The beta Phase of Multiferroic Bismuth Ferrite and its beta-gamma Metal-Insulator Transition

TL;DR

This study reveals the phase transitions and metal-insulator transition in epitaxial BiFeO3 thin films, showing a monoclinic to orthorhombic transition and an abrupt bandgap collapse at high temperature.

Contribution

It reports the existence of a beta-phase in BiFeO3 thin films and details the phase transition sequence and metal-insulator transition, which were previously uncharacterized.

Findings

Epitaxial BiFeO3 thin films are monoclinic at room temperature.

An orthorhombic beta-phase exists between 820°C and 950°C.

The bandgap collapses at the cubic gamma-phase, indicating a metal-insulator transition.

Abstract

We show that epitaxial (001) thin films of multiferroic bismuth ferrite BiFeO3 are monoclinic at room temperature instead of tetragonal or Rhombohedral as reported earlier . We report a orthorhombic order-disorder beta-phase between 820C and 950C contrary to the earlier report. The transition sequence monoclinic-orthorhombic phase in (001)BiFeO3 thin film (rhombohedral-orthorhombic transition in single crystal) resembles that of BaTiO3 or PbSc1/2Ta1/2O3. The transition to the cubic $\gamma$-phase causes an abrupt collapse of the bandgap toward zero (insulator-metal transition) at the orthorhombic-cubic beta-gamma transition around 950C. This transition is similar to the metal-insulator transition in Ba0.6K0.4BiO3.