Theoretical investigation of novel half Heusler compounds MRhSb (M = Nb & Ta): For optoelectronic and thermoelectric applications

Abstract

The main objective of this work is to make a detailed study on a new class of half heuslers which possess a 19 valence electron and which are sought for perpetually because of their thermoelectric performances. The mechanical, electronic structures, optical, and electrical transport properties are studied using full potential linearized augmented plane wave (LAPW) + local orbitals (lo) scheme, in the framework of density functional theory (DFT) with generalized gradient approximation (GGA) for the purpose of exchange correlation energy functional. The electronic structure is treated by the TB-mBJ exchange-correlation potentials. The independent elastic constants and the related mechanical properties are investigated. From the energy bands and density of states it is observed that the 3d-states of Nb, Ta, and Rh atoms contribute mainly to the conduction band, which results in increase in electrical and thermal conductivity of NbRhSb and TaRhSb . The optical constants as the dielectric function, refractive index, optical reflectivity, and absorption coefficient were calculated and discussed in detail. The dependence of Seebeck coefficient, electrical conductivity, and power factor on the Fermi level is investigated.electronic structure, half-Heusler alloys, optical properties, thermoelectric transport properties | INTRODUCTIONThermoelectricity is proposed as one of the possible ways to address energy problems (the energy crisis and the environmental impact) [1-3]. For this reason, research in recent decades is concentrated on the thermoelectric half-Heusler materials, since they possess a wide variety of interesting physical properties [4][5][6][7][8]. The advantage of these materials is to convert heat directly into electricity without any other means. For reaching such material it is necessary that the material bears a maximum value of figure of merit ZT (At this time, the best reliable maximum thermoelectric figure of merit ZT max for MNiSn-related compounds is about .7 [9] with one reported ZT max >1 [10]). Generally, half-Heusler materials are considered to be promising thermoelectric materials due to their high temperature stability and because of their relatively large Seebeck coefficient and electrical conductivity, and therefore, high power factors. Among the half-Heusler alloys which possess a maximum power factor and which are determined experimentally, the alloys based on TiCoSb, TiFe x Co 1-x Sb [11], and the ZrNiSn-based alloys, [9] their power factor are 23 μW/cmK 2 at 850 K and 34 μW/cmK 2 at 750 K, respectively. Based on the first-principles calculations J. Yang et al [12] concludes that the HHs of Co, Rh, Fe, and LaPdBi are good the thermoelectric materials due their maximum power factors. On purely experimental techniques Lkhagvasuren et al