Unexpected Phonon Behaviour in BiFe_xCr_1-xO₃, a Material System Different from Its BiFeO₃ and BiCrO₃ Parents

Area of Science:

• Materials Science
• Solid State Physics
• Spectroscopy

Background:

• Bismuth ferrite (BiFeO$_{3}$) and bismuth chromite (BiCrO$_{3}$) are parent compounds with distinct properties.
• Doping with both Fe and Cr in BiFeO$_{3}$ can lead to novel material characteristics.
• Understanding the electronic and vibrational properties of mixed compounds is crucial for potential applications.

Purpose of the Study:

• To determine the dielectric function and bandgap of BiFe$_{0.5}$Cr$_{0.5}$O$_{3}$ thin films.
• To investigate the origin of the reduced bandgap in the mixed compound.
• To explore new phonon modes and temperature-induced structural phase transitions using Raman spectroscopy.

Main Methods:

• Spectroscopic ellipsometry was employed to measure the dielectric function and bandgap.
• Raman spectroscopy with multi-wavelengths excitation was used to study phonon modes.
• Temperature-dependent Raman spectroscopy was applied to analyze structural phase transitions.

Main Results:

• The bandgap of BiFe$_{0.5}$Cr$_{0.5}$O$_{3}$ is lower than that of BiFeO$_{3}$ and BiCrO$_{3}$, attributed to Cr-Fe charge transfer at ~2.27 eV.
• A new Raman mode at ~670 cm$^{-1}$ was observed in BiFe$_{0.5}$Cr$_{0.5}$O$_{3}$ thin films and BiFe$_{x}$Cr$_{1-x}$O$_{3}$ bulk samples, showing excitation-dependent intensity and higher-order scattering.
• Temperature-induced structural phase transitions were identified in BiFe$_{0.3}$Cr$_{0.7}$O$_{3}$.

Conclusions:

• The reduced bandgap in BiFe$_{0.5}$Cr$_{0.5}$O$_{3}$ is linked to charge transfer excitations.
• New phonon modes activated by these excitations suggest unique vibrational properties.
• Raman spectroscopy is effective in probing both electronic transitions and structural dynamics in these materials.