Band gap tuning and orbital mediated electron–phonon coupling in HoFe{sub 1−x}Cr{sub x}O{sub 3} (0 ≤ x ≤ 1)

We report on the evidenced orbital mediated electron–phonon coupling and band gap tuning in HoFe{sub 1−x}Cr{sub x}O{sub 3} (0 ≤ x ≤ 1) compounds. From the room temperature Raman scattering, it is apparent that the electron-phonon coupling is sensitive to the presence of both the Fe and Cr at the B-site. Essentially, an A{sub g} like local oxygen breathing mode is activated due to the charge transfer between Fe{sup 3+} and Cr{sup 3+} at around 670 cm{sup −1}, this observation is explained on the basis of Franck-Condon mechanism. Optical absorption studies infer that there exists a direct band gap in the HoFe{sub 1−x}Cr{sub x}O{sub 3} (0 ≤ x ≤ 1) compounds. Decrease in band gap until x = 0.5 is ascribed to the broadening of the oxygen p-orbitals as a result of the induced spin disorder due to Fe{sup 3+} and Cr{sup 3+} at B-site. In contrast, the increase in band gap above x = 0.5 is explained on the basis of the reduction in the available unoccupied d-orbitals of Fe{sup 3+} at the conduction band. We believe that above results would be helpful for the development of the optoelectronic devices based on the ortho-ferrites.