STUDIES OF ELECTRONIC STRUCTURE AND THERMAL PROPERTIES OF HEUSLER THERMOELECTRIC MATERIALS

Abstract

Thermoelectricity can address the global energy crisis and climate changes by directly converting the waste heat into electricity. Despite the extensive research, Heusler alloys as potential thermoelectric materials are yet to meet the threshold e ciency for commercialization. Therefore, in addition to improving the existing materials, the search for new potential Heusler alloys is of key importance { the motivation of the thesis. In search of new prospects for thermoelectric materials, using ab initio calculations, semiclassical Boltzmann transport theory, and constant relaxation time approach, the systematic investigation of the ground state properties, electronic structure, and electrical and thermal transport properties of six cobalt-based (CoVSn, CoNbSn, CoTaSn, CoCrIn, CoMoIn, and CoWIn), two iron-based (FeTaSb and FeMnTiSb) Heusler alloys, and a special derivative of half-Heusler alloys LiZnX (X = N, P, As, Sb, and Bi) has been carried out. Further, the polytypism and its impact on transport properties have been questioned in LiZnX family. The investigations on 18 valence electron count cobalt-based systems revealed some interesting features. It was found that all the cobalt-based systems (except metallic CoCrIn) were promising as p-type thermoelectric material and more competitive than the well-known CoTiSb. The proposed doping levels in all the cases are quite pragmatic and could be realized experimentally. This assures that the calculated numbers are not entirely hypothetical but achievable in reality. In particular, the better thermoelectric properties of p-type CoNbSn, as compared to the existing n-type, can be seen as an important implication in this class. This would motivate the experimentalists to consider the hole-doped compositions of CoNbSn. Further, the electronic band structure of CoCrIn indicates the possiblity v vi Abstract of a topological insulator, thus, may interest researchers. In addition to cobalt-based half-Heusler alloys, the two iron-based Heusler alloys, FeTaSb and FeMnTiSb were found to be promising thermoelectric candidates. The 18 valence electron count FeTaSb was expected to show the similar properties as that of FeNbSb on account of similar sizes of Nb and Ta. Indeed, it was found that n-type FeTaSb can actually complement the best performing half-Heusler alloy FeNbSb. Whereas the lowcost FeMnTiSb was found to have comparable thermoelectric properties on either n-type or p-type doping. Thus, the FeMnTiSb can be used as both n-type and p-type legs in a thermoelectric module, which is rare. As 24-valence electron count Heusler alloys have not been much explored in the context of thermoelectricity, the ndings of FeMnTiSb could be an interesting perspective. Most interesting features were obtained in a special derivative of half-Heusler alloys, LiZnX systems. The rst instance of polytypism was recently reported in LiZnSb system. In order to understand the driving force behind polytypism and its possible existence in other Li-based systems with particular emphasis on thermoelectric properties, the LiZnX (X = N, P, As, Sb, and Bi) systems were investigated in cubic and hexagonal symmetry. Interestingly, the hitherto hexagonal LiZnBi was found to be more stable in cubic geometry. Though metallic, the band structure of cubic-LiZnBi indicates the topological properties. Nevertheless, the cubic-hexagonal phase transition was not limited to LiZnBi but could be achieved in other systems on the application of hydrostatic and internal pressure. Further, the transport properties were improved on cubic-hexagonal phase transition in LiZnX family which validates that polytypism may a ect better thermoelectric performance. The most promising system turned out to be LiZnSb as the gure of merit values were quite impressive in both cubic and hexagonal symmetry. Since the required pressure for cubic-hexagonal phase transition in LiZnSb is 4.3 GPa, the cubic-hexagonal phase transition might be possible in realistic application conditions. However, the high e ciency of both cubic and hexagonal LiZnSb ensures that the overall e ciency of the device is not Abstract vii a ected. All in all, some of the ndings of the present work can be seen as the signi - cant contribution toward the thermoelectric research and will motivate researchers for their experimental realization in the quest of materials with the better gure of merit.