Stabilization of α-BiFeO3 structure by Sr2+ and its effect on multiferroic properties

We report a study on the effect of the substitution of Bi³⁺ by Sr²⁺ on the stabilization of R3c structure of Bi_1?xSr_xFeO₃ (0 ≤ x ≤ 0.3, Δx = 0.05), and its effect in the magnetic and dielectric behavior. Stoichiometric mixtures of Bi₂O₃, Fe₂O₃ and SrO were mixed and milled for 5?h using a ball to powder weight ratio of 10:1 by high-energy ball milling. The obtained powder were pressed at 900?MPa to obtain cylindrical pellets and sintered at 800?°C for 2?h. X-ray diffraction and Rietveld refinement were used to evaluate the effect of Sr²⁺ on the crystal structure. In addition, vibrating sample magnetometry (VSM) and dielectric tests were used for describing the multiferroic behavior. The results show that Sr-doped BiFeO₃ particles present rhombohedral structure (R3c) characteristic of α-BiFeO₃ when the doping is below 0.10?mol of Sr. Additionally, a gradual decrease in the amount of secondary phases with the increase of the amount of strontium is observed. For doping concentration higher than 0.15?mol of Sr, a phase transition to an orthorhombic symmetry (β-BiFeO₃, Pbnm) is detected. Besides, changes in relative intensities of reflection peaks planes (110) and (104) are associated with the phase transformations and with the magnetic and dielectric behavior. The α-BiFeO₃ phase show antiferromagnetic behavior and high values of dielectric permittivity, whereas the β-BiFeO₃ phase show a ferromagnetic behavior and low dielectric permittivity.     

Structural,magnetic and photocatalytic properties of Sr2+ doped BiFeO3 nanofibres fabricated by electrospinning

Sr²⁺ doped BiFeO₃ (Bi_1-xSr_xFeO₃, 0?≤x?≤?0.35) nanofibres were fabricated by a sol-gel based electrospinning method. The as-spun BiFeO₃ (BFO) nanofibres consist of fine grained particles with high crystallinity. With Sr²⁺ doping, both the magnetic and photocatalytic properties of BFO are effectively improved. The best photocatalytic property for degradation of the methylene blue (MB) is obtained in Bi_0.75Sr_0.25FeO₃ nanofibres due to their weakest photoluminescence (PL) intensity. Meanwhile, the photocatalytic property of Bi_0.75Sr_0.25FeO₃ nanofibres is much higher than that of nanoparticles with the same constituent, which is attributed to the unique one-dimension fibrous structure benefiting the separation and decreased recombination of e^-/h⁺ pairs. This work proposes an effective approach for the degradation of organic pollutes.     

Synthesis,optical, and magnetic properties of six-layered Aurivillius bismuth ferrititanate

This work reports on the preparation, structure, photochemical, and magnetic properties of six-layered Aurivillius bismuth ferrititanates, that is, Bi₇Ti₃Fe₃O₂₁, Bi₇(Ti₂Nb)Fe₃O_21+δ, and Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticles. The samples were prepared through the modified citrate complexation and precursor film process. The XRD Rietveld refinements were conducted to study the phase formations and crystal structure. The morphological and chemical component characteristics were investigated using SEM, TEM, and EDX analyses. Bi₇Ti₃Fe₃O₂₁, Bi₇(Ti₂Nb)Fe₃O_21+δ, and Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticles present an indirect allowed transitions with band energies of 2.04, 2.03, and 2.02 eV, respectively. The hybridized (O2p+Fet_2g+Bi6s) formed the valence band (VB) and electronic components of (Ti–3d+Fe–e_g) formed the conduction band (CB) of this six-layered Aurivillius bismuth ferrititanate. The three samples showed efficient photocatalytic degradation of Rhodamine B (RhB) dyes with the excitation wavelength λ > 420 nm. The optical absorption, photodegradation, and magnetic abilities were improved through microstructural modification on “B” site via partial substitution of Mg²⁺ and Nb⁵⁺ for Ti⁴⁺. The photocatalytic results were discussed based on the layer structure and multivalent Fe ions. Fe^3+/2+ in the perovskite slabs (Bi₅Fe₃Ti₃O₁₉)²⁻ could act as the catalytic mediators in the photocatalysis process. As a photocatalyst, Aurivillius Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticle is advantageous due to its photocatalytic and magnetically recoverable abilities.     

Reaction pathways in the solid state synthesis of multiferroic BiFeO3

The obtaining of multiferroic BiFeO₃ as a pure single-phase product is particularly complex since the formation of secondary phases seems to be unavoidable. The process by which these secondary impurities are formed is studied by analyzing the diffusion and solid state reactivity of the Bi₂O₃-Fe₂O₃ system. Experimental evidence is reported which indicates that the progressive diffusion of Bi³⁺ ions into the Fe₂O₃ particles governs the solid state synthesis of the perovskite BiFeO₃ phase. However a competition is established between the diffusion process which tends to complete the formation of BiFeO₃, and the crystallization of stable Bi₂Fe₄O₉ mullite crystals, which tend to block that formation reaction.     

Neutron diffraction study and temperature variation of magnetic anisotropy in Bi substituted nickel ferrite

Bismuth doped NiFe₂O₄ (NFO) polycrystalline samples: Ni_1-xBi_xFe₂O₄ with x = 0.00, 0.05 and 0.10, were synthesized using solid-state reaction method. The crystal structural, magnetic structure and magnetic properties of parent NFO and Bi-doped NFO samples were characterized using X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetization (M) versus magnetic field (H) isotherms, respectively. Rietveld refinement of XRD and NPD patterns confirms the formation of a single-phase cubic spinel crystal structure. No observable change in the lattice parameter was found (within the error limits) with the 5 at % Bi doping to NFO, whereas ≈0.15% change in the lattice parameter was observed in case of 10 at % Bi-doped NFO. Room temperature magnetic structure studies using NPD reveal a net magnetic moment of 1.55(3) μ_B/f. u., in the case of NiFe₂O₄. The net magnetic moment at room temperature changes to 1.61(4) μ_B/f.u. and 1.47(5) μ_B/f.u. for Ni_0.95Bi_0.05Fe₂O₄ and Ni_0.90Bi_0.10Fe₂O₄, respectively. The same observation was also inferred from the spontaneous magnetization (M_sp) obtained from approach-to-saturation (ATS) analysis of M-H isotherms. Temperature (T) dependence of M_sp follows a T² decline. Approximately 14% increase in magnetic anisotropy was observed in Ni_0.95Bi_0.05Fe₂O₄ compared to NiFe₂O₄ in the temperature range 5 K -300 K.     

Effect of La and Ni substitution on structure,dielectric and ferroelectric properties of BiFeO3 ceramics

In this communication, we present the results on Bi_1−xLa_xFe_1−yNi_yO₃ (x=0.0, 0.1; y=0.0, 0.05) samples processed by solid-state reaction route in order to study crystalline and electronic structure, dielectric and ferroelectric properties. The best refinement was achieved by choosing rhombohedral structure (R3c) for BiFeO₃ and Bi_0.9La_0.1FeO₃ samples. Whereas, the XRD pattern of BiFe_0.95Ni_0.05O₃ and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ samples were refined by choosing rhombohedral (R3c) and cubic (I23) structure. Raman scattering measurement infers nine Raman active phonon modes for all the as prepared samples. The substitution of Ni ion at Fe-site in BiFeO₃ essentially changes the modes position i.e. all the modes are observed to shift to lower wave number. Dielectric constant (ε′) and dielectric loss (tan δ) as a function of frequency have been investigated and they decreases with increasing frequency of the applied alternating field and become constant at high frequencies. This feature is a characteristic of Maxwell Wagner type of interfacial polarization. The remnant polarization (P_r) for Bi_0.9La_0.1FeO_3, BiFe_0.95Ni_0.05O₃, and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ are 0.08, 0.11, 0.69 μC/cm², respectively and the value of coercive field for Bi_0.9La_0.1FeO_3, BiFe_0.95Ni_0.05O₃, and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ are 0.53, 0.67, 0.68 kV/cm, respectively. X-ray absorption spectroscopy (XAS) experiments at Fe L₂,₃ and O K-edges are performed to investigate the electronic structure of well-characterized Bi_1−xLa_xFe_1−yNi_yO₃ (x=0.0, 0.1; y=0.0, 0.05) samples. The presence of reasonable ferroelectric polarization at room temperature in Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ ceramics makes it suitable for technological applications.     

Visible Light Photocatalytic Activity of Bismuth Ferrites Tuned by Bi/Fe Ratio

Pure perovskite BiFeO₃, sillenite Bi₂Fe₄O₉, and BiFeO₃/Bi₂Fe₄O₉ were synthesized facilely by controlling the precursor Bi/Fe ion ratio through a hydrothermal method. The phase composition, morphology, optical properties, and photocatalytic activity of as‐prepared samples were investigated in detail. Our results indicate that the morphology and properties of products strongly dependent on the precursor Bi/Fe ion ratio. Photocatalysis of Congo Red reveals that sillenite Bi₂Fe₄O₉ shows superior activity to perovskite BiFeO₃, and BiFeO₃/Bi₂Fe₄O₉ exhibited higher activity with 71.45% degradation rate in 90 min. It provides an easy and efficient way to tune the composition and photocatalytic activity of Bi‐based oxides.     

Enhanced multiferroic properties in Pr-doped BiFe0.97Mn0.03O3 films

Bi_1−xPr_xFe_0.97Mn_0.03O₃ (x=0.00, 0.05, 0.10, 0.15, 0.20) thin films were deposited on FTO/glass substrate using chemical solution deposition. The influences of Pr doping on the crystalline structure and multiferroic properties were investigated. In the X-ray diffraction and Raman spectra results, the crystal structures of Bi_1−xPr_xFe_0.97Mn_0.03O₃ films revealed a gradual transformation from the trigonal structure to the tetragonal structure. The leakage current densities of Bi_1−xPr_xFe_0.97Mn_0.03O₃ films are one order of magnitude lower than that of BiFeO₃. Compared with unsaturated polarization-electric field hysteresis loop of BiFeO₃ film, the Pr and Mn co-doped BFO films have significantly improved ferroelectric properties. The improved remnant polarization (Pr=91.3 µC/cm²) and the positive switching current (I=0.028 A) have been observed in Bi_0.85Pr_0.15Fe_0.97Mn_0.03O₃ film. The improved electrical properties are attributed to the structure transformation, increasing grain boundaries, low oxygen vacancies ratio and increasing Fe³⁺ concentration. In addition, the saturation magnetization of Bi_0.85Pr_0.15Fe_0.97Mn_0.03O₃ film is 1.81 emu/cm³, which is approximately three times higher than pure BiFeO₃ (M_s=0.67 emu/cm³).     

Enhanced electrocatalytic activity and CO2 tolerant Bi0.5Sr0.5Fe1-xTaxO3-δ as cobalt-free cathode for intermediate-temperature solid oxide fuel cells

One of the significant motivations in developing intermediate-temperature solid oxide fuel cells (IT-SOFCs) is to design cobalt-free cathodes with high electrocatalytic activity and CO₂ tolerance ability. In this work, iron-based perovskite materials Bi_0.5Sr_0.5Fe_1-xTa_xO_3-δ are investigated as potential cathodes for IT-SOFCs. The effects of Ta doping on crystal structure, thermal expansion coefficients and electrocatalytic activities are systematically evaluated. Among the Ta-doped oxides, Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3-δ exhibits the highest electrochemical performance with the lowest polarization resistance (R_p) of 0.124 Ω cm² at 700 °C in air. The peak power density of the single cell with Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3-δ cathode reaches 1.36 W cm⁻² at 700 °C. Compared to Bi_0.5Sr_0.5FeO_3-δ, the improved CO₂ tolerance of Ta-doped oxides can be attributed to the high acidity of Ta⁵⁺ cations and the increased average metal bond energy (ABE) within the material. Further study proves that the adsorption-dissociation process of molecular oxygen is the limiting step for oxygen reduction reaction (ORR) on Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3-δ cathode.     

Hierarchical spinel NixCo1-xFe2O4 microcubes derived from Fe-based MOF for high-sensitive acetone sensor

Hierarchically spinel Ni_xCo_1-xFe₂O₄ (0.0?≤?x?≤?0.5) microcubes were successfully prepared through a combined solvent evaporation strategy and morphology-inherited annealing treatment. By using the Fe-based metal-organic frameworks (MOFs), Fe₄(Fe(CN)₆)₃, and inorganic Ni²⁺and Co²⁺ acetate salts as co-templated precursors, hierarchical Ni-doped spinel CoFe₂O₄ architecture can be obtained, and the Ni/Co substitution ratios can be controlled systematically. The obtained cubic hierarchical Ni_xCo_1-xFe₂O₄, (0.0?≤?x?≤?0.5) composites presented highly acetone sensing properties, among which the Ni_0.1Co_0.9Fe₂O₄ composite showed the strongest response performance (with gas response (R_g/R_a) of 1.67 at 240?°C) and excellent reproducibility (for at least 14 cycles), and the proposed acetone sensing mechanism was also discussed. The presented solvent evaporation and co-templated strategy may allow precisely access to fabricating other heteroatom doped inorganic materials with intriguing morphologies, architectures, chemical compositions and tunable sensing properties.     

Magnetic Properties of FeMn<Subscript>y</Subscript>Co<Subscript>y</Subscript>Fe<Subscript>2−2y</Subscript>O<Subscript>4</Subscript>@Oleylamine Nanocomposite with Cation Distribution

In this study, oleylamine (OAm) capped FeMn_yCo_yFe_2?2yO₄ (0.0?≤?y?≤?0.4) nanocomposites (NCs) were prepared via the polyol route and the impact of bimetallic Co³⁺ and Mn³⁺ ions on the structural and magnetic properties of Fe₃O₄ was investigated. The complete characterization of FeMn_yCo_yFe_2?2yO₄@OAm NCs were done by different techniques such as XRD, SEM, TGA, FT-IR, TEM, and VSM. XRD analyses proved the successful formation of mono-phase MnFe₂O₄ spinel cubic products free from any impurity. The average crystallite sizes were calculated in the range of 9.4–26.4 nm using Sherrer’s formula. Both SEM and TEM results confirmed that products are nanoparticles like structures having spherical morphology with small agglomeration. Ms continued to decrease up to Co³⁺ and Mn³⁺ content of y?=?0.4. Although Mössbauer analysis reveals that the nanocomposites consist three magnetic sextets and superparamagnetic particles are also formed for Fe₃O₄, Co_0.2Mn_0.2Fe_2.6O₄ and Co_0.4Mn_0.4Fe_2.2O₄. Cation distributions calculation was reported that Co³⁺ ions prefer to replace Fe²⁺ ions on tetrahedral side up to all the concentration while Mn³⁺ ions prefer to replace Fe³⁺ ions on the octahedral.     

Effects of Ho and Ti Doping on Structural and Electrical Properties of BiFeO3 Thin Films

Effects of Ho and Ti ions individual doping and co‐doping on the structural, electrical, and ferroelectric properties of the BiFeO₃ thin films are reported. Pure BiFeO₃, (Bi_0.9Ho_0.1)FeO₃, Bi(Fe_0.98Ti_0.02)O_3+δ, and (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. All thin films were crystallized in distorted rhombohedral structure containing no secondary or impurity phases confirmed by using an X‐ray diffraction study. Changes in microstructural features, such as grain morphology and grain size distribution, for the doped samples were analyzed by a scanning electron microscopy. From the experimental results, a low electrical leakage (1.2 × 10^?5 A/cm² at 100 kV) and improved ferroelectric properties, such as a large remnant polarization (2P_r) of 52 μC/cm² and a low coercive field (2E_c) of 886 kV/cm, were observed for the (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ thin film. Fast current relaxation and stabilization observed in the (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ imply effective reduction and neutralization of charged free carriers.     

A review of the structure,magnetic and electrical properties of bismuth ferrite (Bi2Fe4O9)

Nowadays, investigations on the materials with multiferroic properties are in progress. These materials compromise simultaneous electric and magnetic properties. Ferrite Bismuth (FB) is one of the ceramic materials that enjoy this property and possesses three different crystalline structures (perovskite BiFeO₃, selenite Bi₂₅FeO₄₀ and mullite Bi₂Fe₄O₉). In this review, first, the crystalline structure and the electric and magnetic properties of Bi₂Fe₄O₉ are studied, and then, the effects of adding dopants to the ferrite are discussed. Mullite-type bismuth ferrite (Bi₂Fe₄O₉) as a spin frustrated multiferroic has potential for magnetoelectric coupling, and it might be an appropriate alternative for some of the multiferroics that suffer from a weak magnetoelectric coupling.     

Oxygen vacancy and grain boundary resistance regulate the intrinsic ferroelectric properties of Bi0.96Sr0.04Fe0.98−xMnxCo0.02O3 thin film

Bi_0.96Sr_0.04Fe_0.98−xMn_xCo_0.02O₃ (BSFM_xCO, x = 0.00–0.05) thin films with ferroelectric properties were successfully prepared by chemical solution deposition (CSD). It is found that BSFM_xCO has a stable trigonal structure. The conversion from Fe³⁺ to Fe²⁺ inhibiting Mn³⁺ and the synergistic effects of Mn³⁺ and Co³⁺ reduce the oxygen vacancies, which prevents the formation of defect complexes. The results show that the leakage current of BSFM_0.04CO is dropped to 0.011 A/cm². Simultaneously, the weakened grain boundaries resistance and the pinning effect by doping Mn³⁺ reduce the built-in electric field effectively, which enhances the polarization reversal current and the capacitance. Under the applied electric field, we have obtained the intrinsic residual polarization value of 151 μC/cm², thanks to the easily reversed domains of BSFM_0.04CO and the enhanced intrinsic polarization. Therefore, ion doping can reduce oxygen vacancies and the grain boundary resistance. As a result, the intrinsic ferroelectricity in BiFeO₃ is enhanced by the improvement of intrinsic polarization.     

Ferroelectric properties and core shell domain structures of Fe-modified 0.77Bi0.5Na0.5TiO3-0.23SrTiO3 ceramics

The crystal structure, domain patterns, and ferroelectric properties of Fe-modified BNT-ST [0.77(Bi_0.5Na_0.5)TiO₃-0.23Sr(Ti_1-xFe_x)O₃] ceramics, fabricated by a conventional solid-state reaction, were investigated. Core-shell structures were observed and the volume fractions of the core-domain and shell (relaxor-matrix) were found to be dependent on Fe-modification content. The crystal structures of the core-domain and the relaxor-matrix were rhombohedral with the space group R3c, and the tetragonal with the space group P4bm, respectively. Compositional inhomogeneity, specifically, the enrichment of Bi³⁺ and Na⁺ and the considerable depletion of Sr²⁺, were observed in the core-domain region, and was reduced by substituting Ti⁴⁺ with Fe³⁺. The Fe-modification of the BNT-23ST ceramics promoted the diffusion of Sr²⁺ ions into the core region and shifted ferroelectric behaviour towards ergodic-relaxor behaviour. This improved the effective d₃₃* of BNT-23ST ceramics to over 500 pm/V at 2 kV/mm.     

Achieving ultra-high electromagnetic wave absorption by anchoring Co0.33Ni0.33Mn0.33Fe2O4 nanoparticles on graphene sheets using microwave-assisted polyol method

The Co_0.33Ni_0.33Mn_0.33Fe₂O₄/graphene nanocomposite for electromagnetic wave absorption was successfully synthesized from metal chlorides solutions and graphite powder by a simple and rapid microwave-assisted polyol method via anchoring the Co_0.33Ni_0.33Mn_0.33Fe₂O₄ nanoparticles on the layered graphene sheets. The Fe³⁺, Co²⁺, Ni²⁺ and Mn²⁺ ions in the solutions were attracted by graphene oxide obtained from graphite and converted to the precursors Fe(OH)₃, Co(OH)₂, Ni(OH)₂, and Mn(OH)₂ under slightly alkaline conditions. After the transformations of the precursors to Co-Ni-Mn ferrites and conversion of graphene oxide to graphene under microwave irradiation at 170?°C in just 25?min, the Co_0.33Ni_0.33Mn_0.33Fe₂O₄/graphene nanocomposite was prepared. The composition and structure of the nanocomposite were characterized by X-ray diffraction (XRD), inductive coupled plasma emission spectroscopy (ICP), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy (RS), transmission electron microscopy (TEM), etc. It was found that with the filling ratio of only 20?wt% and the thickness of 2.3?mm, the nanocomposite showed an ultra-wide effective absorption bandwidth (less than ?10?dB) of 8.48?GHz (from 9.52 to 18.00?GHz) with the minimum reflection loss of ??24.29?dB. Compared to pure graphene sheets, Co_0.33Ni_0.33Mn_0.33Fe₂O₄ nanoparticles and the counterparts reported in literature, the nanocomposite exhibited much better electromagnetic wave absorption, mainly attributed to strong wave attenuation, as a result of synergistic effects of dielectric loss, conductive loss and magnetic loss, and to good impedance matching. In view of its thin thickness, light weight and outstanding electromagnetic wave absorption property, the nanocomposite could be used as a very promising electromagnetic wave absorber.     

Structural and thermoelectric properties of hydrothermal-processed Ag-doped Fe2O3

Fe_2-xAg_xO₃ (0?≤?x?≤?0.04) nanopowders with various Ag contents were synthesized at different hydrothermal reaction temperatures (150?°C and 180?°C). Their structural properties were fully investigated through an X-ray diffraction, a Fourier transform infrared spectroscopy, and an X-ray photoelectron spectroscopy. The hydrothermal reaction temperature, time, and Ag content remarkably affected the morphological characteristics and crystal structure of the synthesized powders. The Fe_2-xAg_xO₃ (0?≤?x?≤?0.04) powders synthesized at 150?°C for 6?h and the Fe_2-xAg_xO₃ (0.02?≤?x?≤?0.04) powders synthesized at 180?°C for 12?h formed the orthorhombic α-FeOOH phase with a rod-like morphology, whereas the Fe_2-xAg_xO₃ (0?≤?x?≤?0.01) powders synthesized at 180?°C for 12?h formed the rhombohedral α-Fe₂O₃ phase with a spherical-like morphology. The Fe_1.98Ag_0.02O₃ fabricated by utilizing Fe_1.98Ag_0.02O₃ powders synthesized at 180?°C showed the largest power factor (0.64?×10^?5 Wm^?1 K^?2) and dimensionless figure-of-merit (0.0036) at 800?°C.     

Synthesis and characterization of yttrium iron garnet nanoparticles doped with cobalt

In this work, we have synthesized and characterized yttrium iron garnet nanoparticles doped with cobalt. The X-ray diffraction data showed a single phase, belonging to the cubic structure of Y₃Fe₅O₁₂. Rietveld refinement revealed variation of the angles and interionic distances (Fe³⁺(a)-O^2-Y³⁺(c) and Fe³⁺(d)-O^2--Y³⁺(c) when Fe³⁺ ions are replaced by Co³⁺ ions in the tetrahedral (d) and octahedral (a) sites of YIG. In addition, the lattice parameter a, decreases from 12.3846?Å to 12.3830?Å with the increasing of cobalt concentration. The analysis by Infrared and Raman spectroscopies has shown a slight stretching at lower wave numbers as the dopant concentration increased. The magnetic measurements confirm the substitution of Fe³⁺ by Co³⁺ in the a-sites and d-sites with the reduction of the saturation magnetization from 26.63?emu/g to 24.92?emu/g, for 0.000?≤?y?≤?0.030. Changes in the coercive field varying the dopant concentration were related to the particle size and pinning centers existence.     

Structural and magnetic studies of A site doped LaRh1−xCuxO3(A=Ca2+, Sr2+, Pb2+ and Bi3+)

A site doped Rh perovskites with general formula La_0.75A_0.25Rh_0.7Cu_0.3O₃ and La_0.75A_0.25Rh_0.5Cu_0.5O₃ (A=Ca²⁺, Sr²⁺, Pb²⁺and Bi³⁺) were synthesized by solid-state methods and their crystallographic, magnetic, and electric properties investigated. Doping by divalent cations resulted in much lower cell volumes and octahedral distortions than doping with a trivalent oxide. The Pb²⁺ and Bi³⁺ (6s²) doped oxides exhibited the lowest magnetic moments and the highest activation energies. Magnetization curves are indicative of antiferromagnetic behavior. The addition of Ca²⁺, Sr²⁺, Pb²⁺and Bi³⁺cations to the A-site decreases the conductivity.     

Effect of Bismuth substitution on the enhancement of thermoelectric power factor of nanostructured BixCo3-xO4

Bismuth Cobalt Oxide (Bi_xCo_3-xO₄) nanoparticles with different compositions (x?=?0, 0.025, 0.05, 0.1, 0.2) were prepared by chemical precipitation method. The structural, morphological and thermal properties of the prepared samples were studied by XRD, SEM, FTIR and TG&DTA analysis. X-ray diffraction analysis shows that pure phase of Cobalt oxide was formed till x?≤?0.05 and while increasing the Bi concentration (0.05?≤ x?≤?0.2) mixed phases of Co₃O₄, Co₂O₃, CoO and separate phase of Bi₂O₃ were formed. The diffraction peaks were reasonably shifted due to substitution of Bi²⁺ ions. XPS analysis conforms the presence of mixed valance states of Co and presence of Bi with their binding states in the samples. The electrical resistivity and Seebeck coefficient were measured for Bi_xCo_3-xO₄ (0?≤ x?≤?0.2) at different temperatures. It was observed that the electrical resistivity decrease till x?≤?0.05 due to the substitution of Bi ions in Cobalt lattice and increases at higher x values (0.05?≤ x?≤?0.2) due to the formation of Bi₂O₃ phase. The Bi substitution has considerably reduced the electrical resistivity by one order when completely dissolved in the cobalt oxide lattice at lower x values. The Seebeck coefficient value gradually increased for all samples of Bi_xCo_3-xO₄ (0?≤ x?≤?0.2). The power factor was calculated from electrical resistivity and Seebeck coefficient and the maximum power factor of 0.025 µWm^?1K^?2 was obtained for Bi_0.2Co_2.8O₄ sample at 530?K. The experimental results revealed that the Bi substitution have promising effect on the thermoelectric properties of nanostructured Bi_xCo_3-xO₄ (0?≤ x?≤?0.2).