[Paper Review] Structural Origin of the Metal-Insulator Transition of Multiferroic BiFeO3

Simon A. T. Redfern, Joanna N. Walsh+3 more|ArXiv.org|Jan 23, 2009

Multiferroics and related materials6 references9 citations

TL;DR

This study identifies the structural origin of the metal-insulator (MI) transition in multiferroic BiFeO3 through high-pressure and high-temperature X-ray diffraction. It demonstrates that the metallic γ-phase is cubic (Pm3m), not rhombohedral, and that the MI transition is a second-order band-type transition driven by semi-metallic band overlap in the cubic phase, not a Mott transition or spin-state change.

ABSTRACT

We report X-ray structural studies of the metal-insulator phase transition in bismuth ferrite, BiFeO3, both as a function of temperature and of pressure (931 oC at atmospheric pressure and ca. 45 GPa at ambient temperature). Based on the experimental results, we argue that the metallic gamma-phase is not rhombohedral but is instead the same cubic Pm3m structure whether obtained via high temperature or high pressure, that the MI transition is second order or very nearly so, that this is a band-type transition due to semi-metal band overlap in the cubic phase and not a Mott transition, and that it is primarily structural and not an S=5/2 to S=1/2 high-spin/low-spin electronic transition. Our data are compatible with the orthorhombic Pbnm structure for the beta-phase determined definitively by the neutron scattering study of Arnold et al .[Phys. Rev. Lett. 2009]; the details of this beta-phase had also been controversial, with a remarkable collection of five crystal classes (cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral!) all claimed in recent publications.

Motivation & Objective

• To resolve long-standing controversy over the crystal structure of the metallic phase in BiFeO3.
• To determine whether the metal-insulator (MI) transition in BiFeO3 is driven by electronic correlations (Mott-like) or band structure effects.
• To clarify the structural phase sequence under temperature and pressure, particularly the nature of the β-phase and γ-phase.
• To distinguish between high-spin (S=5/2) and low-spin (S=1/2) Fe3+ states as the origin of the MI transition.
• To provide definitive structural evidence for the MI transition mechanism in multiferroic BiFeO3 using in situ X-ray diffraction.

Proposed method

• Performed in situ X-ray diffraction on BiFeO3 under high temperature (up to 931 °C) and high pressure (up to 45 GPa).
• Used structural refinement to identify the crystal symmetry of the metallic phase at high temperature and high pressure.
• Compared experimental data with neutron diffraction results from Arnold et al. (2009) to validate the β-phase structure as orthorhombic Pbnm.
• Analyzed the evolution of lattice parameters and bond lengths to infer electronic structure changes.
• Assessed the role of Fe3+ spin state transitions (S=5/2 vs. S=1/2) by comparing structural trends with magnetic and electronic models.
• Evaluated the MI transition as a band-type process by examining band overlap in the cubic phase.

Experimental results

Research questions

• RQ1 What is the true crystal structure of the metallic γ-phase in BiFeO3 under high temperature and high pressure?
• RQ2 Is the metal-insulator transition in BiFeO3 a Mott transition driven by electron correlation or a band-type transition due to semi-metallic band overlap?
• RQ3 Does the transition involve a change in the spin state of Fe3+ ions (from high-spin to low-spin)?
• RQ4 How does the structural sequence (α → β → γ) evolve under temperature and pressure, and what is the symmetry of the β-phase?
• RQ5 Is the metal-insulator transition second order or first order, and what structural parameters control its nature?

Key findings

• The metallic γ-phase of BiFeO3 is cubic with space group Pm3m, not rhombohedral, under both high-temperature and high-pressure conditions.
• The metal-insulator transition is second order or very nearly so, indicating a continuous structural and electronic evolution.
• The transition is a band-type mechanism driven by semi-metallic band overlap in the cubic Pm3m phase, not a Mott transition.
• The β-phase, observed at intermediate temperatures and pressures, is definitively orthorhombic with space group Pbnm, resolving prior controversy.
• There is no evidence for a high-spin (S=5/2) to low-spin (S=1/2) transition of Fe3+ ions as the origin of the MI transition.
• The structural evolution under pressure and temperature supports a continuous, symmetry-driven transition rather than a first-order phase change.