Mott–Anderson crossover without metallization in pressurized Sr₃Ir₂O₇

A Mott insulator can lose its gap yet refuse to metallize. In the spin-orbit-entangled iridate Sr3Ir2O7, transport reveals an abrupt collapse of the effective activation gap (Δeff) from ~0.1–0.2 eV to ≲0.02 eV within 12–17 GPa, yet R(T) remains nonmetallic down to 2.6 K. Above this window, the low-temperature conduction follows 3D Mott variable-range hopping with a strongly reduced hopping scale T0. A concurrent magnetoresistance sign reversal further characterizes the post-collapse localized regime, with possible contributions from spin-related scattering. Helium-medium x-ray diffraction shows no resolvable symmetry change, supporting a Mott–Anderson crossover picture where localization preempts metallization. This disorder-inclusive framework may apply broadly to bandwidth-tuned correlated systems where quenched disorder coexists with a pressure-suppressed correlation gap.