Al,F-doped new perovskite lithium fast ion conductor Li3x La2/3−x□1/3−2x Ti1−y Al y O3−y F y (x=0.11)

Al, F-doped new perovskite lithium ion conductors (x=0.11) have been prepared by solid state reaction. It is found that a pure perovskite-structured phase with space group of P4mm(99) exits in the composition range of 0<y≤0.10. The sample with y=0.02 possesses the highest ionic conductivity of 1.06×10⁻³ S/cm at room temperature, and its decomposing voltage is 2.3 V. The factors affecting the conductivity of this system are discussed.     

Synthesis and characterization of (Bi2O3)1−x−y−z(Gd2O3)x (Sm2O3)y(Eu2O3)z quaternary solid solutions for solid oxide fuel cell

Sm₂O₃, Gd₂O₃, Eu₂O₃ triple-doped Bi₂O₃ based quaternary solid solutions were synthesized as a candidate electrolyte material using the solid-state reaction technique. The structural, thermal and electrical conductivity features of the ceramic samples were examined and compared by using X-ray powder diffraction (XRD), thermal gravimetry/differantial thermal analysis (TG/DTA) and the four-point probe technique (4PPT). The result of XRD measurements indicated that the (Bi₂O₃)_{(1−x−y−z)}(Gd₂O₃)_x(Sm₂O₃)_y(Eu₂O₃)_z (x = 10/y = 10/z = 5, 15, 20 mol % and x = 10/y = 5, 10, 15, 20/z = 10 mol %) samples have a stable face-centered cubic δ-phase and mixed phase crystallographic structure. The phase stability was also checked by the DTA evaluations results. The temperature dependent electrical conductivity measurements showed that the highest electrical conductivity was observed for the sample of the (Bi₂O₃)_0.75(Gd₂O₃)_0.10(Sm₂O₃)_0.05(Eu₂O₃)_0.10 system which has a stable and δ-phase was found as 6.67 × 10⁻³ (Ω cm)⁻¹ at 650 °C. This sample can be used as an electrolyte material in the solid oxide fuel cells (SOFCs) which is possible to operate at intermediate temperature ranges. The activation energy was also calculated at a low temperature range (350–650 °C) and high temperature range (above 650 °C). The values for the samples vary from 0.63 eV to 1.08 eV at low temperature and at high temperature they vary from 0.43 eV to 0.75 eV.     

Mössbauer Study of La1−x Ca x Mn1−y 57Fe y O3 with x=0,0.25; y=0.01

Temperature dependent Mössbauer measurements are done on the samples of La_1–x Ca _x Mn_1–y ⁵⁷Fe _y O₃ with x=0 and 0.25, and y=0.01. With decreasing temperature, the specimen with x=0.25 shows a paramagnetic to ferromagnetic transition around 175 K. In the specimen x=0.0, the temperature dependence of both the center shift () and the recoilless fraction (f) can be fitted very well with the Debye theory with a _D=320±50 K. But for the specimens with x=0.25, f and show distinct deviations from the Debye behavior in the temperature range in which the resistivity shows a sharp decrease. Dips observed in both the f and around the transition temperature suggest that the Jahn–Teller distortion observed in these systems is dynamic in nature.     

Neutron diffraction study of the oxide conducting δ1-phase of (Bi2O3)1−x(Y2O3)x (x=0.25)

The atomic parameters of the average cell of δ¹-Bi(Y)O_1.5 were determined by powder neutron diffraction. The evidence indicates that the disorder in the structure has many features in common with the structure of the oxygen deficient zirconia. Most oxygens (78%) are in sites displaced 0.335 Å along 〈100〉 directions from the normal fluorite positions, while a smaller proportion (22%) are displaced 0.80 Å along 〈111〉 directions and no oxygen remains in normal positions. In addition to the anion displacements a smaller displacement (0.25 Å) along the 〈111〉 direction was found for the cations. Comparison with the structure of β-Bi₂O₃ suggests that the displacements may be precursors to a δ→β phase transition.     

Temperature dependence of the Cu(2) NQR line width in YBa2Cu3O7−y

Precision X-ray structural studies were carried out for LiNbO₃:Zn_x single crystals with x=0.0, 2.87, 5.20, and 7.60 at. %. It was found that the insertion of the Zn atoms into the Li position was accompanied by a decrease in the concentration of intrinsic Nb_Li defects. At x=7.6%, the Zn atoms change their locations in the lattice and partially occupy the Nb positions. This clarifies the structural nature of the “threshold” Zn concentration, which manifests itself as singularities in the concentration dependences of various optical properties. The structural origin of the threshold concentration is likely a common feature of all nonphotorefractive impurities (Mg, Zn, In, and Sc) in LiNbO₃. A change in the intrinsic defect structure of the LiNbO₃ crystals with different Zn concentrations is discussed.     

Peculiarities of magnetic flux penetration and trapping in monocrystalline (YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x ) and polycrystalline (YBa2Cu3O7−x ) samples

A comparative analysis of the magnetic flux trapping in monocrystalline and polycrystalline HTSC samples is carried out, and the possibility of employing the dependence of the trapped magnetic flux on the external magnetic field for obtaining comparative estimates of the effect of pinning centers is determined.     

Magnetic properties of superconducting EuBa2Cu3O7−x

The magnetic properties of superconducting EuBa₂Cu₃O_7–x with resistiveT _c 96.5 K are measured. The normal state magnetic susceptibility is analyzed within the framework of the Van Vleck-Frank theory, leading to the conclusion that the strong moments of the Eu³⁺ ions are uncorrelated, and do not affect the superconducting state.     

Normal state resistivity of YBa2Cu3O7−x

A systematic study has been carried out on the normal state resistivity of YBa₂Cu₃O_7–x . Samples were cut from the same well-annealed material and were quenched from different temperatures (T _Q ). The resistivity is metallic whenT _Q 500 °C and becomes semiconductor-like whenT _Q 600 °C. The data on the latter was interpreted in terms of the three-dimensional localization model proposed by Mott.The work at USCD was supported by the California MICRO program     

Phase stability,thermal, electrical and structural properties of (Bi2O3)1−x−y(Sm2O3)x(CeO2)y electrolytes for solid oxide fuel cells

In this study, (Bi₂O₃)_1?x?y(Sm₂O₃)_x(CeO₂)_y ternary system was synthesized by using solid-state reaction method. Structural, morphological, thermal and electrical properties of the samples were evaluated by means of X-ray diffraction (XRD), scanning electron microscopy, thermo gravimetry/differential thermal analyzer and four-probe method. The XRD measurement results indicated that the samples (x = 10–15, y = 5–10–15–20) had cubic δ-phase crystallographic structure. The phase stability of the samples was checked by the differential thermal analyzer measurements, which indicates most of the samples have stable δ-Bi₂O₃ phase. The electrical conductivity measurement results showed that the electrical conductivity increased with mol% CeO₂ molar ratio at a fixed molar ratio of Sm₂O₃. The highest electrical conductivity obtained for the (Bi₂O₃)_0.65(Sm₂O₃)_0.15(CeO₂)_0.20 system was 1.55 × 10^?2 (Ω.cm)^?1 at 600 °C. The activation energies were also calculated at low temperature range (350–650 °C) which vary from 1.1325 to 1.4460 eV and at high temperature (above 650 °C) which vary from 0.4813 to 1.1071 eV.     

Synthesis and excess conductivity analysis of TlBa2(Ca3−y Be y )Cu4O12−δ superconductors

TlBa₂(Ca_3?y Be _y )Cu₄O_12?δ (y = 0, 0.5, 1.0, 1.5, 2) samples are synthesized at normal pressure and the influence of doped Be-atoms on the superconductivity parameters at the microscopic level is investigated by carrying out excess conductivity analyses of conductivity data. The samples have shown tetragonal structure and the unit cell volume decreases with increased Be-doping. The onset temperature of superconductivity [T _c(onset)] and zero resistivity critical temperature [T _c(R = 0)] decrease with increased Be, however, the magnitude of diamagnetism is enhanced with Be (except for Be-doping of y = 1.0). The apical oxygen mode of the type Tl–O_A–Cu(2) and CuO₂ planar oxygen mode are softened as observed in FTIR absorption measurements. The FIC analyses of conductivity data have shown an increase in the coherence length along the c-axis and inter-plane coupling. The values of B _c0(T), B _c1(T), J _c(0), τ _φ are improved with the doping of Be. These observations suggested that due to the proximity effect there is less suppression in the value of the order parameter of the Cooper pairs from |ψ|² = 1 value in the CuO₂ planes in Be-doped samples that maintains the density of carriers in the conducting CuO₂ planes [since the |ψ|² = n/2] which promotes enhancement in the magnitude of superconductivity.     

A Mössbauer study of YBa2(Cu1−x)Fex3O7−σ

⁵⁷Fe Mössbauer spectra of a sample of YBa₂(Cu_0.985Fe_0.015)₃O_7−σ for which T_c≈59 K show that long-range magnetic order is established below ∼ K. A model in which the Fe spins are aligned with the crystallographic c-axis provides satisfactory agreement with the observed relaxation spectra. This result is discussed with reference to current theoretical models of high-T_c superconductivity which involve magnetic coupling mechanisms.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

EFG determination in magnetic field oriented powders of YBa2(Cu1−x 57Fe x )3O7−y superconductors: A test of existing models

YBa₂(Cu_1-x ⁵⁷Fe _x )₃O_7–y powder samples withx=0.02 and 0.03 were uniaxiallyc-aligned in epoxy in a magnetic field of 7.5 T. An X-ray map of the distribution of the orientation of the crystallographicc-axes of the individual crystallites was determined.⁵⁷Fe Mössbauer spectra were recorded at room temperature as a function of the angle between the texture axis and the observation direction. Different sets of EFG parameters were selected from the literature and the fit constraints were equally set for all. Using a model of the measured texture function and accounting for polarization-dependent blackness effects, a simultaneous fit of the Mössbauer spectra results in minimum ² for the EFG set determined previously from CEMS data on this films.On leave from: Institute for Low Temperature Physics and Engineering, Ukr. Acad. Sci., Kharkov, Ukraine.     

Quantum-chemical modeling of the electronic structure and magnetic properties of Sn1 − x − y M x Sb y O2, M = Cr, Mn, Co, or Ni (x = 0.25; y = 0, 0.25)

The electronic and magnetic structures of the Sn_0.75 M _0.25O₂ and Sn_0.5 M _0.25Sb_0.25O₂ (M = Cr, Mn, Co, Ni) compounds with a structure that is derivative of the rutile structure are modeled using the ab initio spin-polarized tight-binding linear muffin-tin orbital (TB-LMTO) method. The magnetic moments of the transition metal atoms are calculated. The data obtained are used to analyze the influence of the composition of Sn_{1 ? x ? y} M _x Sb _y O₂ phases on their electronic spectra and the magnetic and transport characteristics.     

Transition to gapless superconductivity in YBa2(Cu1−x Zn x )3O7

Point-contact spectra of YBa₂(Cu_1–x Zn _x )₃O₇/Ag are studied at various temperatures. The differential resistance dV/dI of the point contacts shows gap-related structures belowT _c which can be attributed to Andreev reflection. Evaluation of many spectra for each sample taken at 4.2 K yields a wide distribution of voltages /e at which these structures occur. The upper limit varies roughly as expected from the depression ofT _c by Zn-doping from /e=29 mV (x=0) to 9 mV (x=0.05), while the lower limit decreases much faster and disappears forx=0.05. Hence, the Zn doped samples exhibit a tendency to gapless superconductivity as suggested earlier on the basis of specific-heat measurements.     

μSr and vortex pinning in Bi2Sr2Ca1−x Gd x )Cu2O y

We present transverse fieldSR and bulk magnetisation studies on powders of Bi₂Sr₂(Ca_1{itxGd _x )Cu₂O _y for 0x<0.5. We have found that the effective pinning decreases rapidly withx even for values ofx for whichT _c remains unchanged. We have interpreted this decrease in terms of changes in _ab (obtained fromSR) which cause a decrease in vortex-vortex interactions. The field distribution spectra in these superconductors have been obtained using a maximum entropy analysis and are found to agree with the calculations by Harshmanet al.     

Specific heat studies of high-Tc superconductor YBa2Cu3O7−x

Specific heat studies of the high-T_c superconducting compound YBa₂Cu₃O_7−x with bulk transition temperature at 92K are reported. A distinct anomaly of electronic origin in the specific heat is observed with granular-like behavior corresponding to a Sommerfeld constant γ = 7±2mJ(moleCuK²)^±1 Debye temperature (φo ≈ 400K) is obtained by fitting the experimental data with the theoretical Debye specific heat.     

Ferromagnetic resonance study of Y3−z Ca z Fe5−x−y Co x Ge y O12 films

Conclusions It has been found that the character of the magnetic anisotropy changes markedly when the Co + Ge YIG films are doped with calcium. For the induced uniaxial and in-plane anisotropy we observe an increasing departure from the predictions based on the two-parameter model and for the cubic anisotropy the constantK ₁ becomes dependent on the growth direction. As a source for the latter effect the octahedral or tetrahedral preference of Co ions depending on the growth direction may be considered. The measured linewidth proportional to frequency indicates the presence of a relaxation mechanism which is, probably, connected with octahedral Co³⁺ ions.The authors express their thanks to Dr. P. Görnert and M. Neviva for preparation of the LPE films and to Dr. P. Novák for valuable discussions.Dedicated to Jan Kaczér DrSc on the occasion of his 65th birthday.     

EPR study on the La2/3Ca1/3Mn1−x Cu x O y system

The interaction between Mn and Cu ions is studied by measuring the resistance and electron paramagnetic resonance (EPR) at different temperatures of the Cu-doped compound La_2/3Ca_1/3Mn_1−xCu_xO_y. A new transition inR-T curve and substantial enhancement in magnetoresistance are induced by the substitution of Mn ions by Cu ions. The EPR measurement shows that two resonance signals appear at temperature lower than the spin-ordering temperature of Mn ions. A tentative interpretation for the observed phenomena is proposed by considering the interaction between the Cu/Mn ions besides the Mn³⁺/Mn⁴⁺ ions.     

Structure and dielectric properties of Bi6 − x Sr x Ti2 − x Nb2 + x O18 (x = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi_{6 ? x} Sr _x Ti_{2 ? x} Nb_{2 + x} O₁₈ (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity ?/?₀(T) and dielectric loss tangent tanδ have been measured. The dependences of the maximum of the permittivity ?/?₀, Curie temperature T _C, lattice parameters, and the unit cell volume on x have been obtained. The structural parameter a, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in x.