Chemical pressure induced red shift in band gap and d-d transition energies in Sr doped BiFeO3

Abstract

The optical response of multiferroic Bi1−xSrxFeO3(0 ≤ x ≤ 0.45) samples is studied in the spectral range from 1 eV to 4 eV. Optical response in the studied spectral range were dominated by two charge transfer transitions and two doubly degenerate d-d transitions (6A1g → 4T2gand6A1g → 4T2g) for all samples. The d-d transitions were found to weaken as the Sr content was increased which may be due to the transformation of the crystal structure from rhombohedral to cubic. Moreover, a red shift in the d-d transition energies were observed as the Sr ion substitutes the BFO host lattice, which indicates the variation in the crystal field strength. The direct band gap was found to decrease from 1.99 eV to 1.89 eV, whereas the indirect band gap decreased from 1.40 to 0.49 eV as the Sr content is increased to x = 0.45. The decrease in the energy band gap values and red shift in d-d transition energies is ascribed to the chemical pressure induced reduction in unit cell volume. The results indicate a promising ability for tuning the BFO band gap to enable functionality as a multiferroic photovoltaic material.