Overlapping Large Polaron Conductivity Mechanism and Dielectric Properties of Al<Subscript>0.2</Subscript>Cd<Subscript>0.8</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> Ferrite Nanocomposite

In this work, the conduction and dielectric properties of Al_0.2Cd_0.8Fe₂O₄ ferrite nanoparticle, which was synthesized by a co-precipitation method, were investigated. Experimental data were taken from 20 Hz to 10 MHz and from 293 to 613 K. AC conductivity of the sample was analyzed within the framework of the overlapping large polaron tunneling (OLPT) mechanism. DC conductivity behavior fits the classical Arrhenius-type conductivity in the examined temperature range. Electrical properties of the material sample have been studied using an impedance spectroscopy technique. The effect of frequency and temperature on dielectric constant (ε ^′), dielectric loss (tan ??), and impedance (Z′ and Z′′) has been discussed in terms of hopping of charge carriers between Fe²⁺ and Fe³⁺ ions. According to results, a relaxation process fits the Cole–Cole model.     

Impedance spectroscopy of Ba<Subscript>3</Subscript>Sr<Subscript>2</Subscript>DyTi<Subscript>3</Subscript>V<Subscript>7</Subscript>O<Subscript>30</Subscript> ceramic

Polycrystalline sample of Ba₃Sr₂DyTi₃V₇O₃₀ was prepared at 950°C using a high-temperature solid-state reaction technique. X-ray structural analysis indicated the formation of a single-phase orthorhombic structure with lattice parameters: a = 12·2719 (39) Å, b = 8·9715(39) Å and c = 19·7812(39) Å. Microstructural study showed densely packed uniform distribution of grains over the surface of the sample. The a.c. impedance plots were used as tools to analyse the electrical response of the sample as a function of frequency at different temperatures (30–500°C). These plots revealed the presence of grain boundary effect, from 200·C onwards. Complex impedance analysis showed non-Debye type of dielectric relaxation. The Nyquist plots showed the negative temperature coefficient of resistance character of Ba₃Sr₂DyTi₃V₇O₃₀. A hopping mechanism of electrical transport processes in the system is evident from the modulus analysis. The activation energy of the compound (calculated both from loss and modulus spectrum) is the same, and hence the relaxation process may be attributed to the same type of charge carrier.     

Ba<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>TiO<Subscript>3</Subscript> ferroelectric nanofilms on silicon buffered with a TiO<Subscript>2</Subscript> layer

Polycrystalline, 50- to 70-nm-thick barium strontium titanate films of composition Ba_0.8Sr_0.2TiO₃ have been grown on single-crystal silicon substrates by rf ion-beam sputtering. We have determined their structure and composition and detected impurities at the film/substrate interface in the form of titanium silicide islands. The deposition of a 4- to 6-nm-thick TiO₂ buffer layer onto Si by ion-beam sputtering before ferroelectric film growth is shown to prevent uncontrolled formation of impurities near the interface. The buffered heterostructures possess high thermal stability.     

Thermodynamic Properties of FeNb<Subscript>2</Subscript>O<Subscript>6</Subscript> and FeTa<Subscript>2</Subscript>O<Subscript>6</Subscript>

Empirical calculational approaches have been used to evaluate the enthalpy, entropy, heat capacity, and melting point of iron(II) niobate and iron(II) tantalate and the coefficients A, B, and C in an equation for the temperature dependence of their heat capacity. The melting point of FeTa₂O₆ has been experimentally determined to be 1891 ± 5 K. The calculated heat capacity (C°_p (298.15 K)) of iron tantalate and the Gibbs energies of formation of FeN₂O₆ and FeTa₂O₆ have been compared to previously reported data.     

Electrical conductivity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Tm<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions

New solid solutions, Bi_2?x?y Tm _x Nb _y O_3+δ, with tetragonal and cubic structures have been synthesized in the Bi₂O₃-Tm₂O₃-Nb₂O₅ system, and their electrical conductivity has been measured at temperatures from 670 to 1020 K. The 1020-K conductivity of the tetragonal solid solution Bi_1.8Tm_0.15Nb_0.05O_3+δ is comparable to that of Bi_1.75Tm_0.25O₃, the best conductor in the Bi₂O₃-Tm₂O₃ system.     

Phase developments in Pb(Mg<Subscript>1/2</Subscript>W<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> and Pb(Zn<Subscript>1/2</Subscript>W<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> via B-site precursor route

Phase formation stages of MgWO₄ and ZnWO₄ (precursor compositions for following steps) were investigated by monitoring the reactions of oxide chemicals at various temperatures. Developed phases were examined by using X-ray diffraction (XRD). Successive attempts were also conducted for Pb(Mg_1/2W_1/2)O₃ (PMW) and Pb(Zn_1/2W_1/2)O₃ (PZW) by reacting PbO with the precursor compounds. Stages of phase development in the two compositions were also analyzed. The results are compared with those of another tungsten-containing perovskite Pb(Fe_2/3W_1/3)O₃ (PFW) and its B-site precursor Fe₂WO₆. After PbO addition to the precursor powders, a perovskite phase formed directly (i.e., without any intermediate phases) in the case of PMW. For PbO + ½ZnWO₄, in contrast, the decomposition of ZnWO₄ and preferential reaction with PbO resulted in Pb₂WO₅ and ZnO, instead of the perovskite PZW.     

Low-temperature synthesis of Sr<Subscript>0.3</Subscript>Ba<Subscript>0.7</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> ultrafine powder and its characteristics

Ultrafine strontium barium niobate (Sr_0.3Ba_0.7Nb₂O₆, SBN30) powders were prepared by urea method starting from a precursor solution constituting of Sr (NO₃)₂, Ba (NO₃)₂, NbF₅, urea and polyvinyl alcohol (PVA) as surfactant. Their structural behavior and morphology were examined by means of X-ray diffractometry (XRD) and Scanning electron microscopy (SEM). The results showed that the SBN30 powders crystallized to a pure tetragonal phase at annealing temperatures as low as 750 °C. The average particle size of SBN powders subjected to 750 °C was of the order of 150–300 nm. With increasing calcination temperature,however, the average particle size of the calcined powders increased. The SBN30 ceramic prepared from urea method can be sintered at temperature as low as 1,225 °C. The transition temperature from the ferroelectric phase to the paraelectric phase and the relative dielectric permittivity of the SBN30 powder were less than the corresponding values of the bulk ceramic. The permittivity and loss tangent (tan δ) at room temperature (1 kHz) was found to be 930 and below 0.025.     

Mechanochemical syntheses of LiFeGe<Subscript>2</Subscript>O<Subscript>6</Subscript>-based nanocomposite and novel nanoglassy LiFeTi<Subscript>2</Subscript>O<Subscript>6</Subscript>

Two members of the pyroxene family, the germanate-type LiFeGe₂O₆ and the titanate-type LiFeTi₂O₆, are prepared for the first time via non-conventional one-step mechanosynthesis. The evolution of the as-prepared materials in the course of mechanosynthesis and their structural state on the long-range and local atomic scales are characterized by X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy, respectively. Both, the nanocrystalline nature of LiFeGe₂O₆ and the nanoglassy state of LiFeTi₂O₆ are revealed.     

Synthesis of Ba<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript>−Sr<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript> solid solution and their dielectric properties

Oxides of the type, Ba_3-xSr_xZnNb₂O₉ (0 ≤x ≤3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4.0938(2) forx = 0 to 4.0067(2) forx = 3. Scanning electron micrographs show maximum grain size for thex = 1 composition (∼ 2 μm) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of x with the maximum dielectric constant atx = 1.     

Fabrication of a plane-parallel interface in Ni/PbZr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>O<Subscript>3</Subscript> heterostructures

A process has been proposed for producing a plane-parallel ferromagnetic/ferroelectric interface, which ensures a reproducible magnetoelectric performance of Ni/PbZr_0.2Ti_0.8O₃ (PZT) heterostructures. Its principle is to smooth the initial surface profile of the PZT ceramic substrate to a submicron level by sequential deposition/sputtering of three layers 0.2, 0.1, and 0.05 μm in thickness through ion beam sputtering of a target having the same composition as the substrate, followed by the growth of a nickel film on the smoothed surface. This allows one to preclude film bulging and spalling and ensures high quality of the interface.     

Fluctuation-Induced Conductivity Analysis of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Layered YBCO Single Crystal

In this work, a cylindrical YBa₂Cu₃O_7?x (Y123) sample having 20 mm diameter and 8 mm height was prepared by the cold-seeding method using an Nd123 seed, and the crystal growth process was performed in an alumina crucible with a Y₂O₃ layer. The structural orientation of the specimens was measured by an X-ray diffractometer (XRD). As a result of the X-ray diffraction data, only (00 l) peaks were observed, indicating that all specimens are highly oriented with the c-axis perpendicular to the top surface. Resistivity measurements of a 1.5-mm-thick rectangular sample were performed by a standard four-probe method at temperatures between 60 and 100 K at a rate of 4 K/min using a Physical Properties Measurement System (PPMS) under various constant magnetic fields from 1 to 5 T with the magnetic field parallel to the c-axis, which is the direction of pressing into the zero-field cooling regime (ZFC). Fluctuation-induced conductivity analysis was performed to investigate the availability of the sample for technology. The analysis showed that the critical exponents were in agreement with theoretical values; thus, it was seen that superconducting properties of the sample were good and the 3D fluctuation was dominant. Because of the large c?axis coherence length (ξ _c(0)) and small anisotropy, the superconducting properties became better. Furthermore, the fluctuations were reduced, and it was understood that the effective availability of the sample in technology will be possible.     

Combustion synthesis and characterization of Ba<Subscript>2</Subscript>NdSbO<Subscript>6</Subscript> nanocrystals

Nanocrystalline Ba₂NdSbO₆, a complex cubic perovskite metal oxide, powders were synthesized by a self-sustained combustion method employing citric acid. The product was characterized by X-ray diffraction, differential thermal analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy, transmission electron microscopy and scanning electron microscopy. The as-prepared powders were single phase Ba₂NdSbO₆ and a mixture of polycrystalline spheroidal particles and single crystalline nanorods. The Ba₂NdSbO₆ sample sintered at 1500°C for 4 h has high density (∼ 95% of theoretical density). Sintered nanocrystalline Ba₂NdSbO₆ had a dielectric constant of ∼ 21; and dielectric loss = 8 × 10⁻³ at 5 MHz.     

Magnetic and Superconducting Properties of Doped and Undoped Double Perovskite Sr<Subscript>2</Subscript>YRuO<Subscript>6</Subscript>

We report here SQUID (magnetization) measurements, along with supporting specific heat, Raman, SEM (scanning electron microscope), EDX (energy dispersive X-ray) and XRD (X-ray diffraction) measurements, on Cu-doped and undoped double perovskite Sr₂²⁺Y³⁺Ru⁵⁺O₆^2-\mathrm{Sr}_{2}^{2+}\mathrm{Y}^{3+}\mathrm{Ru}^{5+}\mathrm{O}_{6}^{2-} (abbreviated as SrY2116) system grown as single crystal using high-temperature solution growth technique. These measurements show the undoped system to be a nonmetallic (insulating) spin glass (SG) and the ∼5–30% Cu-doped (i.e. Cu-concentration/(Cu + Ru-concentration) ∼5–30%) system to be a spin glass superconductor (SGSC) with T _c (critical temperature) ∼28–31 K and superconducting volume fraction, f _sc∼2.2–9%. To mention, similar measurements done on undoped and Cu-doped BaY2116 and BaPr2116 systems show for them the same (SG, SGSC) behaviors. However they show a decrease in T _c and f _sc when diamagnetic Y³⁺ ions are replaced by Pr³⁺ spins, presumably due to enhanced internal pair breaking, and also decreased Cu–O–Cu overlap, owing to Pr³⁺ presence; these phenomena are known to exist in the Pr123 compound, PrBa₂Cu₃O_7−δ (δ∼0), due to ∼10% of Pr³⁺ ions having tendency to occupy Ba²⁺ sites. Measurements done on undoped and Cu-doped SrHo2116 show similar SG and SGSC properties. Further, the undoped and Cu-doped SrY2116 crystals grown by hydrothermal growth technique (i.e., grown using lower temperature and high pressure) show same behaviors. From these investigations it can be said that the undoped Ru-double perovskites (A₂BB′O₆, B′=Ru) are SG systems and that Cu-doped Ru-double perovskites (A₂BB′O _δ , δ∼6, B′=Ru_1−x Cu _x , 0<x≲0.3) are SG superconductors (SGSCs). Results are discussed.     

Investigation of the Thermodynamic and Electronic Properties of Double Perovskite Ca<Subscript>2</Subscript>CoNbO<Subscript>6</Subscript>

In the present work, a self-consistent ab initio calculation using the full-potential linearized augmented plane wave (FP-LAPW) method within the framework of the spin-polarized density functional theory (DFT) was used to study the structural, electronic, and thermodynamic properties of Ca₂CoNbO₆ double perovskite compound. The generalized gradient approximation (GGA) described by Perdew–Burke–Ernzerhof (PBE) and GGA + U were used. The results obtained for the electronic properties show a ferrimagnetic and half-metallic behavior of the compound. The novelty of our work is the study of the thermodynamic properties of Ca₂CoNbO₆ double perovskite such as heat capacity and Debye temperature which showed an important effect of pressure compared to the temperature.     

Effect of Mn doping on the dielectric properties of BaZr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>O<Subscript>3</Subscript> ceramics

Pure and Mn-doped BaZr_0.2Ti_0.8O₃ ceramics are prepared via the conventional solid state reaction method. The microstructures, dielectric properties, and diffuse transition of Mn-doped BaZr_0.2Ti_0.8O₃ ceramics have been investigated. The results indicate that manganese ions enter the unit cell maintaining the perovskite structure of solid solution. The addition of manganese leads to the decrease of the Curie temperature. The dielectric loss of the Mn-doped BZT ceramics is lower than that of pure BZT ceramics, and decreases as Mn content increases. The diffuseness of the phase transition of Mn-doped BZT ceramics decreases with the increase of Mn content. There is no obvious frequency dispersion around the dielectric constant peaks for Mn-doped BZT ceramics. The coercive electric field and the remanent polarization decreases as Mn content increases.     

Synthesis and microwave absorption property of ternary composites: polyaniline/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Ba<Subscript>3</Subscript>Co<Subscript>2</Subscript>Fe<Subscript>24</Subscript>O<Subscript>41</Subscript>

Polyaniline (PANI)/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite was prepared by an in-situ polymerization method. The phase structure, morphology and magnetic properties of the as-prepared PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite were characterized by XRD, FT-IR, SEM, TEM, and VSM, respectively. The microwave absorption properties of the composite were investigated by using a vector network analyzer in the 2–18 GHz frequency range. The results show that the maximum reflection loss value of the PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite reaches ?30.5 dB at 10.5 GHz with a thickness of 3 mm and the bandwidth of reflection loss below ?10 dB reaches up to 1.2 GHz. The excellent microwave absorption properties of the as-prepared PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite due to the enhanced impedance match between dielectric loss and magnetic loss.     

In-field Conductivity Fluctuations in Ba<Subscript>0.72</Subscript>K<Subscript>0.28</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Fluctuations in the conductivity of Ba_0.72K_0.28Fe₂As₂ single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki?Thompson and Aslamakov?Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T _c. 3D fluctuations in superconductivity are realized near T _c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T ₀, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε ₀), as functions of magnetic field were used to determine the coherence length and the lifetime, τ _φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba_0.72K_0.28Fe₂As₂ single crystal gives a value of 3.76 × 10^??12 s for the τ _φ in the absence of magnetic field and it decreases to 2.4 × 10^??12 s in magnetic field of 13 T.     

Electrical properties of perovskite-like compounds in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> system

The electrical properties of layered perovskite-like compounds with the general formula Bi _{m + 1}Fe _{m − 3}Ti₃O_{3m + 3} have been studied in relation to their physicochemical properties and structure.     

Conductivity of Sm<Subscript>2</Subscript>TiO<Subscript>5</Subscript> and Sm<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>

The oxygen-ion conductivity of porous materials, the coarse-grained pyrochlore-like Sm₂Ti₂O₇ and fine-grained Sm₂TiO₅ compounds, produced by mechanical activation of initial oxides is studied at 400–1000 °C. The Sm₂TiO₅ samples contain ～15 wt % of the nanosized pyrochlore-like Sm₂TiO₅ phase in addition to the rhombic phase. As determined by impedance spectroscopy, the ionic conductivities of Sm₂TiO₅ and Sm₂Ti₂O₇ at 1000°C are 1.3 × 10^?3 and 1.8 × 10^?4 S cm^?1, and the activation energies of the bulk and grainboundary conductivities of the materials are 1.04 and 1.24 eV for Sm₂TiO₅ and 1.69 and 1.80 eV for Sm₂Ti₂O₇.