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This study systematically investigates structural, optoelectronic, photocatalytic and thermoelectric properties of A2-xA'xBiSbO6 (A = Ca, Sr, Ba; A' = Mg) (x = 0, 1) DPs using DFT approach. The primary objective is to elucidate the influence of partial Mg substitution at the A-site on the multifunctional properties of parent compounds. Structural stability criterions confirm the cubic phase through tolerance, octahedral factor and formation energy, while Birch-Murnaghan equation of state demonstrates a stable ground state with negative minimum energy. Owing to the smaller ionic radius of Mg, substitution at A-site reduces the Bi-O and Sb-O bond lengths. The Bandstructure and density of states (DOS) calculation performed using TB-mBJ approximation, revealed the semiconducting nature with indirect band gap of 2.66, 2.65, and 2.58 eV for A2BiSbO6 (A = Ca, Ba, Sr). Mg-inclusion certainly reduced the band gap to 2.01, 2.15, and 2.19 for AMgBiSbO6 (A = Ca, Ba, Sr), respectively due to downward shifting of conduction band minima toward Fermi energy driven by strong Mg-2s & O-2p and Bi-6p & Sb-5s hybridization. The optical properties are further investigated through the complex dielectric function ε (ω), absorption α (ω), and optical losses L (ω) with strong absorption predominantly in UV to far UV regions. Furthermore, the photocatalytic study reveals, A2BiSbO6 (A = Ca, Sr, Ba) possesses suitable band-edge positions for overall water splitting across the entire pH range (0-14). In contrast, AMgBiSbO6 (A = Ca, Sr, Ba) materials only possesses favorable band-edge alignment for oxygen evolution reaction (OER) at all pH value. Besides that, the qualitative analysis using solar to hydrogen conversion efficiency (ηSTH) is estimated to be 4.83%, 5.47%, and 6.43% for A2BiSbO6 (A = Ca, Sr, Ba), respectively and simultaneously possess lower weak exciton binding energy. Besides that, temperature-dependent thermoelectric calculations over the 300–1000 K range confirm the semiconducting nature of the investigated compounds, as evidenced by the behavior of electrical conductivity (σ/τ) and the Seebeck coefficient (S), with holes as dominant charge carriers. The lattice thermal conductivity (κₗ) was estimated using the Slack model, and the resulting thermoelectric figure of merit (ZT) demonstrates excellent thermoelectric performance at elevated temperatures. Notably, CBSO, SBSO, BBSO, and SMBSO exhibit maximum ZT values of 2.47, 2.475, 2.474, and 2.482 respectively, attain ZT > 2 at 1000K, highlighting considerable potential for high-temperature thermoelectric applications.
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