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Graphene and Multi Walled Carbon Nanotubes (MWCNT) admixtured Bismuth Iron Oxide (hereafter referred to BIO) nanocomposite ceramics have been synthesized using the sol-gel process. A comparative study on their structural, optical, dielectrical and electrochemical sensing properties has also been made for the first time in an “as-prepared” state. The X-ray diffraction (XRD) peaks clearly indicate the existence of mixed crystalline phases of BIO with graphene and MWCNT. SEM images reveal the distinct morphologies of BIO nano structures, even in the admixtured phase with graphene and MWCNT. The characteristic vibrations and possible scattering modes of BIO admixtured carbonaceous materials (graphene & MWCNT) have been identified through FT-IR and Raman spectra. Band gap (Eg) variation was noticed in the synthesized composites upon loading graphene and MWCNT on the BIO matrix. Enhancement of relative permitivity (er) has been witnessed in the graphene admixtured BIO composite. This increase is significant, when compared to the MWCNT-BIO composite and it may be due to the effective dispersion of graphene over BIO. The pseudo capacitive behaviour of the BIO composite powder with graphene and MWCNT was also ascertained from the Cyclic Voltammetry (CV) studies.

High-performance Ba(Ti0.8Zr0.2)O3–(Ba0.7Ca0.3)TiO3 (BCZT) fiber crystals were grown using laser-heated pedestal growth (LHPG) for a systematic investigation of processing effects on structure and physical properties. By changing the growth rate (200−5 mm/h), fiber crystals with varying stoichiometry were realized: controlled composition, Ba0.872Ca0.128Ti0.925Zr0.075O3-δ, for the optimized crystal (5 mm/h), relative to the ceramic sample (SS) synthesized by the ceramic method. SS and optimized fibers exhibited larger energy storage densities of 700 and 868 mJ/cm3, respectively. Electrocaloric (EC) measurements by direct and indirect methods yielded ΔT of 1.3 K & 0.95 K and EC responsivity (ΔT/ΔE) of 0.3 and 0.43 K mm/kV for SS and 5 mm/h respectively, at 30 kV/cm, at least two-folds higher compared to 200 mm/h fiber. The EC responsivity of our samples is comparable to that of Pb(Mg,Nb)O3 crystals. These results demonstrate that BCZT has potential application as EC elements when the composition is controlled by fine-tuning of growth parameters.

We report an investigation of the structural, morphological, the ac-dc electrical, magnetic, and Mössbauer spectroscopy properties of 20MnFe2O4-80SiO2 nanocomposite prepared using a one-step and facile auto-combustion approach. XRD pattern shows the formation of MnFe2O4 nanocrystallite without any crystallization of the SiO2 phase. However, the presence of a secondary phase of the nanosized α-Fe2O3 particles was also detected. The morphological analysis showed aggregation of polygonal magnetic nano-crystallites dispersed non-uniformly in a silica matrix. The dc electrical measurements performed on a wide range of temperatures from 120 to 400 K showed the semiconducting nature of the nanocomposite. The temperature dependence of dc conductivity could be perfectly fitted to the nearest neighborhood hopping model with activation energy, ΔENNH, of 0.45 eV. The Nyquist plots demonstrated a nonmonotonous thermally activated trend and non-Debye relaxation behavior. An equivalent circuit was successfully fitted to the complex impedance spectra. The variation of both grain and grain boundary conductivities as a function of temperature exhibited three distinct regions, semiconducting-metallic-semiconducting with different activation energies over the measured temperature window. The provided description of such behavior is further advocated by the ac conductivity and dielectric modulus studies. VSM measurements revealed that the nanocomposite magnetic behavior deviates from the ideal non-interacting superparamagnetic picture, due to the presence of α-Fe2O3 nanocrystalline impurities and relatively intensive exchange interactions between ions. Mössbauer spectra showed the presence of Fe3+ ions with sixfold environment and also confirmed the existence of a sextet related to α-Fe2O3 with a quantity of 28% out of magnetic components. Fe3+ superparamagnetic doublets were also found within the fitting procedure.

Lead-free Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) bulk ceramics have recently gained attention as one of the attractive candidates for electrocaloric cooling due to their large heat absorption capacity induced by electro-structural phase transition near room temperature. However, adiabatic temperature change (ΔT) at room temperature in ceramics has been limited by their relatively low dielectric breakdown strength. Thick films, in this respect, possess the advantages of ceramics (volume) and thin films (thickness). In this work, a systematic ferroelectric and electrocaloric investigation of BCZT thick films, fabricated by electrophoretic deposition combined with laser annealing, has been carried out. Laser annealing at an optimized energy density of 398 W/cm2 has resulted in densely packed grain morphology with no compositional heterogeneities. Analysis of the scaling behavior of dynamic hysteresis revealed that ferroelectric domain reversal in BCZT/Pt has good stability and low energy consumption in the saturation region. The present thick films exhibited larger breakdown strength of 294 kV/cm and recoverable energy storage density of ~7.3 J/cm3 , at least five-fold enhancement compared to the bulk which is stable up to a 4 mm bending radius. The indirect electrocaloric measurements displayed parameters such as a ΔT and entropy change (ΔS) of 2.94 K and 3.1 J kg− 1 K− 1 respectively, for 205 kV/cm which are more than two orders increased compared to the bulk. Higher values of ΔT (1.9 K) at 25 ◦C, refrigerant capacity of 249 J/kg, and responsivity of 0.143 K mm/kV for BCZT/Pt film compared to relevant lead-free ferroelectric thick films elucidate the potential use of these films for solid-state refrigeration applications. The work also presents electrophoretic deposition with laser annealing as an alternative technique to widely studied tape-casting of thick films for electrocaloric studies. published

A methodical study on structural, electrical and dielectric properties of NiFeCoO4 nanoparticles, synthesized via sol-gel technique has been reported in this article. X-ray diffractogram confirmed phase purity of the synthesized sample. Raman spectra show the five predicted Raman bands that appear around 600–720, 250–360, 500–590, 450–520 and 180–220 cm−1 corresponding to A1g, Eg, T2g (3), T2g (2) and T2g (1), respectively. From the dielectric measurements, we have determined the different dielectric parameters such as the conductivity σac, complex permittivity e*, complex impedance Z*, and the tangent loss (tanδ). The electrical properties strongly depend on temperature and frequency. The relaxation activation energy deduced from the Z” vs. frequency plots was similar to the conduction activation energy obtained from the conductivity. Hence, the same type of charge carriers are attributed to the relaxation process and the conduction mechanism. The complex impedance plots have revealed the presence of only one semicircular arc corresponding to grains and grain boundaries contributions at all the temperatures and an equivalent electric circuit was proposed as a model of the sample.

In this paper, we investigated the impact of the substitution of barium (Ba) by strontium (Sr) in site A of the BaTiO3 perovskite lattice on physical properties. The studied materials were synthesized via the conventional solid-state method. Then, the purity and the crystallographic structure of the prepared materials were carried out at room temperature by X-ray powder diffraction (XRD) proving an orthorhombic system with a P4mm space group. Their morphology and the particle size were evaluated by scanning electron microscope (SEM) which shows a nanoparticle agglomeration of about 2.4 µm and 2.7 µm respectively for BT and BST samples. The investigation of the optical properties by Fourier transform infrared spectroscopy confirms the successful preparation of the perovskite materials. The study of the dielectric properties of ceramic materials performed by impedance spectroscopy in a frequency range [100-106 Hz] and temperature range [150−400 K] proves the appearance of the relaxation process in the modulus spectra after Sr substitution. The dc conductivity shows a VRH conduction process for both samples at lower temperatures and an SPH one at higher temperature with an increase in the activation energy for BT material and a decrease in the BST one. The ac conductivity data were described by Jonscher’s power law indicating a correlated barrier hopping (CBH) conduction process in the BT sample and an overlapping large polaron tunneling (OLPT) one for the BST compound.

This work involves the synthesis and study of physical properties of the La0.8Ca0.1Pb0.1Fe0.975Mg0.025O3 compound, which has been characterized by various experimental techniques, such as X-ray diffraction, SEM and complex impedance spectroscopy. The structural study showed that the La0.8Ca0.1Pb0.1Fe0.975Mg0.025O3 compound crystallized in the orthorhombic structure with the Pnma space group. The particle size and the surface morphology of this compound have been analysed using SEM. The particle size was found to be around 120 nm and we confirmed that one particle contains more than one crystallite. Importantly, the studied compound presented a giant dielectric permittivity (e′ of around 9 × 104 at high temperature and low frequencies). An equivalent electric circuit has been deduced from the Nyquist plots of the complex impedance parts (Z′′ vs. Z′) to correctly describe the electrical behavior of the La0.8Ca0.1Pb0.1Fe0.975Mg0.025O3 compound. The chosen circuit consists of two cells mounted in series corresponding to the grain and grain boundary contributions. The electrode contribution has been detected from the frequency dependence of the imaginary part of modulus where the activation energy of each constitution has been calculated. The relaxation process and the electrical conductivity are attributed to the same type of charge carriers characterized by similar values of the activation energy determined from loss factor tangent (tg(δ)), the imaginary part of the permittivity and the modulus spectrum.

The hexaferrite Ba1−xSrxFe12O19 compounds with x = 0, 0.5 and 1 were synthesized by the autocombustion method. X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) were used for structural and morphological studies.

La1−2xBaxBixFeO3 (0.00 ≤ x ≤ 0.20) nanoparticles were prepared by the auto-combustion method using glycine as a combustion fuel. X-ray diffractometry (XRD) measurements confirmed the orthorhombic structure of the synthesized compounds with the Pnma space group as a principal majority phase and showed the presence of a very minor secondary phase when x > 0.1. The nanosize criterion of the prepared compounds was confirmed from the crystallite size values calculated using the Williamson–Hall formalism. The relaxation process has been studied by the frequency dependence of the imaginary parts of impedance and modulus (Z′′ and M′′) which satisfied the Arrhenius law. Nyquist plots allowed us to obtain an adequate equivalent circuit involving the grains and grain boundary contributions. The activation energies calculated from Z′′, M′′ and the resistance of both contributions deduced from the Nyquist plots are found to be very similar. The conduction mechanism has been analyzed using the temperature dependence of the exponent Jonscher's power law parameter which confirms the NSPT conduction mechanism type for all compounds with an enhancement of the binding energy of the charge carrier (WH) with the substitution.

The goal of this work was to demonstrate the viability of using laser irradiation to promote the development of a magnetite/hematite bi-layer structure. Local heating by laser irradiation induces a hematite redox mechanism, promoting a phase transition of hematite (Fe2O3) into magnetite (Fe3O4). A relationship between laser feed rate, laser power and the top layer grain size was established. The average grain size increased from 3 μm to 10 μm when the laser power increased from 50 W to 100 W. Raman spectroscopy results demonstrated that residual hematite coexists on the magnetite top layer, distributed mostly along the grain boundaries. Electrical conductivity measurements of the irradiated regions confirmed the effectiveness of the laser surface treatment in promoting the local phase transformation of hematite into magnetite, with consequent enhancement of electrical conductivity. The maximum conductivity, 2.8 mS/m, was thus reached for the highest laser power used, 100 W. This phase transformation can be used on other materials to produce bi-layer structure, presenting an important factor for future applications and development using this technique.

The La0.8Ba0.1Ce0.1FeO3 compounds were prepared by the auto-combustion route and annealed at two different temperatures to study their effect on the structural, morphological and dielectric properties of the Barium- and Cerium-substituted LaFeO3 compound. The X-ray diffraction analysis revealed that both compounds crystallized in the orthorhombic structure belonging to the Pnma space group. The rise of the annealing temperature from 700 to 900 °C was found to lead to the increase of the average grain size value as characterized by SEM. Furthermore, two clear relaxations phenomena have been detected using the Nyquist and Argand’s plots of dielectric impedance and Modulus curves at different temperatures, which are attributed to both grain and grain boundary contributions. Their activation energies have been calculated not only from the frequency dependence of both imaginary parts of impedance (Z″) and modulus (M″) but also from the contribution of the resistances deduced from the Nyquist plots. These plots have been adjusted using two circuits in series, each of which containing a resistance in parallel to a CEP capacitance. The conduction mechanism was analyzed by ac conduction using the Jonscher’s power law. The NSPT conduction model has been confirmed for both compounds in which the rise in heat treatment decreases the binding energy of carriers.

In this work we investigate the effect of Ag+ doping at Sr2+ site in La0.57Nd0.1Sr0.33−xAgxMnO3 (LNSAMOx, x = 0.00 and 0.15) system on the magnetic and magneto-electrical properties. The variation of the magnetization M versus temperature T, at 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. All samples undergo a sharp metal–semiconductor transition at a temperature Tρ, accompanying the ferromagnetic–paramagnetic transition. The maximum values of resistivity temperature coefficient (TCR) and magneto-resistance were 2.5% K−1 and 63% at room temperature of 290 K and 305 K, respectively. The TCR value is considered as an important factor in determining the sensitivity of uncooled infrared bolometers. Most interesting is that the adjustment of the low-temperature range ρ–T revealed the presence of low-temperature peaks in the resistivity curves, attributed to the diffusion effects of grain boundaries, electron–electrons, electron–magnon, electron–phonons and the interactions between them. Overall, LNSAMOx polycrystalline ceramics appear promising for uncooled infrared detectors, magnetic memories or magnetic storage devices.

Barium Zirconate Titanate-Barium Calcium Titanate (BCT-BZT) is one of the lead-free systems, which exhibits remarkable ferroelectric properties akin to lead-based perovskite materials. In this stud...

Two compounds with the formula La0.8Ba0.1Bi0.1FeO3 have been prepared by the sol–gel (LBBFO-SG) and the auto-combustion (LBBFO-AC) preparation methods. The effect of the preparation method on structural, morphology, vibrational, electrical, and dielectric properties have been carried out using the X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Raman and impedance spectroscopy. The Rietveld refinement of the XRD diffractograms confirms the crystallization of both compounds through the Pnma space group. Furthermore, the nano-size scale of the prepared compounds was verified by the XRD and the SEM microscopy. From the Raman study, we conclude that the vibration modes are similar to those of the pure LaFeO3 with a slight shape change in some modes. The relaxation processes were analyzed with the Bergmann formalism to validate the presence of two relaxation processes corresponding to grains and boundary grains contributions. These relaxations have been confirmed by studying the dielectric constant also the dielectric loss tangent is used to study the ferroelectric–paraelectric phase transition at around room temperature. The activation energy for both contributions has been calculated from the impedance, the modulus, and the dielectric loss tangent and they have almost the same values. The Nyquist plots (Z″ vs. Z′) have been adjusted using two circuits in series containing each one a resistance in parallel to a CEP capacitance.

In this study, our central focus is to investigate the magnetocaloric characteristics of a La1−x□xMnO3 (x = 0.1, 0.2 and 0.3) series prepared by a sol–gel technique published in Prog. Mater. Sci., 93, 2018, 112–232. The crystallographic study revealed that our compounds crystallize in a rhombohedral structure with Rc. Ferromagnetic (FM) and paramagnetic (PM) characters were detected from the variation in magnetization as a function of magnetic fields at different temperatures. The second order transition was verified from the Arrott plots (M2 vs. (μ0H/M)), where the slopes have a positive value. In order to verify the second order, we traced the variation of magnetization vs. temperature at different magnetic fields for x = 0.2. This revealed a ferromagnetic (FM)–paramagnetic (PM) transition when temperature increases. Relying on the indirect method while using the Maxwell formula, we determined the variation in the entropy (−ΔSM) as a function of temperature for different magnetic fields for the three samples. We note that all the studied systems stand as good candidates for magnetic refrigeration with relative cooling power (RCP) values of around 131.4, 83.38 and 57.26 J kg−1 with magnetic fields below 2 T, respectively. Subsequently, the magnetocaloric effect was investigated by a phenomenological model for x = 0.2. The extracted data confirm that this phenomenological model is appropriate for the prediction of magnetocaloric properties. The study also demonstrated that this La0.8□0.2MnO3 system exhibits a universal behaviour.

The effect of variant high boiling point solvent combinations in the synthesis and photo-sensing characteristics of tin disulfide (SnS2) thin nanosheets were investigated. Solvent dependent single-phase formation was observed by powder X-ray diffractometry (XRD), and results were verified with Raman spectroscopy measurements. Particles synthesized with only oleylamine show a mixed-phase, and two other solvent combinations such as oleylamine + oleic acid + n-octadecene, and oleylamine + oleic acid, show single-phase 2H–SnS2, without phase contamination. Scanning and transmission electron microscopes were used to verify the orbicular and hexagonal shape of synthesized SnS2 nanosheets. Well-characterized samples were employed to fabricate heterojunction photosensitive diodes using a p-type silicon substrate. Light-responsive measurement results of fabricated diodes indicate that the oleylamine + oleic acid synthesized sample show very high light-sensitivity, and the fill factor value of a fabricated diode is closer to the ideal diode.

(La0.8Ca0.2)1−xBixFeO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) (LCBFO) multiferroic compounds have been prepared by the sol–gel method and calcined at 800 °C. X-ray diffraction results have shown that all samples crystallise in the orthorhombic structure with the Pnma space group. Electrical and dielectric characterizations of the synthesized materials have been performed using complex impedance spectroscopy techniques in the frequency range from 100 Hz to 1 MHz and in a temperature range from 170 to 300 K. The ac-conductivity spectra have been analysed using Jonscher's power law σ(ω) = σdc + Aωs, where the power law exponent (s) increases with the temperature. The imaginary part of the complex impedance (Z′′) was found to be frequency dependent and shows relaxation peaks that move towards higher frequencies with the increase of the temperature. The relaxation activation energy deduced from the Z′′ vs. frequency plots was similar to the conduction activation energy obtained from the conductivity. Hence, the relaxation process and the conduction mechanism may be attributed to the same type of charge carriers. The Nyquist plots (Z′′ vs. Z′) at different temperatures revealed the appearance of two semi-circular arcs corresponding to grain and grain boundary contributions.

ZnO nanoparticles have been prepared by auto-combustion method. Morphological and structural properties of the prepared samples were investigated by SEM, XRD, Raman and XPS characterizations. The XRD diffractogram of the sample indicates that ZnO has a hexagonal wurtzite structure. The crystallites average size, calculated from the Williamson–Hall plot, was 69.3 nm and the estimated by the SEM image (76.86 nm). Raman investigation indicates different modes of atomic displacement which correspond to longitudinal/transversal optical components with different frequencies. These modes are due to the macroscopic electric fields associated with the basic phonon of hexagonal ZnO. The XPS spectra indicate the presence of Zn and O in the structure with a small number of interstitial Zn2+, oxygen vacancies (Vo), and a negligible amount of chemical bonds with carbon (=CO …), confirmed by FTIR spectroscopy. The UV absorbance and reflectance spectra show a high absorbance with gap energy of 3.17 eV, estimated by Tauc's model. The a.c. electrical spectroscopy can be described by the Jonscher universal power-law. The charge carriers move according to the correlated barrier hopping pattern over the dispersive region. At room temperature, the conductivity of ZnO is high (~ 8 × 10–6 S.m−1) making it promises for gas detection applications. The sensor was prepared by spraying the suspension of ZnO nanopowders on alumina substrates with pre-deposited gold interdigitated electrodes. The sensor responses of NO2, for concentrations of 0.5, 0.75 and 1 ppm, were investigated at room temperature under illumination with different wavelengths. The best response of the sensor was obtained for a concentration of 1 ppm NO2 excited by 400 nm (purple) and 380 nm (UV) wavelengths, which were 91 and 88 with response/recovery times equal to 4/6.7 min and 4.4/3.3 min. Higher responses at the lower wavelength are due to the higher excitation energy which tends to excite more electrons, at the material surface, subsequently participating in the detection mechanism with gas molecules

Background Cancer remains as one of the major causes of mortality worldwide. Recent advances in nanoparticles based therapy mark a new era on cancer treatment. Many groups have investigated biological/physical effects of nanoparticles on tumour cells and how these vary with physical parameters such as particle size, shape, concentration and distribution. Magnetic hyperthermia (MHT) can be an alternative or an add-value therapy with demonstrated effectiveness. MHT uses magnetic nanoparticles, which can be directly applied to the tumour, where, by applying an external ac magnetic field, will promote a localized temperature increment that can be controlled. Methods LiFe5O8 nanocrystallites were synthesized by a novel and eco-friendly sol-gel process, using powder coconut water (PCW) has a mediated reaction medium. The dried powders were heat-treated between 400 and 1000 °C, being analysed their structure, morphology and magnetic properties; cytotoxicity and magnetic hyperthermia assays were performed. Results The heat-treated LiFe5O8 powder led to crystallite sizes from 50 nm to 200 nm. Increasing the temperature of the heat treatment, secondary phases tend to disappear. The cytotoxicity results revealed a non-cytotoxicity property, for concentrations below 5 mg/mL. The sample HT at 1000 °C, revealed hysteresis and magnetic saturation of 73 emu.g−1 at 300 K and a specific absorption rate(SAR) of about 80 W/g. Conclusions It was possible to synthesize LiFe5O8 by a novel, eco-friendly and sustainable route. The nanoparticles have a high heat efficiency, especially the sample heat-treated at 1000 °C, with SAR values similar to Fe3O4, showing potential to be used in the treatment of cancer, by MHT.

In this study, La0.67Ca0.2Ba0.13Fe1−xCrxO3 (x = 0 and 0.03) powders were prepared by auto-combustion method. The examination of the purity and crystalline structure of the samples by X-ray diffraction showed a cubic structure formed with the $$Pm\overline{3}m$$ space group. Scanning electron microscopy was used to study the morphology and grains size, indicating an agglomeration of nanometric particles. Impedance spectroscopy in the temperature range from 200 to 360 K was used in order to understand the evolution of the electrical behavior of the substituted perovskites. The real and imaginary parts of the impedance were adjusted with two different equivalent circuits. The activation energies extracted from the imaginary part of the impedance, and the electrical conductivity was very closed indicating a transformation of the electrical behavior. This was confirmed by a decrease in the resistance after substitution of Fe cations by Cr ones. AC conductivity studies showed a Jonscher’s behavior and a change of conduction process from a succession of NSPT and CBH model to the existence of only CBH one after the substitution.

In this research paper, our central focus is upon the investigation of the mean-field approach based on the Bean-Rodbell model applied to the magnetic properties of the compound La0.8MnO3. Using the isotherm, we determined the value of the mean-field exchange parameter λ. After using a MATLAB software, the values of J, g, and M0 were identified and compared with the experimental values. Another MFSS software running with MATLAB program was invested to determine the theoretical variation of magnetization as a function of magnetic field at different temperatures, the variation of magnetic entropy, and the nature of transition compared with the experimental values provided by data and Maxwell relation. A good accordance has been recorded between the experimental and theoretical curves. The values of ∆Smax and RCP which are equal to 3.63 J kg−1 K−1 and 83.38 J kg−1, respectively. They were determined by the Maxwell relation and were compared with those determined by the Bean-Rodbell model which are equal to 4.01 J kg−1 K−1 for ∆Smax and to 97.36 J kg−1 for the RCP at 2 T respectively.

The effect of 5% magnesium doping on the structural, magnetic and gas sensing properties of La0.8Ca0.1Pb0.1Fe0.95Mg0.05O3 (LCPFMO) has been investigated. The nanosize compound was prepared by the Sol–Gel method, using the citric acid route. The Structural study confirms that this compound crystallizes in the orthorhombic structure with the Pbnm space group. From the magnetic measurements, we were able to show that the effect of the Mg doping is to increase the saturated magnetization and to decrease the Ferromagnetic–Paramagnetic magnetic phase transition temperature (TC). A theoretical modulation was derived in order to confirm the domination of the ferromagnetic contribution over the Antiferromagnetic one. The AC-conductivity dependence on both temperature and frequency was studied; the study confirmed the presence of two behaviors; at high-temperature region, it was found to be only temperature dependent, while at low temperatures, it depends on both frequency and temperature parameters. At low temperature, the conduction mechanism is studied according to the NSPT model, where all parameters were discussed. The electrical Sensitivity (S) of the prepared sample to different ethanol gas concentration was investigated, using a broadband dielectric spectroscopy. It was found that 5% of magnesium insertion leads to an improvement in the sensitivity of the LCPFMO sample. This sensitivity was found to be ethanol amount dependent.

Niobium oxides are presently very important materials for electronic device applications. In this work, a new approach, using the proteic sol–gel route, was used to prepare niobium pentoxide powders. This approach uses powdered coconut water (PCW) for the powders production. The obtained powders, with an amorphous structure, were heat-treated at different temperatures and their structural and morphological properties were studied and related to their dielectric properties. Dielectric measurements as a function of frequency (100 Hz–1 MHz) and temperature (200–400 K) were performed. The heat-treatment promotes the formation of crystalline structures, mainly Nb2O5 with orthorhombic structure, which transforms to monoclinic with treatments at 1000 °C. The formation of CaNb2O6 and NaNbO3 crystallites, in the samples treated at 800 and 1000 °C is related to the PCW base composition. The samples revealed a quasi-independence of the dielectric constant with temperature and frequency, presenting a $$\varepsilon^{\prime}$$ ≈ 70 and tan ≈ 0.006, at room temperature and 100 kHz.

We report on the magnetic properties of disordered Li2Co2 – xNix(MoO4)3 (0 ≤ x ≤ 2) system with a lyonsite structure, where Li+, Ni2+, and Co2+ ions are distributed in a disordered manner in the trigonal prism and octahedral sites. All compositions were found to exhibit paramagnetic behavior at high temperature; the magnetic susceptibility was described by the modified Curie-Weiss law. The experimental effective moments were explained by spin and orbital contributions of the Co2+ ions and only the spin contribution for Ni2+ ions. The negative Curie-Weiss temperature θ indicates the existence of antiferromagnetic coupling for all compositions. At low temperature, the compounds with Co2+ ions (x = 0, 1, 1.6, and 1.8) exhibit a weak ferromagnetic state below 4 K due to intrinsic canting of antiferromagnetically coupled lattices. The compound containing only Ni2+ ions, Li2Ni2(MoO4)3, exhibits a pure antiferromagnetic behavior.

A spinel ferrite sample Zn0.4Ni0.6Fe2O4 was prepared by a conventional solid-state reaction method at high temperature. This compound was found to exhibit a single phase and crystallize in a cubic structure with Pm3 m Space group. The complex impedance was investigated in the temperature range 400–700 K and in the frequency range 100 Hz - 1 MHz. The Z-impedance analysis revealed that the relaxation phenomenon is strongly dependent on temperature and frequency. The impedance plane plot showed semi-circle arcs at different temperatures, and a proposed electrical equivalent circuit explained its results. The temperature and frequency dependence of the ac conductivity was shown to obey the universal Jonscher's power law. The activation energy, inferred from the impedance spectrum, was confirmed to match very well with those estimated from the conduction mechanism, indicating that the relaxation process and conductivity have the same origin. The magnetization is due to the ferromagnetic interactions between iron ions.

To explore the relationship between optical properties, oxygen vacancies and magnetism in SrTiO3, we prepared a series of SrTiO3-δ (δ = 0.00, 0.125 and 0.250) samples by solid state reaction method followed by the creation of oxygen vacancies-δ. Surface composition and chemical valence state in the synthesized compounds were examined by X-ray photoelectron spectroscopy (XPS). The Fourier Transform Infrared spectroscopy (FT-IR) spectra showed that the transmission properties change with oxygen vacancies introduction. Ultraviolet–visible absorption revealed that optical properties were highly dependent on oxygen vacancies concentrations. When the magnetic field was applied, magnetization showed weak-ferromagnetism at room temperature on SrTiO3-δ non-stoichiometric systems. Yet, it was reversed showing diamagnetism on SrTiO3 perfect crystal. The large concentration of oxygen vacancies (δ = 0.250) reduced ferromagnetism and optical absorption, implying the possible presence of a relationship between the magnetism and the optical properties. Theoretical results showed two types of electronic contributions to magnetism, one of which emanates from localized states below the conduction band minimum, and the other comes from itinerant electrons of Ti atoms lying further away from the oxygen-vacancy. The electronic structures and magnetic properties were calculated by Full-Potential Linear Augmented Plane Wave (FP-LAPW) method within the Generalized Gradient Approximation (GGA) and the atomic relaxations introduced by the oxygen vacancies host in a fully self-consistent way. The results shed some light on the promising magneto-optical properties of the SrTiO3-δ materials.

The ethanol sensing properties of SrTiO3-δ (δ = 0.075 and 0.125) ceramics was analyzed by dielectric measurements. The ceramics were prepared by solid state reaction method followed by the creation of oxygen vacancies- δ , through a thermal activated process. The crystal symmetry, space group and unit cell dimensions were derived from the X-ray diffraction (XRD) data using FullProf software whereas grain's size distribution was assessed by scanning electron microscopy (SEM). The prepared samples have been analyzed by impedance spectroscopy over the frequency range from 100 Hz to 1 MHz and temperature range from 240 to 340 K. The dielectric properties of SrTiO 3-δ ceramics showed a quite remarkable stability of giant permittivity (>104 ) as well as a low dielectric loss , which open ways for several applications such as over voltage protections of electronic devices. A low-frequency dielectric relaxation behavior was found, and the carriers for electrical conduction result from the first-ionization of oxygen vacancies . The conductivity and gas sensitivity of SrTiO 3-δ-based sensors were investigated. Results demonstrated that the conductivity decreases after the introduction of the ethanol gas, and p-type semiconductor gas-sensing materials were obtained. Both characteristics present higher responses at lower optimal operating temperatures.

La0.75Ba0.25−xSrxFeO3 perovskite compounds with different strontium concentrations were synthesized via the sol–gel method. X-ray diffraction (XRD) data indicated that all obtained samples crystallize in the orthorhombic structure with the Pnma space group. The dielectric properties of these samples, using complex impedance spectroscopy technique have been carried out as function of frequency and temperature as well. An adequate electrical equivalent circuit has been used to evaluate the grain and grain boundary contributions in complex impedance results. Furthermore, the AC conductivity spectra obey to Jonscher’s universal power law. The behavior of the exponent “S” suggests that the conduction mechanism follows the overlapping large polaron tunneling (OLPT) process for x = 0.05, while for both compounds x = 0.10 and x = 0.15 the non-overlapping small polaron tunneling (NSPT) is the applicable model. The behavior of e″ as a function of both frequency and temperature has been described by Giuntini model.

We have investigated the effect of multi-doping in A and B site of the multiferroic prepared by the Sol-gel method Bi0.8Er0.1Ba0.1Fe0.96Co0.02Cr0.02O3 (BEBFCC) on the structural, morphological and electric properties. X-Ray diffraction and Rietveld refinement confirm that the BEBFCC compound presents a rhombohedral structure with the R3C space group. The Ac electrical properties have been measured from 331 to 735 K as a function of frequency (100 kHz - 1 MHz). Debye's theory and Arrhenius relations were used to study the relaxation phenomenon. The frequency dependence of the impedance (Z”: imaginary part) reveals the existence of one relaxation peak, while the Modulus formalism shows two peaks. The presence of two contributions, related to the dielectric relaxations visible in this compound, was confirmed by the Impedance Nyquist plots. The study of the ac conductivity based on Jonsher's power law confirms that the conduction is associated to the Non-Overlapping Small Polaron Tunneling model.

We applied a general non-aqueous route for the formation of mono-, bi-, tri-, tetra, hexapod and multipod magnetic spinel ferrite metal oxide (CoFe2O4) nanocrystals. The magnetic CoFe2O4 nanocrystals were characterized by X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy, magnetometry and Mossbauer spectroscopy, to understand their structure and magnetic properties. These CoFe2O4 nanoparticles have the smallest particle size reported ever, with a mean diameter of ∼3 nm. The coercivity was found to be larger compared to that reported in the literature for spherical particles. Mossbauer spectroscopy indicated a magnetic transition above room temperature, where 20% of the Fe ions transit to a paramagnetic state, around 400 K. From the thermal dependence of magnetic parameters, the blocking temperature was estimated to be around 425 K. These studies indicate that our CoFe2O4 nanocrystals are different from their cubic/spherical counterparts, which generally display a single domain character. These CoFe2O4 nanocrystals display a strong shape anisotropy dependent magnetic property.

In this study, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80–330 nm thick) via pulsed-laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirmed the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phases. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with the coexistence of four-layer stacking. Such stacking defects are found to be pertinent to the high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of the orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr of 35 μC cm−2) with a fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization as opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1–5 nm), distributed across the film, with Bi and Fe-rich compositions and oxygen vacancies that contribute to the weak ferromagnetic behavior mediated by the Dzyaloshinskii–Moriya interactions. Subtle changes in the structural strain and lattice distortions of thin films with varied thicknesses led to distinct ferroic properties. Stronger ferroelectric polarization of 80 nm and 120 nm films compared to that of thicker ones can be due to structural strain and the possible rearrangement of BO6 octahedra. The observation of the improved magnetoelectric coefficient of 50 mV cm−1 Oe−1 for 120 nm film, as compared to that of several Aurivillius oxides, indicates that the structural strain modification in SBLFT is beneficial for the fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions was found to be responsible for the improved multiferroic behaviour of the SBLFT films.

The magnetic properties, Mossbauer and the critical behavior of the compound Bi0.8Ba0.1Er0.1Fe0.96Cr0.02Co0.02O3 prepared by sol-gel are the subject of this study. The compound has an R3c structure and exhibits a ferromagnetic to paramagnetic phase transition c.a. 574 K. A theoretical adjustment of the hysteresis cycles at room temperature show two magnetic contributions: Ferromagnetic (FM) and Antiferromagnetic (AFM) with a predominance of the first contribution. Mossbauer spectroscopy revealed two Fe3+ cations differently coordinated. Magnetic measurements versus magnetic field reveal that the sample exhibits a second order magnetic phase transition. The Curie temperature ​​and the critical exponents (β, γ and δ) were determined by different techniques. These values are in agreement with the ones predicted by the mean field model.

The La0.67Ca0.2Ba0.13Fe1−xMnxO3 perovskite compounds with different manganese concentrations (x = 0.0, 0.03, 0.06) were synthesized by the autocombustion method and annealed at 700 °C. The X-ray diffraction (XRD) data indicated that all obtained compounds crystallize in the cubic structure with the Pm $$\overline{3}$$ m space group. The increase in the substitution rate has been found to lead to the decrease in the crystallite size value characterized by X-ray diffraction. The dielectric properties of these samples, using a complex impedance spectroscopy technique, were also performed as a function of frequency and temperature. A suitable equivalent electrical circuit was used to assess the contributions of electrode, grains, and grain boundaries in the complex impedance results. Activation energies were determined from different methods. Nyquist plots were consistent with three circuits in series, each one with a capacitance in parallel with a resistance. The conduction mechanism follows the NSPT model for the three samples at T

The physical properties of the cubic and ferrimagnetic spinel ferrite LiFe5O8 has made it an attractive material for electronic and medical applications. In this work, LiFe5O8 nanosized crystallites were synthesized by a novel and eco-friendly sol-gel process, by using powder coconut water as a mediated reaction medium. The dried powders were heat-treated (HT) at temperatures between 400 and 1000 &deg, C, and their structure, morphology, electrical and magnetic characteristics, cytotoxicity, and magnetic hyperthermia assays were performed. The heat treatment of the LiFe5O8 powder tunes the crystallite sizes between 50 nm and 200 nm. When increasing the temperature of the HT, secondary phases start to form. The dielectric analysis revealed, at 300 K and 10 kHz, an increase of &epsilon, &prime, (&asymp, 10 up to &asymp, 14) with a tan&delta, almost constant (&asymp, 0.3) with the increase of the HT temperature. The cytotoxicity results reveal, for concentrations below 2.5 mg/mL, that all samples have a non-cytotoxicity property. The sample heat-treated at 1000 &deg, C, which revealed hysteresis and magnetic saturation of 73 emu g&minus, 1 at 300 K, showed a heating profile adequate for magnetic hyperthermia applications, showing the potential for biomedical applications.

Ni0.5Zn0.5FeCoO4 spinel ferrite was elaborated using sol-gel technique. X-ray diffraction patterns indicate that sample has a cubic spinel type structure with Fd-3m space group. The change in Raman modes and relative intensity were observed due to ball milling and consequently to the decrease of particle size and cationic redistribution. The Raman spectra show peaks appearing at 450 and 490 cm−1 corresponding to T2g (2) and T2g (3), respectively. It may be noted that both of these modes shift toward the higher wavenumber with the substitution of Zn by Ni in Ni0.5Zn0.5FeCoO4 ferrite. Mossbauer spectroscopy analysis shows one tetrahedral A-site and two octahedral B-sites. Due to Zn doping, the hyperfine magnetic fields are much smaller than for CoFe2O4 and NiFe2O4 ferrites. The Jonscher’s power low was used to describe the ac-conductivity measurements. Frequency dependence of dielectric constant (e″) and tangent loss (tan) display a dispersive behavior at low frequencies that can be explained by the Maxwell Wagner model and Koop's theory. Electric modulus formalism has used to study the relaxation dynamics of charge carriers. The complex impedance spectra (Nyquist plots) show well-defined semicircles which are strongly dependent on the temperature.

The effect of induced crystallization on changes in electrical transport of two types of glass‐ceramics, pure polaron conductive 40Fe2O3‐60P2O5 (F40) (mol%) and predominantly polaronic 5Li2O‐5ZnO‐40P2O5‐ 50WO3 (Li‐50W) (mol%) was investigated. F40 glass‐ceramics produced at low heat‐treatment temperatures contain single‐phase Fe3(P2O7)2 whereas at higher temperatures two more phases Fe4(P2O7)3 and Fe(PO3)3 are formed. Structural modifications strongly depend on the crystallization temperature and time. The appearance of crystalline phases studied by Mössbauer spectroscopy exhibits changes in Fe2+/Fetot ratio in crystalline/glassy phases. The detailed analysis of different iron sites allows their correlation with changes in electrical conductivity as crystallization progresses. Depending on the course of crystallization, the contribution of each phase to the overall conductivity is determined by the frequency dependence of Zʺ(ω) and Mʺ(ω). DC conductivity shows a sharp decrease as Fe3(P2O7)2 phase appears and consequently glass matrix remains impoverished in Fe2+‐Fe3+ pairs. In the multiphase systems prepared at higher crystallization temperatures, the overall electrical conductivity increases although the continuous grain boundaries along different crystalline grains play a limiting factor. In contrast, the slight conductivity change in Li‐ 50W glass‐ceramics upon crystallization is a result of remaining W5+‐W6+ pairs in the residual glassy phase. Independence of electrical transport on Li+ ions confirms predominantly polaronic transport in Li‐50W glass‐ceramics.

The control of the oxidation state represents an important challenge for materials engineering. It is fairly known that iron oxide can be present in several oxidation states each one with unique physical and chemical properties. In the present work the effects of the different oxygen partial pressure on the growth of Fe3O4 fibres, using hematite (Fe2O3) as a precursor material was studied. The fibres were grown by the Laser Floating Zone (LFZ) method at a constant pulling rate (100 mm/h) under different atmospheres (air, vacuum, argon and nitrogen). The structure and morphology of the as-grown fibres were studied by X-ray diffraction, SEM/EDS/EBSD and optical microscopy. The fibres show a core/shell structure, where Fe3O4 is the core and Fe2O3 the shell, putting in evidence that the partial pressure of oxygen is a crucial parameter to control the thickness and grains morphology of the shell. Low oxygen partial pressure increases the width of the Fe2O3 insulating phase shell. The electrical conductivity, of the samples grown under an atmosphere with low oxygen partial pressure, is improved due to the morphology of the grains in the shell. However, no significant changes in the magnetization measurements were detected. From the results, we can conclude that applying an atmosphere during the LFZ growth allows tunning the transport properties envisaging electrical applications of the as-grown material.