org/cgi-bin/paper?S2052252520005977 |
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