Role of R-f states on electronic structure and thermoelectric performance of RNiSb (R = Gd, Er and Lu) half Heusler compounds: narrow gap thermoelectric materials.

We combine the first-principle calculations, based on the density functional theory, and Boltzmann transport theory to investigate the structural, electronic and thermoelectric of narrow gap RNiSb (R = Gd, Er and Lu) half Heusler (HH) compounds. Coulomb corrected generalized gradient approximation (GGA + U) has been employed which is the most competent method to treat rare earth compound. We find spin up/down Ni-3d and spin down R-4f states as main contributors to the total density of states (DOS) near Fermi energy (E_F). All the investigated compounds exhibit the indirect narrow energy gaps (0.246–0.279 eV) that are in agreement with the corresponding experimental values. We obtained fairly high values of the Seebeck coefficient (S) in agreement with available experimental data. The calculated value of total S (from spin-up and spin-down channels), using two current models, increases from 50 to 202 µV/K at 300 K when Gd with half-filled f-states is replaced by Lu with completely filled f-states in these RNiSb HH compounds. We also find the maximum value of ZT is 0.32 and 0.76 at 800 K for Gd and Er compounds, whereas 0.68 is the maximum ZT for Lu compound at 450 K. We obtained favorable and stable ZT values (ZT > 0.7) for a wide temperature range only in ErNiSb compound in agreement with the experiment. This proposes ErNiSb as a promising candidate for thermoelectric applications. [ABSTRACT FROM AUTHOR]