Origin of pseudo gap and thermoelectric signatures of semimetallic Ru2TaGa: Structural stability from phonon dynamics, mechanical, and thermodynamic predictions

Abstract

We have studied the origin of the semimetallic pseudo gap and the role of site preference in the L21-type Heusler compound Ru2TaGa. A detailed account of the structural stability, phonon-dependent properties, electronic structure, and thermoelectric properties of the host compound is presented herein. Stability assessments have been made on the basis of phonon band structure, formation energy, mechanical criteria, and hull energy calculations. From the calculated density of states (DOS) and band structure, a semimetallic scenario is evident, with the Fermi level trapped in the pseudo gap. HSE calculations provide a decisive picture, with an indirect overlap between electron and hole pockets near the Fermi level of Ru2TaGa being responsible for the formation of a negative or pseudo gap. Comparison of the Ru eg and Ta t2g states reveals the role of d-d hybridization in pseudo gap formation in Ru2TaGa-like compounds. Additionally, we endeavor to discuss the transport coefficients of Ru2TaGa. The room temperature Seebeck coefficient calculated for Ru2TaGa is 66.42 μV/K, considerably larger than those of other semimetallic compounds of the same family. Considering the lattice thermal conductivity (κL = 2.5 W/mK) at 800 K, we obtain a significant figure of merit of ZT = 0.025 in Ru2TaGa. Although the absolute thermoelectric figure of merit (0.002) is relatively low in Ru2TaGa, it is still the highest among the non-doped full Heuslers of the same family hitherto reported.