Enhanced ferroelectric properties of (Pb0.90La0.10)Ti0.975O3 multilayered thin films prepared by RF magnetron sputtering

The (Pb_0.90La_0.10)Ti_0.975O₃/PbTiO₃ (PLT/PT), PbTiO₃/(Pb_0.90La_0.10)Ti_0.975O₃/PbTiO₃ (PT/PLT/PT) multilayered thin films with a PbO_x buffer layer were in situ deposited by RF magnetron sputtering at the substrate temperature of 600 °C. With this method, highly (1 0 0)-oriented PLT/PT and PT/PLT/PT multilayered thin films were obtained. The PbO_x buffer layer leads to the (1 0 0) orientation of the films. The dielectric, ferroelectric and pyroelectric properties of the PLT multilayered thin films were investigated. It is found that highly (1 0 0)-oriented PT/PLT/PT multilayered thin films possess higher remnant polarization 2P_r (44.1 μC/cm²) and better pyroelectric coefficient at room temperature p (p = 2.425 × 10⁻⁸ C/cm² K) than these of PLT and PLT/PT thin films. These results indicate that the design of the PT/PLT/PT multilayered thin films with a PbO_x buffer layer should be an effective way to enhance the dielectric, ferroelectric and pyroelectric properties. The mechanism of the enhanced ferroelectric properties was also discussed.     

Thermal, electrical, and electrochemical properties of Lanthanum-doped Ba0.5Sr0.5 Co0.8Fe0.2O3-δ

Crystal structure, thermogravimetry (TG), thermal expansion coefficient (TEC), electrical conductivity and AC impedance of (Ba_0.5Sr_0.5)_1-xLa_xCo_0.8Fe_0.2O_3-δ (BSLCF; 0.05?x?0.20) were studied in relation to their potential use as intermediate temperature solid oxide fuel cell (IT-SOFC) cathode. A single cubic pervoskite was observed by X-ray diffraction (XRD). The TEC of BSLCF was increasing slightly with the increasing content of La, and all the compounds showed abnormal expansion at high temperature. Proved by the TG result, it was associated with the loss of lattice oxygen. The electrical conductivity, which is the main defect of Ba_0.5Sr_0.5 Co_0.8Fe_0.2O_3-δ (BSCF), was improved by La doping, e.g., the compound of x=0.20 demonstrated a conductivity of σ=376 S cm⁻¹ at 392 °C. The increase of electrical conductivity resulted from the increased concentration of charge carrier induced by La doping. In addition, the AC impedance revealed the better electrochemical performance of BSLCF. For example, at 500 °C, the sample with composition x=0.15 yielded the resistance values of 2.12 Ω cm², which was only 46% of BSCF.     

Magnetic and transport properties of Ln1–xSrxCoO3 perovskite materials (Ln= La, Nd, Gd) prepared by low-temperature synthetic routes

We have used three soft-chemistry methods for the efficient preparation of Ln_1−xSr_xCoO₃ samples, adapting the combustion and the liquid-mix methods for the synthesis of the Ln = La and Gd compounds, respectively, and the preparation of Nd_1−xSr_xCoO₃ by the nitrate decomposition method. We report the magnetic and electrical properties of these relatively small particle-size materials, specially in the case of the Ln = La and Ln = Nd series (d≈0.2 μm and 0.5 μm, respectively), and we compare them to those displayed by the corresponding compounds prepared at higher temperatures. The compounds here obtained are ferromagnetic for x≥0.15 when Ln = La and for x≥0.20 when Ln = Nd and Gd. Their resistivity decreases as the doping degree increases. And, very interestingly, for compositions 0.20<x≤0.45, when Ln = La, and for x=0.40, when Ln = Nd, they show M-I transitions as the temperature rises. These are very sensitive to the application of electrical current and its polarity and the presence of magnetic fields, displaying peculiar behaviors. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Impedance spectroscopy of Gd-doped BiFeO3 multiferroics

The polycrystalline Bi_1?x Gd _x FeO₃ (BGFO) (x=0.0, 0.05, 0.10, 0.15, 0.20) materials were synthesized by a solid-state reaction (mixed oxide) technique. Preliminary X-ray structural analysis of the compounds confirmed the formation of single-phase polycrystalline samples. Room temperature scanning electron micrographs of the materials revealed the size, type and distribution of grains on the surface of samples. Studies of impedance, electrical modulus and electric conductivity of the materials in a wide frequency (10–1000 kHz) and temperature (30–500 ^°C) range using a complex impedance spectroscopy technique have provided considerable vital information on contribution of grains, grain boundary and interface in these parameters. A strong correlation between these electrical parameters and microstructures (bulk, grain boundary, nature of charge carrier, etc.) of the materials was established. The frequency dependence of electric modulus and impedance of the material shows the presence of non-Debye type of relaxation.     

Synthesis and enhanced low-field magnetoresistance in La0.67Ca0.33MnO3 /CuO composites

The (1−x)La_0.67Ca_0.33MnO₃+xCuO composites have been synthesized by a new liquid phase method. The XRD and SEM measurements reveal that little CuO is soluble in the structure of La_0.67Ca_0.33MnO₃ and is mainly distributed at the grain boundary of La_0.67Ca_0.33MnO₃. As CuO content x increases, the magnetization M values increase until x=0.05 and M values decrease when x further increases at low temperature. For x=0.10, 0.20 and 0.30 composites, double metal-insulator transitions accompanying a single ferromagnetic transition are observed. Large low-field magnetoresistance is achieved for the composites and the largest magnetoresistance appeared when x=0.20.     

57Fe Mössbauer spectroscopy studies of Sr2Fe1−x Cr x Mo1−x W x O6 double perovskite compounds

Polycrystalline double perovskites Sr₂Fe_1?x Cr _x Mo_1?x W _x O₆ with x = 0, 0.05, 0.10, 0.15, 0.20, and 0.30 have been prepared by sold state reactions. A continuous decrease of the tetragonal unit cell parameters α and c with increasing x values is observed. The highest Curie temperature T _C = 426 K is recorded for the x = 0.10 compound. ⁵⁷Fe Mössbauer spectroscopy measurements indicate a non-integral electronic configuration of ～3d^5.3 for the Fe ions at the ordered double perovskite structure for x ≤ 0.20, which reaches ～3d^5.4 for x = 0.30. Fe–Mo/W anti-site and anti-phase boundary defects are observed in all samples in equal concentrations of around 3% of the total number of Fe ions in their structure.     

Heat capacity and dielectric properties of multiferroics Bi1 − x Gd x FeO3 (x = 0–0.20)

The heat capacity and the permittivity of multiferroics Bi_{1 ? x} Gd _x FeO₃ (x = 0, 0.05, 0.10, 0.15, 0.20) have been studied in the temperature range 130–800 K. It has been found that insignificant substitution of gadolinium for bismuth markedly shifts the temperature of antiferromagnetic phase transition and increases the heat capacity over a wide temperature range. It has been shown that the temperature dependence of the excess heat capacity is due to the manifestation of three-level states. Additional anomalies characteristic of the phase transitions have been revealed in the temperature dependences of the heat capacity for the compositions with x = 0.1 and 0.15 at T ≈ 680 K and T ≈ 430 K, respectively. The results of studies of the heat capacity have been discussed simultaneously with the data of structural studies.     

Effect of SrTiO3 on dielectric and piezoelectric properties of NBT

Composites of (1 ? x)Na_0.5Bi_0.5TiO₃-(x)SrTiO₃, where x = 0.05, 0.10, 0.15, 0.2, 0.3 and 0.9 are studied. Individual compounds are synthesized by sol gel, and composites are prepared by solid-state sintering process. Through the analysis of X-ray diffraction, lattice parameters are obtained and, from scanning electron microscope (SEM), micro-structure of the samples is observed. The depolarization temperature (T_d) and the Curie temperature (T_c) are determined from dielectric studies. Relaxor behavior of the samples is interpreted using modified Curie Weiss law. Control of polarization in sodium bismuth titanate (NBT) is achieved using strontium titanate (SrTiO₃-ST) and studied through polarization vs. electric field (PE) loops and piezoelectric measurements. The intra-granual and inter-granual effects on the electrical properties of the ceramics are studied from impedance analysis.     

Luminescence of Eu2+-activated Cs(Ca,Mg)F3 and Rb(Ca,Mg)F3

In the system CsCa_0.95?xMg_xEu_0.05F₃ a new phase has been found with x ≈ 0.7, which gives efficient blue luminescence. For RbCa_0.95?xMg_xEu_0.05F₃ efficient green luminescence is obtained at x ≈ 0.5.     

Spontaneous phase transition from relaxor to macrodomain ferroelectric state in single-crystal PbSc0.5Nb0.5O3-BaSc0.5Nb0.5O3 solid solutions

The cooling of Pb_1?x Ba_xSc_0.5Nb_0.5O₃ solid solutions with x≤0.04 leads to a spontaneous transition from a relaxor to a macrodomain ferroelectric state, accompanied by anomalous variation of the dielectric and optical properties of the material. As the barium content in the system increases, the relaxor state becomes more stable and eventually “freezes” at x≈0.05. The crystals with x=0.06 exhibited the appearance of a macrodomain ferroelectric phase induced both by an external electric field with a strength of 1.5 kV/cm and by an internal electric field formed in the course of dielectric aging.     

Magnetic ordering of YBa2(Cu1−x Fe x )3O7

The Mössbauer spectra of YBa₂(Cu_1?x Fe _x )₃O₇ at room temperature show several doublets attributed to Fe in Cu(1) sites with different oxygen configurations. Here we present a systematic study performed at 4.2 K forx=0.005, 0.01, 0.03, 0.05, 0.10, 0.15. To obtain information about the magnetic ordered state two samples, withx=0.005 andx=0.15, have been studied at 4.2 K underB _ext=5 T. The Mössbauer spectra indicate that the iron moments are polarized forx=0.005, while in the ordered state (x=0.15) they have an antiferromagnetic or spin-glass-like arrangement with high anisotropy.     

Ordering of polarons in Pr0.5Ca0.5MnO3

We report detailed dielectric investigations of Pr_1?xCa_xMnO₃ (PCMO, x=0.5) ceramics, a member of the perovskite manganite family that exhibits a colossal magnetoresistance. Analysis of the electrical conductivity and dielectric constant data revealed that in the low temperature phase, below 42 K, small polarons are responsible for the charge transport in the system. This provides a new understanding of the phase diagrams in PCMO systems     

Magnetocaloric properties of (La0.67−xGdx)Sr0.33MnO3 polycrystalline nanoparticles

In this paper, magnetic property and magnetocaloric effect (MCE) in nanoparticles perovskite manganites of the type (La_0.67−xGd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) synthesized by using an amorphous molecular alloy as precursor have been reported. From the magnetic measurements as function of temperature and magnetic applied field, we have discovered that the Curie temperature (T_C) of the prepared samples is found to be strongly dependent on Gd content. The Curie temperature of samples is 358.4, 343.2, and 285.9 K for x=0.1, 0.15, and 0.2, respectively. A large magnetocaloric effect close to T_C has been observed with a maximum of magnetoentropy change in all the samples, ∣ΔS_M∣_max of 1.96 and 4.90 J/kg K at 2 and 5 T, respectively, for a substitution rate of 0.15. In addition, the maximum magnetic entropy change observed for samples with different concentration of Gd, exhibits a linear dependence with the applied high magnetic field. These results suggest that (La_0.67−x Gd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) compounds could be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Dielectric relaxation in NBT–ST ceramic composite materials

Lead-free piezoelectric ceramic composites (1-x) Na_0.5Bi_0.5TiO₃-xSrTiO₃, where x?=?0.05, 0.10, 0.15 and 0.2, are prepared by mixing independently-prepared individual phases through sol–gel method. X-ray diffraction revealed the coexistence of rhombohedral and cubic phases. Surface morphology of the composites is observed using SEM, and both the phases are observed in the samples. Spectroscopic studies of the composites are characterized based on Fourier transform infrared, and vibration bands are analyzed at room temperature in the wave number region 500–3,000 cm^?1. Dielectric properties of these composites are measured from room temperature to 400 °C in the frequency range 1–10 kHz, and these studies indicate the presence of defect clusters within the composite materials. P–E hysteresis loops of these samples were measured as a function of temperature and observed large anomalies in the behavior compared to pure sodium bismuth titanate (NBT) (the host material). Unlike NBT, all the composites exhibited excellent P–E loops. In addition, decrease in remanence polarization is observed, with increase of SrTiO₃ at room temperature. Piezoelectric parameter d₃₃ is measured on poled composites.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by substituting La3+ with Sm3+

The microwave dielectric properties of La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. Apparent density of 6.59 g/cm³, dielectric constant (ε_r) of 19.9, quality factor (Q×f) of 70,200 GHz, and temperature coefficient of resonant frequency (τ_f) of ?77 ppm/°C were obtained for La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h. The dielectric constant, and τ_f of La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics were almost independent with the sintering temperature as the sintering temperature varied from 1450 to 1600 °C.     

Structural properties and electrical behaviour in the polycrystalline lanthanum-deficiency La1−x□xMnO3 manganites

Electrical conductance and X-ray diffraction (XRD) measurements of lanthanum-deficiency La_1−x□_xMnO₃ (x=0.05, 0.10 and 0.20) polycrystalline samples were performed to examine the effect of the internal pressure at B-site on the conduction mechanism. The structural study reveals that all samples crystallize in the rhombohedral system. The electronic conduction appears to be thermally activated at high temperature, which indicates the presence of semiconductor behaviour. The increase of the x content converts 3x Mn³⁺ to 3x Mn⁴⁺ ions with smaller ionic radius, which reduces the internal pressure and leads to the increase of the one-electron bandwidth W. This increase causes the appearance of metallic behaviour at low temperature for x=0.10 and 0.20 content.     

Weak ferromagnetism and spin density distributions in thin films of Gd x Bi1 − x FeO3 solid solutions

Polycrystalline thin-film Gd _x Bi_{1 ? x} FeO₃ (x = 0, 0.05, 0.10, 0.15, or 0.20) samples are synthesized by means of thermal vacuum deposition. The concentrations, temperatures, and magnetic field dependences of specific magnetization are studied. Self-consistent calculations of the spin density distribution are performed for R3c BiFeO₃ and Pnma Gd_0.25Bi_0.75FeO₃ using the density functional theory in the LSDA approximation.     

Disorder induced ferromagnetism in the Cu-doped molybdate SrMo1−xCuxO3 (0≤x≤0.20)

The effect of Cu-doping at Mo-site on structural, magnetic, electrical transport and specific heat properties in molybdates SrMo_1−xCu_xO₃ (0≤x≤0.2) has been investigated. The Cu-doping at Mo-site does not change the space group of the samples, but decreases the structural parameter a monotonously. The magnetic properties change from Pauli-paramagnetism for x=0 to exchange-enhanced Pauli-paramagnetism for x=0.05 and 0.10, and then ferromagnetism for x=0.15 and 0.20. All samples exhibit metallic-like transport behavior in the whole temperature range studied. The magnitude of resistivity increases initially to x=0.10 and then decreases with increasing Cu-doping concentration. The results are discussed according to the electron localization due to the disorder effect induced by the random distribution of Cu at Mo site in the samples. In addition, the temperature dependence of specific heat for the Cu-doped sample has also been studied.     

Dipolar and magnetic ordering in Nd-modified BiFeO3 nanoceramics

The polycrystalline samples of Bi_1−xNd_xFeO₃ (BNFO) with x=0, 0.05, 0.1, and 0.15 were prepared by a chemical route (metal ion complex precursor solution method). A systematic change in crystal structure from rhombohedral to tetragonal on increasing Nd substitution was observed. The reduction of low-frequency dispersion in permittivity and loss pattern due to Nd substitution in BiFeO₃ (BFO) was observed in its dielectric response curve. The loss tangent (tan δ) was found to be reduced on increased Nd concentration in BNFO. Dielectric anomaly near the reported Neel temperature of BFO and peaks in all the BNFO samples were observed in the permittivity vs. temperature graph. Significant opening in the room temperature M–H loop was observed on Nd substitution, which was more prominent for higher concentration of Nd.     

Investigation on optical transmission spectra of (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 single crystals

The optical transmission spectra from 0.3 to 11 μm of relaxor ferroelectric single crystals (1−x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-xPT) were systematically studied at room temperature in this paper. The crystal is transparent between 0.45 and 5.5 μm and becomes completely absorbing around 0.4 μm in near UV region and 10 μm in infrared region. But the wavelength cutoff in near UV is much sharper than the long wavelength cutoff. As compared with other configurations, tetragonal single crystals possess the optimal transmission properties. The optical transmittance in the wavelength region from 0.45 to 5.5 μm is about 70%. The results show that tetragonal PMN-xPT single crystals are promising for a wide range of optical applications. Some discussions about the oxygen-octahedra structure that determines the basic energy level of the crystals are also presented on the optical properties of PMN-xPT single crystals.