Ab Initio Exploration of Promising Double Half-Heusler Ti ₂ AlY ₂ Sb (Y=Pd, Pt) Alloys: Unveiling Stability, Electronic, Mechanical, and Superior Thermoelectric Performance

Double half-Heusler (DHH) compounds exhibit intrinsically low lattice thermal conductivity ([Formula: see text]), outperforming ternary half-Heuslers and making them promising candidates for waste-heat recovery. Using the FP-LAPW method (WIEN2k) with GGA-PBEsol and LDA functionals, we systematically investigate the structural, electronic, optical, elastic, and thermoelectric properties of body-centered tetragonal Ti 2 Y 2 AlSb (Y = Pd, Pt; space group I-42d). Negative formation energies confirm thermodynamic stability: [Formula: see text](Ti 2 Pt 2 AlSb) = - 540.372 kJ/mol and [Formula: see text] (Ti 2 Pd 2 AlSb) = -358.009 kJ/mol. Both compounds are p-type direct-bandgap semiconductors with GGA bandgaps of 0.764 eV (Pt) and 0.474 eV (Pd). Optical spectra (0–13.65 eV) reveal slight anisotropy, with characteristic peaks in the dielectric function, optical conductivity, refractive index ([Formula: see text]=5.96/6.22 at 1.67 eV for the Pt compound; 4.62/4.354.62/4.35 at 1.70 eV for the Pd compound), reflectivity (maxima 58.8% at 8.66 eV and 57.6% at 1.53 eV), and absorption coefficient (maxima [Formula: see text] at 8.09 eV and [Formula: see text] at 7.03 eV). The calculated elastic constants satisfy all mechanical stability criteria. High bulk, shear, and Young’s moduli indicate good compressibility resistance, while Pugh’s ratio (B/G) shows that Ti 2 Pd 2 AlSb is ductile and Ti 2 Pt 2 AlSb lies near the brittle-ductile boundary; shear strength is higher on the (010) plane than on the (100) plane. Thermoelectrically, both materials exhibit low [Formula: see text] (room-temperature values: 1.02 W/m⋅K for Pt, 0.612 W/m⋅K for Pd), high Seebeck coefficients (216 [Formula: see text]V/K and 235 [Formula: see text]V/K, respectively), and peak ZT values of 0.932 (700 K, n = [Formula: see text]) for Ti 2 Pt 2 AlSb and 0.821 (700 K, n = [Formula: see text]) for Ti 2 Pd 2 AlSb, using a temperature-dependent Lorenz number that accounts for bipolar contributions. These DHH compounds therefore serve as a rational design platform for next-generation thermoelectrics.