Lattice-Strain-Guided Cooperative A- and B-Site Engineering for Enhanced Site-Selective Doping and Bandgap Narrowing in Sr _{1– <i>y</i>} Ba _<i>y</i> Ga _{12– <i>x</i>} In _<i>x</i> O ₁₉

Site-selective doping in complex oxides is often constrained by local lattice strain and limited structural tolerance, which restricts dopant solubility at electronically critical sites and hinders further band engineering. Building on our previous understanding of In 3+ site preference in magnetoplumbite-type SrGa 12 O 19, we demonstrate here that cooperative A- and B-site engineering provides an effective route to overcome these limitations. Partial substitution of Sr 2+ by the larger Ba 2+ ions expands the R-block and associated structural voids, thereby buffering lattice strain induced by In 3+ incorporation and enabling substantially higher dopant solubility. As a result, the In 3+ occupancy at the electronically active Ga3 (M3) tetrahedral site increases from 27.1% in SrGa 10 In 2 O 19 to 49.6% in Sr 0.5 Ba 0.5 Ga 9 In 3 O 19, accompanied by minor occupation of the Ga2 site at high doping levels. Detailed structural analysis reveals that Ba 2+ substitution modulates interlayer spacing, polyhedral distortions, and stress propagation within the lattice, collectively facilitating higher In 3+ solubility. The increased In 3+ population at the M3 site progressive narrows bandgap from 4.54 eV (SrGa 12 O 19 ) to 4.00 eV, with a near-linear correlation between bandgap reduction and M3 occupancy. Consequently, the optimized Sr 0.5 Ba 0.5 Ga 9 In 3 O 19 photocatalyst exhibits a significantly enhanced H 2 evolution rate of 79(3) μmol/h after cocatalyst optimization and reduction treatment, together with apparent quantum yields of 4.07% (295 nm) and 6.68% (254 nm). This work highlights the importance of lattice-level stress management for controlling dopant distribution in multisite oxides and provides a generalizable structural-chemical strategy for achieving deep band engineering.