Seminarium Oddziału Badań Magnetyków
Microsoft Teams
Thermoelectric performance and peculiarities of structural disorder in rare-earth bearing half-Heusler phases
mgr Kamil Ciesielski
Oddział Badań Magnetyków INTiBS PAN we Wrocławiu (oraz z Colorado School of Mines)
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Streszczenie:
Half-Heusler (HH) phases are excellent thermoelectric materials recognized for severe crystallographic disorder. The talk presents comprehensive studies of underexplored HH subgroup based on rare-earth (RE) elements. It is shown that RE HH compounds possess unprecedented type of atomic imperfections: split positions of late transition metal from nominal Wyckoff site 4c(¼, ¼, ¼) to quarterly occupied slot 16e(x, x, x), x = 0.256-0.260. Significant amount of atomic defects results in Altshuler-Aronov correction to electrical resistivity below 25 K, and anomalous linear-in-T resistivity decreasing with rising temperature, which can be described with Cote-Meisel model (50-300 K). From perspective of phononic properties, lanthanide HH compounds were previously reported to exhibit the smallest thermal conductivity among the whole half-Heusler family, yet origin of this feature was not known. We show, that the peculiarity stems from combination of low Debye temperature and the lowest speed of sound for RE HH among any half-Heusler phases, additionally supported by severe scattering of phonons on point defects. We also demonstrate, that analysis of bipolar thermal conductivity might deliver important hints for optimization of thermoelectric performance. Using its product – mobility ratio between electrons and holes – it is possible to pave the correct direction of doping and choose the most favorable compound from the series. For all studied RE HH phases, we predict high mobility ratio in favor of electrons. The prognosis was verified experimentally by n-type doping in ScNiSb1-xTex. High mobility and strongly anisotropic conduction band allowing for enhanced effective masses, led to excellent thermoelectric performance in ScNiSb0.85Te0.15. Based on the results of our thermal transport measurements and electronic band structures calculations, and our prediction of very large charge mobility ratio in n-type doped RE HH compounds, we expect that some representatives of this series may prove to be the most efficient thermoelectrics for the moderate temperature range.
