Tailoring Mechanical Stability and Electronic Structure in TiNiSn-Based Half-Heusler Alloys via Sn-Site Doping: A DFT Study

Abstract

Half-Heusler compounds are promising quantum materials for thermoelectric applications due to their excellent electronic properties, including semiconductivity, thermoelectricity and topological insulating behavior. However, optimizing their mechanical stability and optical phenomena remains a challenge. This study employs density functional theory (DFT) with the generalized gradient approximation (GGA-PBE) for the exchange-correlation functional to characterize the novel quaternary Half-Heusler alloys ANiSn_0.5M_0.5 (A = Zr, Ti; M = Si, Ge, Sb). The structural, electronic, thermodynamic, mechanical and optical properties were systematically analyzed. Although the compositions via fractional substitution are unexplored, this investigation reported a comparison analysis alongside prior articles on related samples. Surprisingly, structural analysis suggests that the samples exhibit a cubic to tetragonal transition, while elastic constants confirm that TiNiSn_0.5Si_0.5 and TiNiSn_0.5Sb_0.5 are highly mechanically stable materials. Moreover, Pugh's ratio of 2.01 (for TiNiSn_0.5Si_0.5) and 3.25 (for TiNiSn_0.5Sb_0.5), alongside Debye temperatures of 360.02 and 231.43 K, respectively, were obtained, indicating a ductile character and strong phonon scattering and potentially low lattice thermal conductivity. Electronic structure analysis shows that Sb doping introduces metallic behavior, while Si/Ge alloying maintains semiconducting character. Furthermore, optical behavior reveals higher absorption at the visible-lower UV regions, fluctuated reflectivity around 0.5 and a lower level of conductivity and dielectric constant. While fractional Sn-site substitution strategies have precedent in related half-Heusler families, the specific ANiSn_0.5M_0.5 compositions examined here, together with their combined electronic, mechanical, and optical characterization, have not been previously reported, and this investigation situates the results within the context of prior work on related compounds.