Unexpected Phonon Behaviour in BiFe(x)Cr(1−x)O(3), a Material System Different from Its BiFeO(3) and BiCrO(3) Parents

The dielectric function and the bandgap of BiFe(0.5)Cr(0.5)O(3) thin films were determined from spectroscopic ellipsometry and compared with that of the parent compounds BiFeO(3) and BiCrO(3). The bandgap value of BiFe(0.5)Cr(0.5)O(3) is lower than that of BiFeO(3) and BiCrO(3), 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 BiFe(0.5)Cr(0.5)O(3) thin films and BiFe(x)Cr(1−x)O(3) polycrystalline bulk samples. Furthermore, Raman spectroscopy was also used to investigate temperature induced structural phase transitions in BiFe(0.3)Cr(0.7)O(3).