Pressure-induced hole delocalization in the strongly correlated quasicubic charge-transfer perovskite $LaBa_2Fe_3O_{8+δ}$d

Analysis of the thermal and baric evolution of resistance in $LaBa_2Fe_3O_{8+δ}$ 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.

💡 Research Summary

In this work the authors investigate the pressure‑driven evolution of the electronic, structural, and magnetic properties of the strongly correlated charge‑transfer perovskite LaBa₂Fe₃O₈₊δ. Bulk polycrystalline samples and epitaxial thin films were prepared by solid‑state synthesis and pulsed‑laser deposition, respectively, and their oxygen non‑stoichiometry (δ) was quantified via the lattice‑parameter calibration established in earlier studies. Electrical resistance measurements were carried out over a wide temperature range (2 K – 400 K) and under hydrostatic pressures up to 30 GPa. In the ambient‑pressure state the material is a narrow‑gap insulator whose conduction follows a variable‑range‑hopping (VRH) mechanism with an extraordinarily large characteristic temperature T₀ (∼10⁵ K), reflecting strong localization of holes in the oxygen‑derived valence band.

When pressure is increased, T₀ decreases dramatically, and between roughly 3 GPa and 8 GPa a sharp drop in resistance signals a metal‑insulator transition (MIT). By tracking the temperature dependence of the critical pressure the authors construct a pressure‑temperature (P‑T) phase diagram that displays a well‑defined boundary P_c^MIT(T). This boundary shifts slightly to higher pressures at lower temperatures, indicating that the transition is not purely thermally activated but involves a genuine change in the electronic ground state.

High‑pressure synchrotron X‑ray diffraction (XRD) was performed at room temperature up to 30 GPa. The diffraction patterns reveal a monotonic shift of Bragg peaks to higher angles, consistent with uniform lattice compression. A modest splitting of the intense (101) reflection appears between 5 GPa and 10 GPa, which the authors interpret as evidence for two slightly different tetragonal sub‑cells (t₁ and t₂) arising from La³⁺/Ba²⁺ intermixing and oxygen non‑stoichiometry. Nevertheless, the overall crystal symmetry remains that of a weakly tetragonally distorted quasicubic perovskite (space group Pm3̅m) throughout the entire pressure range; no superlattice reflections or symmetry‑lowering transitions are observed. The pressure dependence of the unit‑cell volume follows the Murnaghan equation of state with V₀≈63 ų, bulk modulus B₀≈155 GPa, and pressure derivative B′≈8, values comparable to those reported for other Fe‑based perovskites such as SrFeO₃. The absence of any structural phase transition at P_c^MIT confirms that the MIT is of electronic origin.

Magnetic measurements under ambient pressure show Curie‑Weiss behavior with an effective moment μ_eff≈3.4 μ_B and a large positive Weiss temperature (θ≈352 K), indicative of strong antiferromagnetic (AFM) correlations. A subtle anomaly near 200 K is associated with a slow spin‑relaxation process, while a more pronounced AFM ordering sets in below ∼100 K, as evidenced by a weak bifurcation between zero‑field‑cooled and field‑cooled susceptibility curves and by Mössbauer spectroscopy. Upon applying pressure the Néel temperature is progressively suppressed, and the magnetic susceptibility becomes less temperature‑dependent, reflecting the delocalization of holes that reduces the local magnetic moments.

The authors interpret the pressure‑induced MIT as a consequence of enhanced Fe 3d–O 2p hybridization, which reduces the charge‑transfer gap Δ_CT and widens the oxygen‑hole bandwidth W. Consequently the ratio Δ_CD/W (charge‑disproportionation gap to bandwidth) decreases, allowing holes to become itinerant. Because the crystal structure remains essentially unchanged, the transition is a pure electronic delocalization rather than a lattice‑driven Peierls‑type instability. The simultaneous suppression of antiferromagnetism suggests that the system approaches a quantum critical point where both charge and spin degrees of freedom become critical.

In summary, the study demonstrates that LaBa₂Fe₃O₈₊δ, a narrow‑gap charge‑transfer insulator, can be driven into a metallic state by modest hydrostatic pressures (3–8 GPa). The transition is marked by a clear P‑T phase boundary, retains the quasicubic perovskite framework, and is accompanied by the weakening of antiferromagnetic order. These findings highlight LaBa₂Fe₃O₈₊δ as an exemplary platform for exploring pressure‑tuned quantum criticality in strongly correlated 3d transition‑metal oxides.