Screening of thermoelectric performance of half-Heusler materials and their alloys from first principles

Abstract

Thermoelectric (TE) generators have seen a lot of renewed interest over the last years. Along with increasing computational power, the approach in the search for new TE materials has seen a shift to first principle calculations. This study employs a highthroughput-like method for first principle calculations on half-Heusler (HH) materials. Based on the high-throughput (HT) result a subgroup is then selected for volumetric band alignment (VBA). VBA approximates alloying by varying the volume of the crystal structure. Since this approximation can be performed on primitive cells it is computationally inexpensive and can be applied to higher numbers of materials than more expensive supercell calculations. By combining HT screening with VBA this study develops a way to search a large group of materials, but also optimize the thermoelectric figure of merit (ZT) by alloying. Finally, supercell calculations are performed to give an indication of the accuracy of the VBA approximation. From the first set of calculations for 67 materials, several showed good TE properties. LaRhTe was found to have both a very low thermal conductivity and a reasonably good power factor giving it a predicted ZT = 2.7, for its relaxed structure. Out of the 67 materials, 11 were chosen for VBA, some based on high ZT and others based on promising features in the band structure. The volume of the 11 materials was adjusted by changing the lattice constant up and down 5% and new first principle calculations were performed. The VBA showed promising results for some compounds. As the volume changed, materials such as LiZnSb and LaPtSb had an impressive increase in ZT, up to as high as 3.5 for the latter. In general, the only materials that did not show an increase in ZT were materials where two or more band valleys were already well aligned. The result from VBA was attempted to be verified by supercell calculations. The unfolded band structures showed a similar trend at the Γ-point compared to the VBA calculations, but the results were inconclusive.

Termoelektriske (TE) generatorer har sett økende interesse de siste˚arene. Med økende datakraft har tilnærmingen i søket etter TE-materialer skiftet mot førsteprinsipp beregninger. Denne studien benyttes en høy-gjennomstrømsmetode for førsteprinsipp beregninger p˚a halv-Heusler-materialer. Basert p˚a resultatene ble en undergruppe valgt til volumetrisk b˚and-tilpasning (VBA). VBA approksimerer legeringer ved ˚a variere volumet til krystall-strukturen. Siden denne approksimasjonen kan utføres p˚a primitive celler er den beregnings-messig billig og kan bli utført p˚a høyere antall materialer enn dyrere supercelleberegninger. Ved ˚a kombinere disse to metodene utvikler denne studien en m˚ate ˚a søke gjennom et stort antall materialer og samtidig optimere det termoelektriske verdien (ZT) ved legering. Til slutt ble supercelleberegninger utført for ˚a gi en indikasjon p˚a nøyaktigheten til VBA-approksimasjonen. Fra de første beregningene p˚a 67 materialer viste flere gode TE-egenskaper. LaRhTe ble funnet til˚a ha b˚ade lav termisk ledningsevne og høy effektfaktor, noe som resulterer i en beregenet ZT p˚a 2.7 for den relakserte krystallstrukturen. Av de 67 materialene ble 11 valgt til VBA, noen basert p˚a høy ZT, andre p˚a grunn av lovende trekk i b˚andstrukturen. Volumet til de 11 utvalgte materialene ble endre ved˚a justere opp og ned gitterkonstanten 5%, s˚a ble nye førsteprinsippsberegninger utført. Den volumetriske b˚and-tilpasningen viste lovende resultater. LaPtSb og LiZnSb viste begge stor økning i ZT, opp til s˚a mye som 3.5 for LaPtSb. Generelt viste alle materialer en økning i ZT med mindre to eller flere b˚and allerede var godt tilpasset i energiniv˚a. Resultatene fra VBA ble forsøkt verifisert med supercelleberegninger. B˚andstrukturene viste likhetstrekk til VBA-beregningene rund Γ-punktet, men dette resultatet ble ansett til˚a være usikkert.