Abnormal Pressure‐Induced Photoluminescence Enhancement and Phase Decomposition in Pyrochlore La₂Sn₂O₇

La₂Sn₂O₇ is a transparent conducting oxide (TCO) material and shows a strong near‐infrared fluorescent at ambient pressure and room temperature. By in situ high‐pressure research, pressure‐induced visible photoluminescence (PL) above 2 GPa near 2 eV is observed. The emergence of unusual visible PL behavior is associated with the seriously trigonal lattice distortion of the SnO₆ octehedra, under which the Sn–O1–Sn exchange angle θ is decreased below 22.1 GPa, thus enhancing the PL quantum yield leading to Sn ³P₁ → ¹S₀ photons transition. Besides, bandgap closing followed by bandgap opening and the visible PL appearing at the point of the gap reversal, which is consistent with high‐pressure phase decomposition, are discovered. The high‐pressure PL results demonstrate a well‐defined pressure window (7–17 GPa) with flat maximum PL yielding and sharp edges at both ends, which may provide a great calibration tool for pressure sensors for operation in the deep sea or at extreme conditions.