Influence of (Ba,F) multidoping on structural, magnetic, optical, and electrical properties as well as performance enhancement of multiferroic BiFeO3

Abstract

Antiferromagnetic (AFM) and high ferroelectric (FE) orderings coexist in the pristine ${\mathrm{BiFeO}}_{3}$. However, its performance is suppressed by complex FE switching originated from its R3c space group and high leakage current due to the volatile nature of Bi. We theoretically predict that the performance can be enhanced by (B,F) codoping. To this end, ${\mathrm{BiFeO}}_{3}$ and its Ba-doped, ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{FeO}}_{3\ensuremath{-}x/2}$, as well as (Ba,F) multidoped, ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{FeO}}_{3\ensuremath{-}x}{\mathrm{F}}_{x}$, are analyzed structurally, magnetically, optically, and electrically for the pure (doped) compound (compositions), where $x=0.25$. The analyses are performed in the framework of density functional theory accompanied by random phase approximation, Berry phase theory, and Hubbard model using $\mathrm{PBE}\text{\ensuremath{-}}\mathrm{GGA}+U$ with ${U}_{\text{eff}}=4(5)\phantom{\rule{4pt}{0ex}}\text{eV}$. Here, we predict that the tetragonal polar distortions of the co-doped compound with an AFM ordering lead to a nonzero spontaneous polarization. Hence, both the magnetic and electric polarizations coexist in the codoped composition. To assess the accuracy of the results, we calculate the spontaneous polarization for the pure ${\mathrm{BiFeO}}_{3}$ in both the $R3c$ and $P4mm$ symmetries and find the results in agreement with the available experimental and theoretical data. Furthermore, our dielectric functions for the pure case are found consistent with the experimental data. Moreover, absorption coefficient spectra, as calculated by $\mathrm{GGA}+U$ with ${U}_{\text{eff}}=4\phantom{\rule{4pt}{0ex}}\text{eV}$ and TB-mBJ with its self-consistently converged $c=1.38$ parameter, using Tauc method also reveal direct optical gaps of 2.66 and 2.80 eV, which agree with the corresponding experimental optical gap of 2.74 eV. To study the impacts of doping on the intrinsic ferroelectricity improvement of ${\mathrm{BiFeO}}_{3}$, we then calculate and analyze the optical absorption edges and loss functions for the pristine and doped compounds. By taking the band structure, partial densities of states, energy loss function, and parallel component of the imaginary part of the dielectric tensor, $\ensuremath{\mathfrak{I}}[{\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}]$, for the pure case into consideration simultaneously, the energies of the prominent peaks for $\ensuremath{\mathfrak{I}}[{\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}]$ spectra and their corresponding permitted absorption (emission) transitions are rigorously analyzed and determined. The analyses reveal that the sources of the prominent peaks occurred in $\ensuremath{\mathfrak{I}}[{\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}]$ mainly originate from the excitation states of the bound electrons of O $2s$, O $2p$, Bi $6s$, Bi $6p$, Fe $3d$, and Fe $4s$ orbitals. Our results in most of the energy ranges show that the intrinsic ferroelectricity can be improved by the (Ba,F) codoping due to the reduction of the leakage current achieving from the calculated electric energy loss function. Further, the rhombohedral ($R3c$) is changed by the codoping to the tetragonal ($P4mm$) structure with more convenient symmetry for polarization switching. Hence, the system not only remains multiferroic after the codoping, but also its performance is enhanced. These evidences show that the codoping can play a key role for the applications of this multiferroic system in various devices.