Research Information System
PHYSICS OF THE SOLID STATE, vol.68, no.4, pp.320-336, 2026 (SCI-Expanded, Scopus)
First-principles calculations (FP-LAPW, WIEN2k) were used to investigate the double perovskites Ca2NaXO6 (X = Cl, Br). Geometry optimization yields the cubic \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Fm\bar {3}m$$\end{document} structure (no. 225) with a = 7.4693 & Aring; (Cl) and a = 7.6236 & Aring; (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 Gamma-Gamma 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, tau = 1 & times; 10-14 s) combined with a Slack-model lattice term gives temperature-rising power factor and ZT approximate to 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.