High Temperature Emissivity, Reflectivity, and X-ray absorption of BiFeO3

TL;DR

This study investigates the temperature-dependent lattice, vibrational, and electronic structure of BiFeO3 using infrared, reflectivity, and X-ray absorption techniques, revealing phase transitions, defect formation, and local structural changes without an insulator-metal transition.

Contribution

It provides new insights into the high-temperature behavior and local structural dynamics of BiFeO3, including the identification of a crossover transition and defect-induced lattice changes.

Findings

Soft phonon behavior indicates a tricritical point.

Crossover transition occurs around 200 K below TN.

No insulator-metal transition observed before melting.

Abstract

We report on the lattice evolution of BiFeO3 as function of temperature using far infrared emissivity, reflectivity, and X-ray absorption local structure. A power law fit to the lowest frequency soft phonon in the magnetic ordered phase yields an exponent {\beta}=0.25 as for a tricritical point. At about 200 K below TN~640 K it ceases softening as consequence of BiFeO3 metastability. We identified this temperature as corresponding to a crossover transition to an order-disorder regime. Above ~700 K strong band overlapping, merging, and smearing of modes are consequence of thermal fluctuations and chemical disorder. Vibrational modes show band splits in the ferroelectric phase as emerging from triple degenerated species as from a paraelectric cubic phase above TC~1090 K. Temperature dependent X-ray absorption near edge structure (XANES) at the Fe K-edge shows that lower temperature Fe3+ turns into Fe2+. While this matches the FeO w\"ustite XANES profile, the Bi LIII-edge downshift suggests a high temperature very complex bond configuration at the distorted A perovskite site. Overall, our local structural measurements reveal high temperature defect-induced irreversible lattice changes, below, and above the ferroelectric transition, in an environment lacking of long-range coherence. We did not find an insulator to metal transition prior to melting.