Magnetic properties of Nd0.5Pb0.5-xSrxMnO3 materials

The structure and magnetic properties of Nd0.5Pb0.5-xSrxMnO3 (0≤x≤0.4) manganites were systematically investigated. Significant changes in Curie temperature and metal-insulator (MI) transition temperature of the samples were observed. All samples exhibited a transition from paramagnetic semiconducting to ferromagnetic metallic state.Curie temperature Tc and the MI transition temperature Tp increased with increasing Sr content. We attributed these behaviours to the enhancing of both the double exchange mechanism and the Jahn-Teller electron-phonon coupling.     

Thermal,Raman and dielectric study of 0.5K0.5Bi0.5TiO3–0.5PbTiO3 ceramics

The 0.5K_0.5Bi_0.5TiO₃–0.5PbTiO₃ ceramics were prepared by following a standard solid-state method. The Raman, thermal and dielectric properties of these ceramics were investigated. The X-ray measurements showed that samples have single perovskite-type structure with tetragonal symmetry. Dielectric study revealed that the dielectric behaviour of the investigated ceramics is rather of normal ferroelectrics with large thermal hysteresis. The transition temperature observed by means of differential scanning calorimetry measurements is in good agreement with that obtained from dielectric study.     

Magnetic properties of Nd0.5Pb0.5—xSrxMnO3 materials

The structure and magnetic properties of Nd_{0.5}Pb_{0.5-x}Sr_xMnO_3 (0≤x≤0.4) manganites were systematically investigated. Significant changes in Curie temperature and metal-insulator (MI) transition temperature of the samples were observed. All samples exhibited a transition from paramagnetic semiconducting to ferromagnetic metallic state. Curie temperature T_C and the MI transition temperature T_p increased with increasing Sr content. We attributed these behaviours to the enhancing of both the double exchange mechanism and the Jahn－Teller electron－phonon coupling.     

Microstructure and electrical properties of Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics of (1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yLiNbO₃ (BNT–BKT–LN-x/y) have been fabricated by a conventional solid-state reaction method, and their microstructure and electrical properties have been investigated. The results of X-ray diffraction (XRD) measurement show that K⁺, Li⁺ and Nb⁵⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. The BKT and LN addition has no remarkable effect on the crystal structure. However, a significant change in grain size took place. Simultaneously, with increasing amount of LN, the temperature for a ferroelectric–antiferroelectric phase transition is clearly reduced. The temperature dependence of dielectric properties suggests that the ceramics have diffuse-type phase transition characteristics. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p of the ceramics attain maximum values of 195 pC/N and 0.336 at x=0.18 and y=0.01.     

Effect of cooling method on the electrochemical performance of 0.5Li2MnO3·0.5LiNi0.5Mn0.5O2 cathodes

0.5Li₂MnO₃·0.5LiNi_0.5Mn_0.5O₂ powders were synthesized by coprecipitation, and high temperature sintered with different cooled methods, such as cooled with furnace material, water material, and in liquid nitrogen (N-material). The effect of cooling methods on physical and electrochemical properties are discussed through the characterizations of X-ray diffraction (XRD), scanning electron microscopy, electrochemical impedance spectroscopy (EIS), and discharge, cyclic, and rate tests. XRD results show that all samples exhibit layered characteristics. The electrochemical performance results indicate that the N-material has the best electrochemical performance. The discharge capacity at 0.1 and 5 C are 279 and 99 mAhg^?1, respectively. The coulomb efficiency is highest, 78.4 %. The capacity retention after 50 cycles at 0.2 C is 97.1 %. EIS results show that the charge transfer resistance of N-materials is lowest, which is responsible for higher rate capacity.     

μ+SR study of Ising spin-glass Fe0.5Mn0.5TiO3

A typical Ising spin-glass Fe_0.5Mn_0.5TiO₃ has been investigated by ⁺ spin relaxation measurements. It is found that the local magnetic fields at the ⁺ stopping sites appear at a temperature twice as high asT _SG. The directional distribution of the local magnetic fields is nearly isotropic, indicating the existence of the transverse spin components. It is clearly demonstrated that the spins fluctuate belowT _SG and even at 4 K. Taking into account the results obtained by other methods, the main part of the fluctuations is inferred to be due to the transverse spin components and the fluctuation limes at 4 K are driven into a range between 10^–5 and 10^–7 s.     

Grain-Size Effect on the Dielectric Properties of Bi0.5Na0.5TiO3

We prepared bismuth sodium titanate (Bi0.5Na0.5TiO3)ultrafine powders by the sol-gel method.The dielctric properties of the pressed pellets and fired ceramics with different grain sizes as a function of tempernature at various frequencies were studied.With decreasing grain size,the dielecric anomaly around 200℃ increases,while the dielectric thermal hysteresis decreases,All the samples with grain sizes larger than 100nm show dielectric peaks at temperature of about 350℃.The very little change in Tm observed down to the critical size indicates that Bi0.5Na0.5TiO3 is an order-disorder system above 200℃,In addition,the dielectric peak becomes lower with decreasing grain size and the ferroelectric critical size of Bi0.5Nan0.5TiO3 was eventually determined to be about 100nm according to the disappearance of dielectric peak.     

First Principles Study on Electronic Structure of PbFe0.5Nb0.5O3

The full potential linearized augmented plane wave (FLAPW) method is used to study the crystal structure and electronic structure properties of PbFeo.5 Nbo.5O3 (PFN). The optimized crystal structure, density of states, band structure and electron density distribution have been obtained to understand the ferroelectric behaviour of PFN.The analysis result of the density of states shows there is an obvious change of Nbd states in the paraelectric-to-ferroelectric phase transition. The polarization result shows that the contribution to ferroelectricity of Nb atoms is larger than that of Fe atoms. In ferroelectric phase there is a hybridization of Fed-Op and Nbd-Opin ferroelectric PFN. This is consistent with the result of the electronic band structure. This hybridization is responsible for the tendency to its ferroelectricity.     

Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics

Lead-free 0.79(Bi_0.5Na_0.5)TiO₃–0.14[Bi_0.5(K_0.5−xLi_x)]TiO₃–0.07BaTiO₃ (BNBK79 + xLi, x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics were prepared by conventional solid state reaction process. The crystalline structures and surface morphologies are investigated by X-ray diffraction method and scanning electron microscopy. Dielectric and piezoelectric properties were measured. With increasing of lithium substitution, the Curie temperatures of BNBK79 + xLi ceramics increase, but the maximum value of the dielectric constant decreases. And a relatively large remnant polarization of 17.6 μC/cm² and 157 pC/N of d₃₃ has been obtained when x = 0.3.     

Diffuse dielectric anomaly of （Na0.5K0.5Bi）0.5TiO3 crystal at high temperature

This paper investigates the dielectric properties of (Na_0.5K_0.5Bi)_0.5TiO₃ crystal at intermediate frequencies (1kHz \le f \le 1MHz) in the temperature range of 30--560℃. A pronounced high-temperature diffuse dielectric anomaly has been observed. This dielectric anomaly is shown to arise from a Debye-like dielectric dispersion that slows down following an Arrhenius law. The activation energy E_r obtained in the fitting process is about 0.69eV. It proposes that the dielectric peak measured at low frequency above 400℃ is not related to the phase transition but to a space-charge relaxation.     

Ferroelectric relaxor properties of (1 − x)K0.5Na0.5NbO3–xBa0.5Ca0.5TiO3 ceramics

The Ba_0.5Ca_0.5TiO₃ (BCT) composition dependent dielectric and structural properties of (1?x)K_0.5Na_0.5NbO₃–xBa_0.5Ca_0.5TiO₃ powders were investigated. Room temperature x-ray diffraction revealed the powder structure to transform from orthorhombic to cubic with increasing BCT composition. The frequency dependent dielectric constant measurements revealed a shift in the temperature of the maximum dielectric constant for at frequencies, suggesting that the system exhibits ferroelectric relaxor behavior. The system containing 15% BCT showed the closest calculated Curie–Weiss exponent to 2, which the exponent for a relaxor ferroelectric.     

Properties of Co-Doped 0.92（Na0.5Bi0.5） TiO3-0.08BaTiO3 Lead-Free Ceramics

Lead-free piezoelectric ceramics 0.92（Bi0.5Na0.5）TiO3-0.08BaTiO3 ＋ xmol% Co3＋ （BNBT-Co, x = 0-8） are prepared by the solid state reaction method. Effects of the amount of Co^3＋ on the electrical properties and phase transition are studied. The results indicate that the addition of Co^3＋ enhances the mechanical quality factor Q^3＋ significantly, whereas the dissipation factor tanδ has a minimum value at x = 3.5. Meanwhile, addition of Co^3＋ leads to small decreases of piezoelectric constant d33, and planar electromechanical coupling kp. The present 0.92（Bio.aNao.5） TiO3-0.08BaTiO3＋3.5 moi% Co^3＋ ceramics exhibit good performance with mechanical quality factor Qm = 910, piezoelectric constant d33 = 106pC/N, planar electromechanical coupling kp =10% and dissipation factor tanδ = 1.1% at 1 kHz. Saturated polarization hysteresis loops have been obtained for BNBT-Co ceramics. Two dielectric peaks at depolarization temperature Td and Tm appear in the curves of ε33^T vs temperature for the pure BNBT ceramics. However, the first dielectric peak Td disappears after the addition of Co^3＋, which means that the transition from ferroelectric to antiferroelectric phase has been eliminated.     

Effect of oxygen content on the magnetic state of La0.5Ca0.5MnO3−γ perovskites

The magnetization and magnetoresistance are studied in La_0.5Ca_0.5MnO_3−γ as a function of oxygen content. It is found that as the oxygen content is decreased, this compound undergoes a sequence of transitions from an antiferromagnetic to a ferromagnetic state (γ⩾0.04), from the ferromagnetic to a spin-glass state (γ ⩾0.14), and from the spin glass to an inhomogeneous ferromagnetic state (γ⩾0.25). Strongly reduced samples (γ⩾0.25) show a large magnetoresistance despite the absence of Mn³⁺–Mn⁴⁺ pairs. It is suggested that the oxygen vacancies in the strongly reduced samples (γ⩾0.25) are long-range ordered. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 583–587 (10 November 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Structure evolution and microwave dielectric response of (Ca0.5+xSr0.5−x)[(Al0.5Nb0.5)0.5Ti0.5]O3 solid solutions

The phase assemblage, crystal structure evolution and microwave dielectric response of (Ca_0.5+xSr_0.5−x)[(Al_0.5Nb_0.5)_0.5Ti_0.5]O₃ ceramics (abbreviated as CSANT hereafter) are investigated. Single perovskite solid solution is formed in the CSANT ceramics in Sr-rich composition range of x < −0.05, however, Ca₄Ti₃O₁₀-type layered perovskite phase begins to segregate after x = −0.05. The CSANT perovskites crystallized in Fm3m cubic symmetry in the composition range of x ≤ −0.2, however, as the Ca²⁺ content in A-site increased, the oxygen octahedral began to be anti-phase tilted at x = −0.1 and the crystal structure transited to P2₁/n pseudo-orthorhombic space group thereafter. The microwave dielectric response of the CSANT ceramics is elaborately discussed in terms of their crystallographic structure and chemical composition. When sintered at 1500 °C for 4 h, a dielectric constant ɛ_r of 52.5, a Qf product of 28000 GHz and a τ_f of +25.4 ppm/°C microwave dielectric ceramic can be obtained in the CSANT ceramics at x = 0.3.     

Phase transition,dielectric and piezoelectric properties of NaNbO3–Bi0.5Li0.5TiO3 lead-free ceramics

Lead-free ceramics (1?x)NaNbO₃–xBi_0.5Li_0.5TiO₃ have been fabricated by an ordinary sintering technique, and their electric properties and temperature characteristics have been studied. All the ceramics possess a perovskite structure with orthorhombic symmetry, indicating that (Bi_0.5Li_0.5)TiO₃ diffuses into NaNbO₃ lattices to form a new solid solution. A low (Bi_0.5Li_0.5)TiO₃ doping level transforms the NaNbO₃ ceramics from antiferroelectric to ferroelectric. The ceramics with x ≤ 0.075 are normal ferroelectric, and the ferroelectric-paraelectric phase become diffusives with the doping level of Bi_0.5Li_0.5TiO₃ increasing. As x increases, the Curie temperature of the ceramics decreases linearly, while the relative permittivity ε_r increases. 0.925NaNbO₃–0.075(Bi_0.5Li_0.5)TiO₃ ceramic exhibits the relatively large piezoelectric constant (d₃₃ = 58 pC/N), high Curie temperature (T_C = 228 °C) and good temperature stability, suggesting that the ceramics are one of new possible candidates for lead-free piezoelectric materials.     

0.5BiFeO3–0.5PbFe0.5Nb0.5O3 Multiferroic Ceramic: Structure,Dielectric and Magnetodielectric Properties

Physics of the Solid State - The structure, dielectric characteristics, and magnetoelectric effect of a 0.5BiFeO3–0.5PbFe0.5Nb0.5O3 multiferroic ceramics are studied. We found that the...     

Phase diagram of (1−x%)(0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3)–x%MnO2 lead-free anti-ferroelectric ceramics

A phase diagram for the lead-free ceramics in the (1?x%)(0.89Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–0.05K_0.5Na_0.5NbO₃)–x%MnO₂ (BNBKN-x%Mn) binary system is constructed for the first time based on the ferroelectric and dielectric measurements. The ferroelectric behaviors under different temperatures suggest that the ceramics are basically of relaxor anti-ferroelectric nature near room temperature. The temperature dependent dielectric properties show that when the addition of MnO₂ increases, the relaxor anti-ferroelectric phase can be stabilized to be close to the Curie point, which corresponds to a relaxor anti-ferroelectric to paraelectric phase transition.     

Effects of A-site disorder on magnetic,electrical and thermal properties of La0.5−xLnxCa0.5−ySryMnO3manganites

The magnetic, electrical and thermal properties in the La_0.5?xLn_xCa_0.5?ySr_yMnO₃ (Ln=Pr, Nd, Sm) bulk system were investigated. Detailed dc magnetization and linear ac susceptibility measurements reveal that the samples first undergo phase transition from paramagnetic to ferromagnetic phase and then to an antiferromagnetic phase upon further cooling. It is found that both the Curie and Neel temperatures decrease systematically with increasing A-site disorder in these manganites. The electrical resistivity exhibits semiconducting behavior throughout the temperature range investigated and the electronic conduction mechanism can be conveniently described within the framework of the variable range hopping model above T=150 K. The Seebeck coefficient (S) in the magnetically ordered regime infers that the complicated temperature dependence of S is an indication of electron–magnon scattering. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The temperature dependence of thermal conductivity, κ(T), reveals a positive dκ/dT in the paramagnetic region, which may be related to the local anharmonic lattice distortions associated with small polarons.