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 .