Accurate Quantification of Vacancies in VCoSb Using Elemental Standards

Session

EMAG - EM Spectroscopy

Authors

Ben Smith (1), Dr Donald Maclaren (1)

Affiliations

1. University of Glasgow

Keywords

Electron energy loss spectroscopy

EELS

Vacancies

Transmission electron microscopy 

Half-Heuslers

Heuslers

Thermoelectrics

Abstract text

Thermoelectric (TE) materials have a potential to help reduce fossil fuel-based power generation, in addition to replacing harmful freon-based refrigerants. (1, 2)  A class of structures being studied in relation to their TE properties is half-Heusler alloys, within which the nominal 19-electron half-Heuslers subset have been found to exhibit enhanced TE performance. These materials contain defective phases including vacancy clusters that are thought to scatter phonons, decreasing the material’s thermal conductivity and improving its TE behaviour. (3) These vacancies effectively alter the 19-electron configuration to an 18-electron structure.  Key to understanding how the TE performance of half-Heuslers might be improved is accurate elemental quantification, in particular the ability to accurately quantify vacancy concentrations and their distribution throughout the alloy.

An example of a half-Heusler that shows promising TE performance is VCoSb. (4) Controlling the presence of V vacancies is known to be key to enhancing TE performance. Our previous work outlined evidence that suggests these V vacancies exist as octahedral clusters spaced aperiodically throughout the crystal lattice. Accurately quantifying these vacancies by electron energy loss spectroscopy (EELS) analysis is challenging: determining an accurate V concentration is difficult using standard methods due to the delayed onset Sb M4,5 peak which overlaps with the V L2,3 peaks significantly. The use of elemental standards to produce experimental cross-section has been shown to be reliable in other systems with overlapping EELS edges. (5)

Results of transmission electron microscopy simulations will be presented whereby the detection limits for the VCoSb system were explored. These results outline the difficulty in detecting low concentrations of vacancies and the maximum thickness needed to detect single vacancies in the crystal lattice. A workflow for more accurate EELS analysis of VCoSb is presented: using a focussed ion beam sub-micron needles of V, Co and Sb elemental standards are made. Measuring the thickness of these needles enables experimental mean free path (mfp) values to be calculated for the three elements. Utilising the mfp values, the thickness of the needles and the zero-loss peak intensity allows the production of experimental cross sections from the high-loss spectrum of each element. We will present the comparison of the fitting of these experimental cross sections to the fitting of standard theoretical cross sections to demonstrate that greater accuracy in quantification of alloys with similar overlapping EELS edges can be achieved using this method. 

Alongside providing a better understanding of vacancy contributions and distribution in VCoSb, the processes outlined here provide a reliable workflow for EELS analysis of systems with similarly challenging EELS edge overlaps. 

References

1.         Bell LE. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science. 2008;321(5895):1457-61.

2.         He J, Tritt TM. Advances in thermoelectric materials research: Looking back and moving forward. Science. 2017;357(6358):9.

3.         Jeong H, Kihoi SK, Kahiu JN, Kim H, Ryu J, Lee KH, et al. Origin of low thermal conductivity in Nb1-xTixFe1.02Sb half-Heusler thermoelectric materials. Journal of the European Ceramic Society. 2021;41(7):4175-81.

4.         Huang LH, Wang JC, Mo XB, Lei XB, Ma SD, Wang C, et al. Improving the Thermoelectric Properties of the Half-Heusler Compound VCoSb by Vanadium Vacancy. Materials. 2019;12(10).

5.         Bobynko J, MacLaren I, Craven AJ. Spectrum imaging of complex nanostructures using DualEELS: I. digital extraction replicas. Ultramicroscopy. 2015;149:9-20.