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

La-doped bilayer iridates provide an important platform for studying the evolution of the spin-orbit-assisted Mott state under electron doping, but La substitution in conventional ambient-pressure-grown bulk samples has been reported only up to x ≈ 0.08. 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 of selected crystals reveal significantly enhanced La incorporation, with nominal x = 0.05 and 0.15 corresponding to compositions of approximately (Sr0.9La0.1)3Ir2O7 and (Sr0.8La0.2)3Ir2O7, respectively. Scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS) confirms La incorporation and an overall increase in local La content across the series. At nominal x = 0.20, the bilayer phase instead transforms into cubic perovskite Sr1–xLaxIrO3. The nominal x = 0.05 product 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, the nominal x = 0.15 product 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 extends the experimentally reported La-doping range of bilayer iridates and reveals electronic and magnetic states not previously reported through conventional synthesis routes.