Topological Chemistry in Ternary Compunds

Project Information

TOPCHEM

Grant agreement ID: 839821

DOI

10.3030/839821

Project closed

EC signature date 11 April 2019

Start date 16 September 2019

End date 15 September 2021

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 162 806,40

EU contribution

€ 162 806,40

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 1 - TOPCHEM (Topological Chemistry in Ternary Compunds)

Reporting period: 2019-09-16 to 2021-09-15

Summary of the context and overall objectives of the project

Since the description of topological electron states in graphene, new emergent physical phenomena found in topological materials have intensively attracted the attention of the scientific community. These are of special interest as they enable new technological applications in quantum computing and spintronics. For example, topological insulators can conduct electrons on the surface but are insulating in their interior. Other topological materials may display quantised conduction, which would allow for more efficient and powerful computer processing units. The main differences between topological materials and trivial ones lay in the energy-dependence of electrons in momentum and real space, which determines their behaviour in the material. This field is mainly driven by theoretical predictions and calculations, being the experimental realization of topological features scarcer. A different class of materials, thermoelectrics, allow the direct transformation of heat currents into usable electrical energy and vice versa, thanks to the thermoelectric effects, known as Seebeck, Peltier, and Thomson effects. This special feature can tackle issues such as climate change and sustainable energy production by waste heat recovery. Interestingly, some iconic topological materials have shown high thermoelectric performance as well, which can be attributed to several factors that relate these two fields with the electronic structure of the materials. Still, this relationship has only been just explored.

This project targets the experimental study of thermoelectric and topological materials to investigate the connection between their transport properties, chemical and crystallographic features with the electronic structure. As such, the main objective of the action is to prepare new topological and thermoelectric materials, of which the atomic crystal and electronic structure will be analysed by different diffraction and spectroscopic techniques. Then, these will be tuned to optimise the thermoelectric properties. This sheds light on the interaction between the materials’ performance, chemical and physical properties, providing a better understanding of thermoelectric efficiency in topological materials and moving the field closer to technological applications. Thus, the implementation of this project contributes to the creation of a single market for knowledge, research, and innovation within EU Horizon 2020 goals.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project is threefold in a traditional manner: synthesis, characterisation, and optimisation. The first step of the project consisted in the preparation of materials whose properties could provide novel results in the topological and thermoelectric field. This has been achieved by the synthesis of high-quality single-crystals by different growth techniques, such as flux and Bridgman methods. The half-Heusler alloys TiCoSb and NbCoSn, which had shown promising thermoelectric properties, were grown as single crystals for the first time. New materials such as Yb1.8Cu9.2Sn2.8 Fe3GeTe2, CoSbSe and Co1-xFexSn, and well-known (Bi,Sb)2(Se,Te)3 were also synthesised as single crystals. The second step focused on the transport properties and the atomic structure of the different crystals. Temperature and field evolution of electrical resistivity, Seebeck coefficient and thermal conductivity have been measured, as well as magnetic moment and susceptibility. These have been complemented with specialised diffraction and spectroscopic studies, as neutron powder diffraction and angle-resolved photoelectron spectroscopy, which have allowed a detailed analysis of the crystallographic and electronic structure. Then, the third step dealt with the optimisation by doping and modified synthesis conditions of selected materials, to study their thermal transport with their topological properties and enhance their thermoelectric performance.

Dissemination of the results has been carried out as two high-impact factor publications in peer-reviewed journals on the properties of the half-Heusler NbCoSn. Furthermore, additional articles reporting on the other materials investigated in the project are close to completion and will be published in peer-reviewed journals. These results have also been presented in different conferences, as the Online-Conference on Inorganic Chemistry 2020 (GDCh) and the Virtual Conference on Thermoelectrics VCT2020, and have been shared with scientific colleagues working on similar fields in online workshops and seminars. This work provides a deeper understanding of the characteristics of topological and thermoelectric materials and will help other scientists and projects in their research for the development of technological applications.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

This project provides a broader panorama of new materials or novel single crystals of known materials, which display topological and thermoelectric properties. Through the project implementation, innovative crystal growth of NbCoSn, TiCoSb, Yb1.8Cu9.2Sn2.8 and CoSbSe has been realised, which enhances the capabilities for further crystal growth of related materials based on the methods employed. Then, investigation of the thermal transport and crystallographic structure has yielded new valuable information on the origin of their interesting properties. The studies on NbCoSn revealed the dominant character of microscale features in thermoelectric performance. The acquired knowledge on the electron mobility dependence with inter-grain energy barriers and how the heavy-element dopant distribution affects thermal transport will benefit other studies on half-Heusler compounds. The experimental study on the electronic structure of TiCoSb unveils the contribution of the valence band features to its transport properties. These, such as heavy electron mass and band convergence, can be used to further improve the state-of-the-art in thermoelectrics, providing a more straightforward path to optimise the thermal transport of this family of materials. The rare-earth derivative Yb1.8Cu9.2Sn2.8 is a promising candidate for further thermoelectric studies which displays surprisingly low values of thermal conductivity. The results on Fe3GeTe2 provide a more comprehensive understanding of its magnetic structure, which is very promising for spintronic applications. The topological materials, (Bi,Sb)2(Se,Te)3 and Co1-xFexSn have displayed interesting magnetic-field-dependent thermopower correlated to the quantisation of electron states in topological systems. These results have an impact on the possible implementation into devices, either for quantum computing or thermoelectricity, which ultimately can be used in waste-heat recovery, contributing to the climate challenges.

Project overview