First principles study on the structural, magnetic, electronic and optical properties of un-doped and La-doped BiFe0.75Mn0.125Ti0.125O3

Based on the generalized gradient approximation (GGA), the structural, magnetic, electronic and optical properties of BiFe0.75Ti0.125Mn0.125O3 (TM) and Bi1 − yLayFe0.75Ti0.125Mn0.125O3 (y=0.125, 0.25) (LTM) were researched by the first principles. Compared with the pure BiFeO3 (zero magnetic moment), the magnetic moment of TM gets improved greatly and the value computed here is ∼3μB, while La doping almost has no effect on that of TM. Ti and Mn co-doping converts the indirect band gap of BiFeO3 (2.20 eV) to a direct band gap of TM (0.95 eV). The bandwidth of TM is similar to that of LTM1 (1 La atom, y=0.125) within the tolerance, but smaller than that of LTM2 (2 La atoms, y=0.25). This is consistent with their gradually increased absorption edges to infrared region, and suggests that doping La into the B sites of TM makes it more potential application on photovoltaic. According to the results of reflectivity, (Ti, Mn)-codoped BiFeO3 is transparent, while LTM is nontransparent in the visible light region which indicates more potential valuable for visible-light-responsive devices. To gain some insight into the optical properties of the samples, three relationships have been discussed: (1) the reflectivity and energy loss function, (2) the extinction coefficient and the imaginary part of the dielectric function, and (3) the refractive index and the real part of the dielectric function.