DFT Study of Ca2NaXO6 (X = Cl or Br) Double Perovskites: Structural, Electronic, Mechanical, Thermoelectric, and Optical Properties

First-principles calculations (FP-LAPW, WIEN2k) were used to investigate the double perovskites Ca2NaXO6 (X = Cl, Br). Geometry optimization yields the cubic $$Fm\bar {3}m$$ structure (no. 225) with a = 7.4693 Å (Cl) and a = 7.6236 Å (Br), indicating thermodynamic stability. Elastic constants satisfy the Born’s mechanical stability criteria, with high resistance to compression and shear; indicators (Pugh’s and Poisson’s ratios, Cauchy’s pressure) point to high hardness and intrinsic brittleness. Zener/percentage anisotropy show mild elastic anisotropy for Ca2NaClO6 (bulk modulus essentially isotropic) and near‑isotropy for Ca2NaBrO6. KTB‑mBJ band structures reveal direct Γ–Γ gaps of 1.75 eV (Cl) and 2.21 eV (Br), corresponding to absorption edges at 709 and 561 nm, respectively. Optical spectra exhibit strong ultraviolet absorption consistent with O‑2p → (Ca‑d, X‑p) transitions. Boltzmann transport (constant relaxation time, τ = 1 × 10‒14 s) combined with a Slack‑model lattice term gives temperature‑rising power factor and ZT ≈ 0.74 at 1000 K for both halides, benchmark‑consistent with published Sr‑halide and iodide analogues. Together, the direct visible‑edge gaps, UV selectivity, and stable high‑T thermoelectric performance identify Ca2NaClO6 and Ca2NaBrO6 as multifunctional energy materials spanning UV‑optoelectronic filtering/detection and high‑temperature waste‑heat harvesting.