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. 2022 May 9;12(9):1607.
doi: 10.3390/nano12091607.

Unexpected Phonon Behaviour in BiFexCr1-xO3, a Material System Different from Its BiFeO3 and BiCrO3 Parents

Cameliu Himcinschi  1 , Felix Drechsler  1 , David Sebastian Walch  2   3 , Akash Bhatnagar  2   3 , Alexei A Belik  4 , Jens Kortus  1
Affiliations

Unexpected Phonon Behaviour in BiFexCr1-xO3, a Material System Different from Its BiFeO3 and BiCrO3 Parents

Cameliu Himcinschi et al. Nanomaterials (Basel). .

Abstract

The dielectric function and the bandgap of BiFe0.5Cr0.5O3 thin films were determined from spectroscopic ellipsometry and compared with that of the parent compounds BiFeO3 and BiCrO3. The bandgap value of BiFe0.5Cr0.5O3 is lower than that of BiFeO3 and BiCrO3, due to an optical transition at ~2.27 eV attributed to a charge transfer excitation between the Cr and Fe ions. This optical transition enables new phonon modes which have been investigated using Raman spectroscopy by employing multi-wavelengths excitation. The appearance of a new Raman mode at ~670 cm-1 with a strong intensity dependence on the excitation line and its higher order scattering activation was found for both BiFe0.5Cr0.5O3 thin films and BiFexCr1-xO3 polycrystalline bulk samples. Furthermore, Raman spectroscopy was also used to investigate temperature induced structural phase transitions in BiFe0.3Cr0.7O3.

Keywords: BiFexCr1−xO3; Raman spectroscopy; charge transfer; multiferroics; phase transition; spectroscopic ellipsometry; thin films.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Real (ε1) and imaginary (ε2) part of BiFe0.5Cr0.5O3 dielectric function. The dotted vertical lines indicate the energy position of the laser lines used for Raman spectroscopy, while the vertical grey arrows show the energy position of the absorption peaks. The inset show the linear extrapolation of (αE)2 to zero absorption which give the value of the optical band gap.
Figure 2
Figure 2
Absorption coefficient of BiFe0.5Cr0.5O3 in comparison with the absorption coefficients of BiFeO3 and BiCrO3. The inset show a zoom of the absorption onset region with the vertical arrow indicating the first absorption peak at ~2.27 eV.
Figure 3
Figure 3
Raman spectra of a 200 nm BiFe0.5Cr0.5O3 thin film on SrTiO3 substrate and of the substrate normalised at the same intensity in the 200–400 cm−1 region, measured with an excitation of 633 nm. The corresponding film signal was calculated as a difference spectrum of the two (upper panel). The difference spectra of the thin film obtained with different excitations lines (633 nm, 532 nm, and 442 nm) and the as measured spectrum with the 325 nm excitation (lower panel).
Figure 4
Figure 4
The difference spectrum of the BiFe0.5Cr0.5O3 thin film compared to the spectra of bulk BiFexCr1−xO3, and of the parent compounds BiCrO3 and BiFeO3 measured with 633 nm excitation. Note the strong peak at 660–670 cm−1 which is detected only for the samples with a Fe/Cr mixture.
Figure 5
Figure 5
Comparison of the Raman spectra of bulk BiFe0.3Cr0.7O3 recorded with different excitation lines in the range of the first, second, and third order of the mode at ~667 cm−1.
Figure 6
Figure 6
Temperature dependence (90–700 K) of the Raman spectra for bulk BiFe0.3Cr0.7O3 showing a reversible phase transition above 475 K.
See this image and copyright information in PMC

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