Symmetry-dependent electronic reconstruction and intrinsic ultraviolet response in Sr₂B'B″O₆ (B' = Ti, Zr; B″ = Sn,...

Understanding the respective roles of crystal symmetry and B-site chemistry in determining functional properties is essential for the rational design of double perovskite oxides. In this study, we systematically investigated a family of Sr₂B'B″O₆ (B' = Ti, Zr; B″ = Sn, Ge) compounds in both the cubic Fm3̄m and distorted P2₁/c phases using density functional theory. All compositions exhibited negative formation energies, confirming thermodynamic stability, with the P2₁/c phase being energetically favored. The elastic constants and derived mechanical parameters demonstrate mechanical stability in both structures, while symmetry lowering generally promotes ductility and modifies the bonding characteristics. Electronic structure calculations revealed that structural distortion significantly reconstructs the band edges, with most compositions undergoing a direct-to-indirect band-gap transition accompanied by band-gap widening. The valence band maximum is dominated by O-2p states and the conduction band minimum by B'-site d orbitals, indicating a potential p-type semiconducting tendency. In contrast, optical responses are only weakly affected by symmetry change: all compounds exhibit intrinsic strong ultraviolet absorption and visible transparency, while B-site substitution induces systematic blue shifts of the absorption edge. Boltzmann transport calculations further show that thermoelectric properties are largely insensitive to symmetry transition and elemental substitution within the constant relaxation time approximation. These results clarify the distinct influences of structural symmetry and chemical composition on multifunctional behavior in Sr₂B'B″O₆ double perovskites and provide theoretical guidance for symmetry-informed materials design.