Co-substituted BiFeO3: electronic, ferroelectric, and thermodynamic properties from first principles (original) (raw)
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Ferroelectrics, 2018
Multiferroic materials with magnetoelectric properties, due to their coupling between the electrical and magnetic properties have attracted the attention of many researchers. Perovskite structured materials based on BiFeO 3 are a class of materials largely considered in these studies. In this work, Bi 1-x Nd x Fe 0.99 Co 0.01 O 3 (x ¼ 0.05, 0.10 and 0.20) compositions were synthesized aiming a better understanding of the crystal and electronic structure role in ferroic properties of these materials. Structural studies were made by X-ray diffraction and Rietveld refinement; evidencing the rhombohedral symmetry of the samples with space group R3c. Electron density calculations were performed by the maximum entropy method. These studies showed that Nd 3þ substitution causes distortions in the unit cell, which have influence on the magnetic properties. Also, the electron density maps showed the bonds between the iron and oxygen ions did not changed a lot. But the Minimum Electron Density observations were enough to conclude that the ferroelectric and the dielectric properties of Bi 1-x Nd x Fe 0.99 Co 0.01 O 3 compositions would be modified with the Nd 3þ substitution.
Journal of Alloys and Compounds, 2017
Single phase multiferroics BiFeO 3 (BFO), Bi 0.99 Sm 0.01 FeO 3 (Sm1) and Bi 0.99 Sm 0.01 Fe 0.99 Co 0.01 O 3 (Sm1Co1) were successively synthesized by tartaric acid assisted sol-gel method. The X-ray diffraction patterns analyzed through Rietveld refinement method confirmed substitution driven structural distortion, which was further supported by William-Hall plot. The morphological analysis of BFO and Sm1Co1 could be effectively controlled to form nanoparticles, due to the smaller ionic radii of the dopants cations. The co-doping of Sm and Co in BFO lattice significantly enhanced the magnetic properties, with the saturation magnetization being 8.4 emu/g, which is about 20 times larger than that of pure BFO (0.40 emu/g). The decrease of Neel transition temperature and irreversible behavior of FC/ZFC curves also support improved magnetic properties. With the codoping of Sm-Co, the band gap was decreased, while interestingly, the leakage current density was significantly reduced to about 10-7 A cm-2. A reduction of oxygen vacancies was confirmed by XPS analysis in the case of Sm1Co1 sample.
Mono-, Di-, and Tri-Valent Cation Doped BiFe0.95Mn0.05O3 Nanoparticles: Ferroelectric Photocatalysts
Advanced Functional Materials, 2022
The ferroelectricity of multivalent codoped Bismuth ferrite (BiFeO3; BFO) nanoparticles (NPs) is revealed and utilized for photocatalysis, exploiting their narrow electronic bandgap. The photocatalytic activity of ferroelectric photocatalysts BiFe0.95Mn0.05O3 (BFM) NPs and mono-, di-, or tri-valent cations (Ag+, Ca2+, Dy3+; MDT) co-incorporated BFM NPs are studied under ultrasonication and in acidic conditions. It is found that such doping enhances the photocatalytic activity of the ferroelectric NPs approximately three times. The correlation of the photocatalytic activity with structural, optical, and electrical properties of the doped NPs is established. The increase of spontaneous polarization by the mono- and tri-valent doping is one of the major factors in enhancing the photocatalytic performance along with other factors such as stronger light absorption in the visible range, low recombination rate of charge carriers, and larger surface area of NPs. A-site doping of BFO NPs by divalent elements suppresses the polarization, whereas trivalent (Dy3+) and monovalent (Ag+) cations provide an increase of polarization. The depolarization field in these single domain NPs acts as a driving force to mitigate recombination of the photoinduced charge carriers.
AIP Advances, 2018
The electronic structures of α-BiFeO 3 are calculated by using a full-potential linearized-augmented-plane-wave method. We employed the local-density approximation (LDA) with the modified Becke-Johnson (mBJ) exchange potential and the LDA + U method. The indirect (direct) bandgap of 2.24 (2.44) eV obtained by LDA + U method is in good agreement with an experiment, while the mBJ potential produces the indirect bandgap of 2.55 eV, and the direct bandgap is slightly larger than the indirect one. The discrepancy between the experimental x-ray spectra and the calculated Fe-3d and O-2p density of states were revealed to be due to the effects of the core hole. The core-hole effects are also responsible for the smaller bandgap in x-ray spectroscopy than the optical spectroscopy. The calculated valencecharge density and the bonding character obtained by LDA + U method also provides the stronger ionic character of the compound than the mBJ potential. Although the mBJ method is very efficient one, it is still very time consuming compared to the LDA + U method. The most suitable exchange-correlation potential for α-BiFeO 3 is the LDA + U. Therefore, it is better to use the LDA + U method for the electronicstructure calculations of BiFeO 3 compound not only for reducing the calculational time but also for better description of bandgaps and some physical properties. From the similar calculations carried out for transition-metal monoxide system it was found that the inadequacy of using the mBJ potential for the description of the localized 3d-states is rather universal.
Journal of Electronic Materials, 2019
BiFeO 3 is a very promising material for future technical applications. The effects of the different size and valance state of Bi 3+ and Ba 2+ as A-site cations in BiFeO 3 (BFO) has been studied. The introduction of large-size Ba ions at Bi site resulted in significant lattice strain and a partial structural phase transformation from rhombohedral R3c to cubic Pm 3m phase. Significant changes in the Raman spectrum, including variation in intensities and fullwidth at half-maxima, shifts in peak positions, and overlapping of Raman modes, were observed on incorporation of Ba into the lattice. The overlapping of various Raman modes for Ba-doped compositions confirmed the structural transformation. The decrease in intensity of A modes can be ascribed to replacement of Bi by Ba atoms, while the softening of E modes indicates presence of oxygen vacancies and suggests substantial changes in and destabilization of FeO 6 octahedra. Destabilization of FeO 6 octahedra was confirmed by Fourier-transform infrared spectroscopy. Optical spectra showed the presence of charge transfer and doubly degenerate d-d transitions. A slight red-shift in the d-d transition was observed, attributed to variation in the crystal field strength. The bandgap increased with increasing Ba incorporation, which is related to weakening of dipolar moments due to the structural transformation from a noncentrosymmetric to centrosymmetric structure. Such better understanding of the mechanisms governing the optical response of BFO could enable researchers to engineer the bandgap and conductivity for enhanced photoferroelectric properties.
Evidence of improved ferroelectric phase stabilization in Nd and Sc co-substituted BiFeO3
Journal of Applied Physics, 2014
9 Nd 0.1 Fe 1Àx Sc x O 3 (x ¼ 0, 0.05, and 0.10) multiferroic compounds were prepared using conventional solid-state route. X-ray diffraction studies and Raman measurements indicated that the compounds were crystallized in rhombohedral structure with R3c space group. Weak ferromagnetism was induced due to the suppression of canted spin structure in the substituted compounds. Both remanent magnetization (M r ) and coercive field (H c ) were enhanced in Nd and Sc substituted compounds. Further, N eel temperature T N was decreased from 644 K for BiFeO 3 to 550 K for Bi 0.9 Nd 0.1 Fe 0.9 Sc 0.1 O 3 compound due to weakening of magnetic exchange interactions between B-site cations in the substituted compounds. Enhanced and nearly well saturated electrical polarizations were observed in Sc substituted compounds, which are attributed to the strengthening of covalent hybridization between Bi and O ions and reduction in oxygen vacancies. The remnant polarization was enhanced to 12.5 lC/cm 2 in Bi 0.9 Nd 0.1 Fe 0.9 Sc 0.1 O 3 compound. Impedance studies revealed that insulating character of substituted compounds is enhanced and electrical relaxations are of non-Debye type. V C 2014 AIP Publishing LLC.
Effect of Mo Doping at the B Site on Structural and Electrical Properties of Multiferroic BiFeO 3
Journal of Superconductivity and Novel Magnetism, 2017
The Mo-doped BiFe 1−x Mo x O 3 , where x = 0 and 0.6, samples were synthesised by the solid-state reaction method. Admirable ferroelectric and piezoelectric properties are expected on substituting solid solutions of bulk BFO with other oxide perovskite compounds. It offers an alternative and is an environment-friendly candidate for lead-free ferroelectric and piezoelectric devices. The prepared samples were investigated by XRD, Fourier transform infrared spectroscopy (FTIR), current density (JE) measurement, PE loop tracer and field emission scanning electron microscopy to examine their crystal structure, bonding nature, current density, ferroelectric hysteresis loop and surface morphology. The Mo-doped BFO shows a change in structure, increased grain size, reduction in current density and enhancement in the PE loop on doping Mo at the Fe site.
Origin of enhanced multiferroic properties in Bi0.85−xLa0.15HoxFeO3 nanopowders
Journal of Magnetism and Magnetic Materials, 2018
Structural, magnetic and ferroelectric properties of polycrystalline BiFeO 3 , Bi 0.9 Dy 0.1 FeO 3 , BiFe 0.97 Co 0.03 O 3 and Bi 0.9 Dy 0.1 Fe 0.97 Co 0.03 O 3 , which were prepared by solid-state method, have been investigated. The X-ray diffraction (XRD) patterns reveal that all the samples are in single phase and show rhombohedrally distorted perovskite structure with R3c space group. Both XRD and Raman-scattering studies show that Dy and Co co-doped sample has a compressive lattice distortion induced by co-substitution of Dy and Co ions at the A and the B sites, respectively. Dy and Co co-doping favors weak ferromagnetism ordering with evident magnetic hysteresis loop and enhances magnetization values at room temperature. Ferroelectric hysteresis loop for Dy and Co co-doped sample shows the nearly saturated polarization at 40 kV/cm and large remnant polarization. Dy dopant is prominent in the reduced leakage current density, while Co dopant is remarkable in the improved remnant magnetization in Bi 0.9 Dy 0.1 Fe 0.97 Co 0.03 O 3 ceramic. By using a simple model, it was found that the anharmonicity of canted spiral cycloidal spin structure was responsible for the weak ferromagnetism of pure BiFeO 3 , and the enhanced magnetization in Co-doped sample was attributed to the transition from the incommensurate cycloidal spiral spin structure towards the G-type canted collinear antiferromagnetic structure.
Physical Review B
The electronic structure and related properties of perovskites ABO 3 are strongly affected by even small modifications in their crystalline structure. In the case of BiFeO 3 , variations in the octahedral rotations and ionic displacements lead to significant changes in the band gap. This effect can possibly explain the wide range of values (2.5-3.1 eV) reported in the literature, obtained from samples of varied structural qualities, including polycrystalline films, epitaxial films grown by pulsed-laser deposition and molecular beam epitaxy, nanowires, nanotubes, and bulk single crystals. Using hybrid density-functional calculations, we investigate the dependence of the electronic structure on the crystal lattice distortions of the ferroelectric-antiferromagnetic BiFeO 3 , disentangling the effects of the ferroelectric ionic displacements and the antiferrodistortive octahedral rotations on the band gap and the band-edge positions. The band gap is shown to vary from 3.39 eV for the rhombohedral ground-state (R3c) structure down to 1.58 eV for the perfect cubic (Pm3m) structure, with changes in the conduction band being much more prominent than in the valence band. The gap varies linearly with the ferroelectric ionic displacements, but nonlinearly with the octahedral rotations around the pseudocubic [111] c axis, and this is explained in terms of the different interactions between Bi 6s, 6p, Fe 3d, and O 2p bands. We argue that such large variation of the band gap with structural changes may well explain the large scattering of the reported values, especially if significant deviations from the equilibrium crystal structure are found near domain boundaries, extended defects, or grain boundaries in polycrystalline films.