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

M. ElMassalami, S. Favre, M. B. Silva Neto

TL;DR

This work investigates how applied pressure tunes the electronic states of the strongly correlated charge-transfer perovskite LaBa2Fe3O8+δ. By combining high-pressure XRD, magnetization, and resistivity measurements, the authors construct a P–T phase diagram and identify a critical boundary $P^{MIT}_c(T)$ marking a transition from localized to hole-delocalized states, occurring at relatively low pressures without a structural phase transition up to at least 30 GPa. The resistivity data, analyzed with Mott VRH theory via $ ho_{VRH}(T)= ho^0_{VRH}\exp[(T_0/T)^{1/4}]$ and $T_0\propto [N(E_F)\xi^{3}]^{-1}$, show $T_0(P)$ decreases with pressure, indicating a pressure-induced growth of the localization length $\xi$ and enhanced hole delocalization; thin films exhibit even stronger pressure sensitivity and a higher $P_c$. The findings, set against evidence of mixed Fe valence and suppressed antiferromagnetism, suggest proximity to quantum criticality and highlight LaBa2Fe3O8+δ as a platform to study emergent phenomena under controlled tuning of pressure, disorder, and strain.

Abstract

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.

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

TL;DR

This work investigates how applied pressure tunes the electronic states of the strongly correlated charge-transfer perovskite LaBa2Fe3O8+δ. By combining high-pressure XRD, magnetization, and resistivity measurements, the authors construct a P–T phase diagram and identify a critical boundary marking a transition from localized to hole-delocalized states, occurring at relatively low pressures without a structural phase transition up to at least 30 GPa. The resistivity data, analyzed with Mott VRH theory via and , show decreases with pressure, indicating a pressure-induced growth of the localization length and enhanced hole delocalization; thin films exhibit even stronger pressure sensitivity and a higher . The findings, set against evidence of mixed Fe valence and suppressed antiferromagnetism, suggest proximity to quantum criticality and highlight LaBa2Fe3O8+δ as a platform to study emergent phenomena under controlled tuning of pressure, disorder, and strain.

Abstract

Analysis of the thermal and baric evolution of resistance in enabled the construction of its pressure-temperature (P-T) phase diagram, which prominently displays a critical boundary, , marking the transition from localized to hole-type extended states. The relatively low critical pressures [-8 GPa] suggest that, as 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 ; 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.

Paper Structure

This paper contains 7 sections, 3 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Experimental
  3. Results and Analysis
  4. Baric Evolution of Crystalline Structure
  5. Ambient-Pressure Magnetization
  6. Baric Evolution of Resistance
  7. Discussion and Conclusions

Figures (5)

  • Figure 1: Baric evolution of room-temperature diffractograms of LaBa2Fe3O_8.5 measured using synchrotron radiation. (a.1)$P = 0$ GPa. (b.1)$P = 5.0$ GPa. (c.1)$P = 10.0$ GPa. (d.1)$P = 20.0$ GPa. (e.1)$P = 30.0$ GPa. Panels (a.2)–(e.2) show expanded views of the intense (101) peak from the corresponding panels (a.1)–(e.1), highlighting its pressure-induced evolution.Note-LaBa2Fe3Ox-2025-Fig1-Impurity-Peaks The + symbols, red continuous lines, and short blue bars represent the measured intensities, calculated patterns, and Bragg reflection positions, respectively. Inset: Schematic representation of the crystallographic unit cell.
  • Figure 2: Baric evolution of the lattice parameters of bulk LaBa2Fe3O_8.5, based on the structural model described in the text. Parameters extracted using Rietveld $R$ (Le Bail $L$) refinement are denoted by filled (empty) symbols. Circles (triangles) correspond to the first $t1$ (second $t2$) tetragonal cell. (a) a-parameters. (b) c-parameters. (c) Unit-cell volumes. Vertical arrow indicates the absence of any structural phase transition at $P^{\mathrm{MIT}}_c(300\,\text{K}) = 5.6(6)$ GPa (see Fig.\ref{['Fig5-P-T-Phase-Diagram']}). (d) Normalized $V/V_0$. Thin curves serve as visual guides, while the solid red curve represents a fit to Eq.\ref{['Eq.V-vs-P-Murnaghan']}. For all parameters, error bars are within the size of plotted symbols.
  • Figure 3: Ambient-pressure magnetic properties of bulk $\ce{LaBa2Fe3O_{8.75}}$ and thin-film LaBa2Fe3O_8.70. (a)$\chi_{dc}(1\,\text{kOe}, T)$ and $\chi^{-1}_{dc}(1\,\text{kOe}, T)$ of bulk sample. Dashed line represents $\frac{13.35}{T + 352}$ emu/mole. (b)$\mu^{fu}(T,H)$ versus $H/T$ of powdered sample. Short vertical arrows indicate a weak but noticeable spin-flop feature, a characteristic of a AFM-ordered state. (c)$\mu^{fu}(T,H)$ of powdered sample. (d)$\mu^{fu}$ versus $1/H$ of powdered sample. (e)$\chi_{dc}(1\,\text{kOe}, T)$ of thin-film.Note-LaBa2Fe3Ox-2025-No-Inverse-Susc-Films(f)$\mu^{fu}(T,H)$ versus $H/T$ of thin-film. (g)$\mu^{fu}(T,H)$ of thin-film. (h)$\mu^{fu}$ versus $1/H$ of thin-film. Dotted lines are linear extrapolation to $\lim_{1/H \to 0} \mu^{fu}(H)$.
  • Figure 4: (a)$R(P,T,H)$ of bulk LaBa2Fe3O_8.75 in a log-log format. (b)$R(P,T,H)$ of thin-film LaBa2Fe3O_8.70 in a log-log format. Downward (upward) arrows in the legend indicate measurements during cooling (warming). Open symbols, filled circles, filled triangles correspond, respectively, to measurements taken during pressure increase, at $P = 2.78\,\mathrm{GPa}$ and $10\,\mathrm{T}$, and after returning to ambient pressure. Fitting $R(P,T,H)$ to Eq.\ref{['Eq.VRH-Resistivity-T-dependent']} give (c)$T_o(P)$ and (d)$R_o(P)$ of thin films while (e)$T_o(P)$ and (f)$R_o(P)$ of bulk samples. Dashed lines in panels (c-f) are guides to the eye, while solid lines in panels (a-b) represent VRH fits. Inset of panel (a): $R(P=2.78\,\text{GPa}, T, H=0\,\text{T})$ shown in circles and $R(P=2.78\,\text{GPa}, T, H=10\,\text{T})$ in triangles.
  • Figure 5: Representative $R(P,T_f=150,200,300\,K)$ of thin-films (a–c) and bulk (d–f), used to determine the critical pressures by extrapolating resistance to zero. (g)P–T phase diagram (see main text and §\ref{['Sec-SM-App-Classification-Fe-Perovskite']} [ Comment-LaBa2Fe3Ox-Nov2025-SM]). The hatched region indicates the temperature range over which $T_{CO}$ and $T_N$ are distributed; its upper range is constrained by the available pressure range. The intermediate $T_{CO}\le T \le T_{CD}$ range is characterized by thermally activated transport, comprising both Arrhenius-type and variable-range hopping (VRH) conduction regimes (see §\ref{['Sec-SM-App-Classification-Fe-Perovskite']} and §\ref{['Sec-SM-Resistivity-Ambient-Pressure']} [ Comment-LaBa2Fe3Ox-Nov2025-SM]); $T_X$ denote the crossover temperature. The low $T \le T_{CO}$ regime is classified as insulating, antiferromagnetic, and charge ordered.