Effect of Gd-substitution on phase transition and conduction mechanism of BiFeO3

The polycrystalline sample of (Bi_0.8Gd_0.2)FeO₃ was prepared by a high-temperature solid-state reaction technique. Preliminary X-ray structural analysis of the sample confirms the formation of the desired compound with rhombohedral phase. The scanning electron micrograph of the sample showed uniform distribution of the plate- and rod-shaped grains. Studies of dielectric and electrical properties of the material were investigated within a wide range of temperature (25–400 °C) and frequency (1 kHz–1 MHz) using complex impedance spectroscopic method. The observation of hysteresis loop of the material confirmed that the material has a ferroelectric property at room temperature. The ac conductivity suggests that the sample obey Jonscher’s universal power law. The dc conductivity follows Arrhenius equation. Detailed studies of ac and dc conductivity show a negative temperature coefficient of resistance (NTCR) behavior of the sample.     

Structural,electrical and magneto-electric characteristics of complex multiferroic perovskite Bi<Subscript>0.5</Subscript>Pb<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Ce<Subscript>0.5</Subscript>O<Subscript>3</Subscript>

The polycrystalline sample of bismuth based-complex multiferroic of a composition Bi_0.5Pb_0.5Fe_0.5Ce_0.5O₃ was prepared by a high-temperature solid-state reaction technique (calcinations temperature = 900 °C, sintering temperature = 960 °C, time = 4 h). Preliminary structural analysis using XRD data exhibits the formation of a single-phase compound. Studies of surface morphology of the ceramic sample of the compound, recorded at room temperature using a scanning electron microscope, show uniform distribution of grains of different size with few voids. Detailed studies of dielectric properties (ε_r, tan δ) supported the existence of multiferroic properties in the above complex system. The analysis of impedance parameters, recorded in a wide frequency (1 kHz–1 MHz) and temperature (room temperature to 450 °C) range of the material provide better understanding of (a) role of grains and grain boundaries in resistive and capacitative characteristics, (c) structure-properties relationship and (b) type of relaxation process occurred in the material. Study of temperature dependence of dc conductivity of the compound shows the existence of negative temperature coefficient of resistance in it. The nature of variation of ac conductivity with temperature of the material follows the Josher’s universal power law. Study of magneto-electric characteristics of the sample at room temperature has provided many useful and new data on magneto-electric coupling coefficient of different orders.     

Dielectric and Structural Properties of Bi<Subscript>5</Subscript>Ti<Subscript>3</Subscript>FeO<Subscript>15</Subscript> Ceramics Obtained by Solid-State Reaction Process from Mechanically Activated Precursors

Bi₅Ti₃FeO₁₅ (BTFO), an Aurivillius compound, was synthesized via sintering the Bi₂O₃ and Fe₂O₃ mixture and TiO₂ oxides. The precursor material was ground in a high-energy attritorial mill for (1, 3, 5, and 10) h. The orthorhombic system Bi₅Ti₃FeO₁₅ ceramics was obtained by a solid-state reaction process at 1313 K. Phase formation behavior was investigated using differential thermal analysis (DTA), thermal gravimetric (TG), and X-ray diffraction (XRD) techniques. The frequency-dependent properties of the material were investigated by impedance spectroscopy. The impedance spectroscopic method is widely used to characterize electrical properties of materials and their interfaces with electronically conducting electrodes. These studies indicate that 1h, 3h, and 5h primary high-energy ball milling followed by sintering is a promising technique for pure Bi₅Ti₃FeO₁₅ ceramic preparation, whereas the ceramics obtained from the substrates after 10h milling is a two-phase material. As the result of this investigation, the model of adjusting the Nyquist charts with a three-element R-CPE (constant phase element) series connection was proposed. It was found that the value of the dielectric constant at the Curie temperature decreases when the milling time of the substrates increases. The decrease in the dielectric constant is influenced by the great dispersion of the grains, their dense packing, and location of particular grains in relation to other grains. Moreover, the change of resistivity with frequency indicates that relaxation processes take place in the material. In conclusion, it was reported that the optimal milling time of precursor oxide powders for carrying out the sintering process is equal to 5 h. Then the obtained ceramics contain one phase and exhibit the highest dielectric properties for practical applications.     

Structural,dielectric and impedance spectroscopy studies of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)(Zr<Subscript>0.025</Subscript>Ti<Subscript>0.975</Subscript>)O<Subscript>3</Subscript> ceramic

Structural, vibrational, dielectric and electrical properties of (Na_0.5Bi_0.5)(Zr_0.025Ti_0.975)O₃ ceramic synthesized by the solid-state reaction technique have been carried out. The X-ray diffraction analysis was indicated as a pure perovskite phase in the rhombohedral structure. The modes of rhombohedral vibrations were appeared in the experimental Raman spectrum at room temperature. The dielectric and electrical properties of the material were investigated by impedance spectroscopy analysis for a broad range of temperatures (50–560 °C) and frequency domain of 10²?10⁶ Hz. The dielectric measurement exhibit two phase transitions: a ferro-antiferroelectric transition followed by an antiferro-paraelectric transition at higher temperatures. Complex impedance analysis was carried out in order to distinct the contribution of the grains and the grain boundaries to the total electrical conduction. The Nyquist plot was proved to be a non-Debye relaxation mechanism. The combined spectroscopic plots of the imaginary part of electric impedance and modulus confirmed the non-Debye type behavior. The frequency dependent ac conductivity obeys the double power law behavior and shows three types of conduction process. The significant decrease of dc conductivity spectrum followed the Arrhenius relationship. The values of calculated activation energy of the compound implied that the electrical conduction is mostly due the high oxygen mobility.     

Electrical and dielectric properties of bismuth holmium cobalt titanate (BiHoCoTiO<Subscript>6</Subscript>): a complex double perovskite

A lead-free bismuth holmium cobalt titanate multiferroic(BiHoCoTiO₆) was synthesized at high temperatures by a solid-state reaction (a mixed oxide) route. Structural analysis of the compound is performed using X-ray diffraction data and an orthorhombic crystal system is suggested for the material. Study of room temperature scanning electron microigrapgh exhibited better morphology for the material. The uniform distribution of the small rod-type of grains with dimension of 1–2 µm length and 0.2–0.3 µm diameters was visible. Analysis of temperature-frequency dielectric data exhibited two dielectric anomalies or phase transitions: first transition temperature(t_c1) at 175?°C and the second one at 325?^°C. Study of frequency and temperature dependence of resistive characteristics (performed using complex impedance spectroscopy) has shown significant contributions of grains and grain boundaries, which in turn, helps in understanding the electrical conduction mechanism and microstructure behaviour of the material in a better way. The impedance or Nyquist plots were modelled with an equivalent circuit containing capacitance, resistance and related parameters due to grain (bulk), grain boundaries and capacitance. The transport properties, AC conductivity and electrical modulus of the material were also investigated and reported here.     

Dielectric and impedance properties of Nd3/2Bi3/2Fe5O12 ceramics

The polycrystalline sample of Nd_3/2Bi_3/2Fe₅O₁₂ was prepared by a high- temperature solid-state reaction technique. Preliminary X-ray structural analysis exhibits the formation of a single-phase tetragonal structure at room temperature. Microstructural analysis by scanning electron microscopy shows that the sintered sample has well defined grains. These grains are distributed uniformly throughout the surface of the sample. Detailed studies of dielectric response at various frequencies and temperatures exhibit a dielectric anomaly at 400 °C. The electrical properties (impedance, modulus and conductivity) of the material were studied using a complex impedance spectroscopy technique. These studies reveal a significant contribution of grain and grain boundary effects in the material. The frequency dependent plots of modulus and the impedance loss show that the conductivity relaxation is of non-Debye type. Studies of electrical conductivity with temperature demonstrate that the compound exhibits Arrhenius-type of electrical conductivity. Study of ac conductivity with frequency suggests that the material obeys Jonscher’s universal power law.     

Dielectric and impedance characteristics of Ba(Bi0.5Nb0.5)O3 ceramics

A lead free polycrystalline material Ba(Bi_0.5Nb_0.5)O₃ was prepared using a high-temperature mixed oxide technique using high purity ingredients. The formation of the material in monoclinic crystal structure was confirmed by an X-ray structural analysis at room temperature. The nature and texture of microstructure by scanning electron microscopy show that the compound has well defined grains uniformly distributed throughout the surface of the sample. Detailed studies of dielectric and impedance properties of the material, carried out in the frequency range of (1 kHz–1 MHz) at different temperatures (30 °C to 475 °C), have shown many interesting properties. Dielectric study showed an existence of diffuse phase transition around 317 °C. The temperature dependence of impedance parameters (impedance, modulus etc.) of the material exhibits a strong correlation of its micro-structure (i.e., bulk, grain boundary, etc.) with the electrical parameters. An existence of negative temperature coefficient of resistance (NTCR) type behavior in the material similar to that of semiconductors was also observed. The complex electric modulus analysis indicates the existence of hopping conduction mechanism in the system with non-exponential type of conductivity relaxation. The nature of variation of dc conductivity with temperature confirms the Arrhenius behavior of the material. The ac conductivity spectra show a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law. The temperature dependent pre-exponential factor (A) shows peak and frequency exponent (n) possesses a minimum at transition temperature.     

Improved electrical and ferroelectric properties of multiferroic Na0.5Bi0.5TiO3/Bi1.07Nd0.03FeO3/Na0.5Bi0.5TiO3 sandwiched structure by a sol–gel process

A ferroelectric/multiferroic/ferroelectric sandwiched structure composed by Na_0.5Bi_0.5TiO₃ (NBT) and Bi_1.07Nd_0.03FeO₃ (BNF) with a LaNiO₃ buffer layer were prepared by a sol–gel method. X-ray diffraction indicated the NBT/BNF/NBT films exhibited a pure perovskite structure. The average grain size of BNF and NBT/BNF/NBT were found to be 40 and 80 nm, respectively. Interestingly, the electrical and ferroelectric properties such as leakage current, dielectric constant, and remnant polarization of NBT/BNF/NBT sandwiched layer, were superior to those of BNF single film. However, the saturation magnetization of NBT/BNF/NBT sandwiched layer was reduced. Our work suggested the NBT/BNF/NBT sandwiched layer with improved multiferroic characteristics have a promising application for future information storage devices.     

Improvement of multiferroic properties of the Bi5Ti3FeO15 thin films by Ni doping

Pentabismuth trititanium iron pentadecaoxide (Bi₅Ti₃FeO₁₅, BTFO), which is a multiferroic four-layer Aurivillius phase compound, has received much attention in recent years. In this work, the BTFO and Bi₅Ti₃Fe_0.5Ni_0.5O₁₅ (BTFNO) films grown on Pt (200 nm)/TiO₂ (50 nm)/SiO₂ (500 nm)/Si substrates by pulsed laser deposition. The microstructure, morphology, ferroelectricity, and ferromagnetism of the films were studied at room temperature. It can be seen from the X-ray diffraction diagrams that BTFO and BTFNO films match well with the perovskite structure without other oxide phases. Scanning electron microscope (SEM) images show that BTFNO films exhibit uniform fine grains and higher density, which is instrumental for the development of ferroelectric properties. Therefore, under the electric field strength of 1116 kV/cm, the remnant polarization of BTFNO films (2P_r?=?23.32 μC/cm²) is larger than that of BTFO thin films at room temperature. Furthermore, compared with BTFO films, the saturation magnetization (2M_s) of BTFNO films is significantly enhanced. It demonstrated that BTFNO films show not only excellent ferroelectricity but also good ferromagnetism, which provides potential applications in information storage.

     

Dielectric and impedance spectroscopy of Sr(Bi0.5Ta0.5)O3 electroceramics

Polycrystalline sample of Sr(Bi_0.5Ta_0.5)O₃ was prepared by a high-temperature solid-state reaction technique. Preliminary X-ray diffraction analysis confirms the formation of single-phase compound of orthorhombic crystal structure at room temperature. The study of microstructure of gold-coated pellet by scanning electron microscopy shows well-defined and homogeneous distribution of grains on the surface of the sample. The dielectric properties of the compound studied in a wide frequency range (1 kHz–1 MHz) at different temperatures (27–500 °C), exhibits that they are temperature dependent. Detailed analysis of impedance spectra showed that the electric properties of the material are strongly dependent on frequency and temperature. Studies of electric modulus show the presence of hopping conduction mechanism in the material with non-exponential type of relaxation. The decrease in value of bulk resistance on increasing temperature suggests the negative temperature co-efficient of resistance behavior of the material. The AC-conductivity spectrum provides a typical-signature of an ionic conducting system, and is found to obey Jonscher’s universal power law.     

A comparative study of structural,electrical and magnetic properties rare-earth (Dy and Nd)-modified BiFeO3

The polycrystalline samples of BiFeO₃ (BFO) and rare earth-modified bismuth iron oxide, Bi_0.95R_0.25FeO₃ (R = Nd, Dy) (BNFO, BDFO) are prepared by a standard high-temperature solid-state reaction technique. A preliminary x-ray structural analysis is carried out to examine the structural deformation and stability of rare earth-modified BFO. Room temperature surface morphologies and textures of the samples are recorded by a scanning electron microscope, which reveals the uniform distribution of the plate-and rod-shaped grains. Studies of dielectric and electric properties in a wide frequency (1 kHz–1 MHz) and temperature (30–400 °C) ranges using complex impedance spectroscopic method have provided many new results. The dielectric constant is found to be increases, and the tangent loss decreases as compared to BFO. The electrical polarizations (spontaneous and remnant) is found to be enhanced on rare-earth substitutions. Studies of ac conductivity suggest that the samples obey Jonscher’s universal power law. The enhancement of magnetization was observed in rare-earth doped samples compared to pure BFO.     

Room-temperature multiferroic properties and magnetoelectric coupling in Bi4−x Sm x Ti3−x Co x O12−δ ceramics

We present the structural, dielectric, ferroelectric, magnetic and magnetoelectric studies of lead free; single phase Bi_4?x Sm _x Ti_3?x Co _x O_12?δ (0 ≤ x ≤ 0.07) ceramics, synthesized using a standard solid-state reaction technique. Raman spectroscopy analysis reveals the relaxation of distortion in TiO₆ octahedron. Field emission scanning electron microscopy confirmed the growth of plate-like grains. It is observed that with the substitution of Sm³⁺ and Co³⁺ ions the dielectric constant, loss tangent and ferroelectric transition temperature decreases. Electrical dc resistivity, remnant polarization and magnetization increases with increasing Sm³⁺ and Co³⁺ contents. The magnetoelectric coupling co-efficient, α = 0.65 mV cm^?1 Oe^?1 is realized for Bi_4?x Sm _x Ti_3?x Co _x O_12?δ (x = 0.07) ceramic sample. Our results clearly demonstrate the lead free, multiferroic nature of Sm/Co-substituted Bi₄Ti₃O₁₂, which may find useful application in designing cost-effective electromagnetic devices.     

Enhanced electric field tunable magnetic properties of lead-free Na0.5Bi0.5TiO3–MnFe2O4 multiferroic composites

Strong magnetoelectric (ME) interaction was exhibited at both dc and microwave frequencies in a lead-free multiferroic particulate composites of Na_0.5Bi_0.5TiO₃ (NBT) and MnFe₂O₄ (MFO) multiferroic, which were prepared by sol–gel route. The room temperature permeability measurements were carried out in the frequency range of 1 MHz–1 GHz. A systematic study of structural, magnetic and ME properties were undertaken. The room temperature ferromagnetic resonance (FMR) was studied. Strong ME coupling is demonstrated in 70NBT–30MFO composite by an electrostatically tunable FMR field shift up to 428 Oe (at E = 4 kV/cm), which increases to a large value of 640 Oe at E = 8 kV/cm. Furthermore, these lead-free multiferroic composites exhibiting electrostatically induced magnetic resonance field at microwave frequencies provide great opportunities for electric field tunable microwave devices.     

Synthesis and characterization of Na2Pb2Pr2W2Ti4Ta4O30

A new complex oxide (Na₂Pb₂Pr₂W₂Ti₄Ta₄O₃₀) of tungsten bronze structural family has been synthesized by a high-temperature solid-state reaction (mixed-oxide) route at 1,050 °C. Room temperature structural analysis shows the formation of a single phase new compound. Study of microstructure of the pellet sample, recorded by scanning electron microscope, exhibits the uniform distribution of different size and shape of grains (with a few small voids) on the surface of the sample. Detailed studies of dielectric properties as a function of frequency and temperature show a dielectric anomaly above room temperature suggesting the existence of a ferroelectric phase transition in the material. Impedance spectroscopic analysis and electrical conductivity of the material exhibit a strong correlation between microstructure and electrical parameters. The temperature dependence of dc conductivity of the compound follows Arrhenius equation. The frequency and temperature dependence of ac conductivity (with fittings) shows the signature of Jonscher’s universal power law. The existence of non-exponential-type of conductivity relaxation in the compound was confirmed.     

Dielectric and impedance spectroscopy of barium orthovanadate ceramics

Barium orthovanadate (Ba₃V₂O₈), a derivative of perovskite family has been prepared using a mixed-oxide technique. The room temperature X-ray diffraction analysis has confirmed the formation of a single phase compound in trigonal crystal structure. The study of microstructure by scanning electron microscopy shows that the compound has well defined grains, distributed uniformly throughout the surface. The studies of dielectric parameters (ε_r and tan δ) of the compound as a function of temperature at three different frequencies (100, 500, 1,000 kHz) exhibit that they are almost temperature independent at low and medium temperature ranges. Detailed studies of impedance and related parameters exhibit that the electrical properties of the material are strongly dependent on temperature, and bear a good correlation with its microstructures. The bulk resistance, evaluated from complex impedance spectra, is found to be decreasing with rise in temperature. It shows that the material has negative temperature co-efficient of resistance similar to that of semiconductors. The same behaviour has also been observed in the study of I–V characteristics of the material. The complex electric modulus analysis indicates the possibility of hopping conduction mechanism in the system with non-exponential type of conductivity relaxation. The nature of variation of dc conductivity with temperature confirms the Arrhenius behavior of the material. The ac conductivity spectra show a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law.     

Heat capacity, thermopower and magnetoresistance effects in multiferroic La0.5Bi0.5Mn0.5Fe0.5O3

We have carried out systematic investigations in perovskite multiferroic La_0.5Bi_0.5Mn_0.5Fe_0.5O₃ by means of X-ray diffraction, magnetisation, electrical resistivity, thermoelectric and heat capacity measurements. The magnetic behaviour of this composition is rather complex, though the magnetisation curve seems to be like a weak ferromagnetic material. However, there is no clear evidence of λ-anomaly in the heat capacity data down to 2 K, yet this behaviour corroborate the trends of semiconducting silicon below room temperature. The sensitivity of magnetic behaviour to the iron-manganese ratio is also demonstrated. In presence of an external field of 7 T, it exhibits a magnetoresistance of ?5 % at 130 K. The thermoelectric value increases linearly with decreasing temperature, and at room temperature the value is +85 μV/K, which is associated with the p-type polaronic conductivity.     

Influence of transition metal element (Co,Ni, Cu) doping on structural,electrical and magnetic properties of Bi<Subscript>0.9</Subscript>Ca<Subscript>0.1</Subscript>FeO<Subscript>3</Subscript> nanoparticles

The multiferroic Bi_0.9Ca_0.1FeO₃, Bi_0.9Ca_0.1Fe_0.9Co_0.1O₃, Bi_0.9Ca_0.1Fe_0.9Ni_0.1O₃, Bi_0.9Ca_0.1Fe_0.9Cu_0.1O₃ samples were prepared by a simple sol–gel method. Rietveld refinement of X-ray diffraction data and Raman spectra reflect a structural phase transition from single phase (rhombohedral, pure BiFeO₃) to two phase coexistence (rhombohedral R3c and cubic Fm-3 m). The structural distortion of Bi_0.9Ca_0.1Fe_0.9Ni_0.1O₃ is very marked. SEM images show the co-doped nanoparticles having an average size of 50 nm. A contribution from the leakage current have been observed in the P–E loops. XPS results reveal that the concentration of Fe²⁺ and oxygen vacancy decreased after transition metal elements (Co, Ni, Cu) doped into Bi_0.9Ca_0.1FeO₃. Moreover Co, Ni doping can enhance the saturation magnetization, while Cu doping can enhance the coercive field in Bi_0.9Ca_0.1FeO₃.     

Magnetism and electrode dependant resistive switching in Ca-doped ceramic bismuth ferrite

Here we report on the preparation and structural, magnetic and electrical characterization of BiFeO₃ and Bi_0.9Ca_0.1FeO₃ ceramic multiferroic samples. We suggest that Ca-doping creates oxygen vacancies and destabilizes the BiFeO₃ spiral magnetic structure. We also study resistive switching effects in Bi_0.9Ca_0.1FeO₃ with metallic electrodes, finding that the appearance of the effect is dependant on the fabrication procedure of the metallic electrode. On the basis of these observations, we critically revise some assumptions in currently available models of resistive switching of complex oxides.     

Structural and electrical properties of La-modified BiFeO3–BaTiO3 composites

In the present investigation, La-modified solid solutions of BiFeO₃ (BFO) and BaTiO₃ (BT) in different molar ratios [i.e., (Bi_0.5?x La _x Ba_0.5)(Fe_0.5Ti_0.5)O₃, with x = 0.0, 0.05, 0.10 and 0.15)] have been synthesized using a solid-state reaction route. Structural and electrical properties of single phase (with minor secondary phase) of BFO–BT system have been studied in details to understand their ferroelectric and other properties. Preliminary X-ray diffraction analysis confirms the formation of a new system, which is different from that of its parent compounds. Substitution of a small amount BaTiO₃ into BiFeO₃ enhances dielectric and ferroelectric responses and reduces electrical leakage or tangent loss. The ac conductivity obeys Jonscher’s universal power law. The electrical behavior of the samples was investigated by impedance spectroscopy in a wide temperature range (25–525 °C) at different frequency (1 kHz–1 MHz). The impedance spectroscopy of the materials also confirms the origin of the relaxation mechanism in the system.     

High-temperature phase transitions in a quaternary lead based perovskite structured materials with negative temperature coefficient of resistance (NTCR) behavior

Impedance spectroscopy measurements were carried out on lead based, 0.25 (PbZr_0.52Ti_0.48O₃) + 0.25 (PbFe_0.50Ta_0.50O₃) + 0.25 (PbFe_0.67W_0.33O₃) + 0.25 (PbFe_0.50Nb_0.50O₃) (PZT–PFT–PFW–PFN) solid solution over a wide range of temperatures (400–650 K) and frequencies (100 Hz–1 MHz). Impedance data showed the presence of both grains and grain boundaries effects in the electrical transport properties of quaternary. The role of the grains and grain boundaries to the impedance become more prominent around the phase transition (~420 K). Two thermally activated processes were found from the temperature dependences of the relaxation time (τ). Activation energies calculated from relaxation times obtained from imaginary part of impedance were estimated ~1.21 and ~1.84 eV over 400–490 K and 490–650 K respectively. The sum of the activation energies for the grain and grain boundary resistances is basically of the same order of magnitude that is from the impedance at high temperatures. A constant phase element is used in the equivalent electrical circuits for fitting of experimental impedance data. The nature of variation of the grain and grain boundary resistance with temperature suggested negative temperature coefficient of resistance behavior.