Extending the La Solubility Limit in Sr₃Ir₂O₇ through High-Pressure High-Temperature Synthesis

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La-doped bilayer iridates provide an important platform for studying the evolution of the spin-orbit-assisted Mott state under electron doping, but the La solubility achieved by conventional ambient-pressure synthesis is limited. Here, we report the synthesis and physical properties of nominally La-doped (Sr₁₋ₓLaₓ)₃Ir₂O₇ (x = 0.05, 0.10, 0.15, and 0.20) prepared using high-pressure high-temperature techniques. Single-crystal X-ray diffraction refinements, supported by scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX), reveal significantly enhanced La incorporation, with nominal x = 0.05 and 0.15 corresponding to actual compositions of approximately (Sr₀.₈₉La₀.₁₁)₃Ir₂O₇ and (Sr₀.₇₇La₀.₂₃)₃Ir₂O₇, respectively. At nominal x = 0.20, the bilayer phase is no longer stabilized and instead transforms into cubic perovskite Sr₁₋ₓLaₓIrO₃. (Sr₀.₈₉La₀.₁₁)₃Ir₂O₇ exhibits a ferromagnetic-like transition near 186 K accompanied by magnetic hysteresis and subtle lattice anomalies indicative of spin-lattice coupling. Despite its high electron-doping level, the compound remains strongly insulating, consistent with a heavily doped localized magnetic insulating state distinct from both parent Sr₃Ir₂O₇ and ambient-pressure La-doped samples. In contrast, (Sr₀.₇₇La₀.₂₃)₃Ir₂O₇ displays metal-like electronic behavior, weakened magnetic order, and enhanced carrier delocalization, although disorder-driven localization persists at low temperatures. These results demonstrate that high-pressure synthesis substantially extends the accessible doping range of bilayer iridates and reveals electronic and magnetic states inaccessible through conventional synthesis routes.