Pressure-induced hole delocalization in the strongly correlated quasicubic charge-transfer perovskite $LaBa_2Fe_3O_{8+\delta}$d
Analysis of the thermal and baric evolution of resistance in $LaBa_2Fe_3O_{8+\delta}$ enabled the construction of its pressure-temperature (P-T) phase diagram, which prominently displays a critical boundary, $P^{MIT}_c(T)$, marking the transition from localized to hole-type extended states. The relatively low critical pressures [$P^{MIT}_c(T) \approx 3$-8 GPa] suggest that, as $P \rightarrow P_c$ in this narrow-gap, strongly correlated charge-transfer system, both the hybridization strength and the charge-transfer character are progressively enhanced - ultimately leading to the emergence of metallicity. Emphasizing the electronic nature of this transition, pressure-dependent structural analyses at room temperature reveal no associated structural phase transition at $P^{MIT}_c(T)$; the system retains a (weakly tetragonally distorted) quasicubic perovskite structure with Murnaghan-type compressibility up to 30\,GPa. The emergence of hole delocalization and metallic conduction, coupled with suppressed antiferromagnetism, suggests proximity to quantum criticality.