Comparison of stability, thermal conductivity and thermoelectric power factor in Heusler alloys CoX′ZrAl with X′=V, Fe, Ir at octahedral site

Abstract

The stability, electronic structures and thermoelectric (TE) properties of quaternary Heusler alloys (CoX${}^{\prime }$ZrAl; X${}^{\prime }$=V, Fe, Ir) are studied in the framework of Density Functional Theory and Boltzmann Transport Theory. All three crystallize in F-43m crystal symmetry and exhibit dynamical stability. The ground state of CoVZrAl, CoFeZrAl and CoIrZrAl are ferromagnetic (FM) semiconductor, non-magnetic semi-metal and FM half-metal respectively. The FM compounds have a mean-field Curie temperature T${}_c\sim$ 670–690 K. The calculated Debye frequency and acoustic phonon velocity are very large in CoFeZrAl indicating strong bonding and phonon-dominated high thermal conductivity ${\kappa }_l\sim$ 41.23 Wm⁻¹K⁻¹ at 200 K. The low electronic carrier density in CoVZrAl at elevated temperatures results in a large Seebeck coefficient and consequently, a high power factor (PF). The steep change in density of states around the Fermi level (D(E${}_F$)) makes PF of CoFeZrAl very high. Large carrier density and not-so-steep D(E${}_F$) in CoIrZrAl renders its Seebeck coefficient and hence its PF smallest. The presence of 5d species at X${}^{\prime }$ site enhances the electronic thermal conductivity to such an extent that the total thermal conductivity increases manifold. Therefore a 3d species at X${}^{\prime }$ site of quaternary XX${}^{\prime }$YZ Heusler alloy gives better TE performance in terms of high PF but the additional demand of overall large TE figure of merit is met by the material with low thermal conductivity. These two conditions are simultaneously satisfied by the material with localized 3d states and weaker covalent bonding as seen in CoVZrAl compared to CoFeZrAl. The ZT value of n-type CoVZrAl is found to reach a high value 1.4 at 600 K.