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.     

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.     

Concentration phase transitions in Li x Na1 − x Ta y Nb1 − y O3 solid solutions

The ferroelectric-antiferroelectric concentration phase transitions in solid solutions of Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃, which are promising environmentally friendly (lead-free) ferropiezoelectric materials, are studied by means of Raman spectroscopy.     

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.     

Thermoelectric properties of n-Bi2Te3 − x − y Se x S y solid solutions under high pressure

The thermoelectric properties of n-Bi₂Te_{3 ? x ? y} Se _x S _y solid solutions with atomic substitutions in the tellurium sublattice (x = 0.27, 0.3, y = 0, and x = y = 0.09) have been studied under a pressure to 8 GPa. It has been found that the Seebeck coefficient and the resistance decrease with increasing P, and power factor χ increases in all compositions and becomes maximal at pressures of 2–4 GPa. It has been shown that the power factor χ, which is proportional to the product of the effective mass of the density of states m/m m/m ₀ and the charge carrier mobility μ₀ in the form (m/m ₀)^3/2μ₀, increases with increasing pressure mainly due to the increase in the mobility and also depends on the solid solution composition. In the composition with substitution Te → Se + S (x = y = 0.09), the peculiarity of the dependence of m/m ₀ on P in the pressure range corresponding to maximal values of the power factor can be explained by the existence of an electronic topological transition. The increase in the power factor under pressure in n-type Bi₂Te_{3 ? x ? y} Se _x S _y solid solutions combined with similar data for p-type Bi_{2 ? x} Sb _x Te₃ solid solutions obtained earlier, including the estimations of possible changes in the thermal conductivity with increasing pressure, give grounds to design thermoelements with improved value of the thermoelectric figure-of-merit, which can be 50–70% at pressures of 2–4 GPa.     

Elastic and thermal properties of Zr z Nb1 − z C x N y solid solutions

The temperature and concentration dependences of the elastic moduli and the thermal linear expansion coefficient of Zr _z Nb_{1 ? z} C _x N _y solid solutions containing from 3 to 8 at % of structural vacancies in a nonmetallic sublattice have been found. The temperature dependences of the Debye temperature Θ_D(T) have been calculated using the elastic data and the data on the heat capacity. It has been shown, using carbide NbC_0.97 as an example, that the Θ_D(T) dependences found from the elastic properties and the heat capacity coincide in the temperature range ～220–300 K. By analogy with the niobium carbide, the heat capacity C _p (300) of Zr _z Nb_{1 ? z} C _x N _y solid solutions of various compositions is calculated based on the values of Θ_D(300) determined from the elastic properties.     

Investigating ionic mobility in solid solutions of Li x Na1 − x Ta y Nb1 − y O3 via Raman spectroscopy

The ionic mobility in Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions (SSes) is investigated by means of Raman spectroscopy. The average lifetime of an Li⁺ ion in the equilibrium position and the magnitude of the potential barrier are estimated from the temperature dependence of the line width corresponding to the vibrations of Li⁺ cations.     

Crystal structure of aluminium containing superconducting oxides: single crystal study of GdBa2Cu3−x AL x O6.88 (x=0.28) and ErBa2Cu3−y Al y O6.6 (y=0.14)

Following the discovery of the high temperature super-conductivity in YBa₂Cu₃O_7– (T _c 93 K) [1] it has been found that Y atoms can be substituted entirely by almost all of the rare-earth elements, except for Ce, Pr, and Tb, without changing the superconducting properties appreciably [2, 3]. The magnetic moments carried by the rare-earth atoms have apparently no influence on the superconducting properties. In Nd-, Sm-, Gd-, Dy- and Er-based compounds the rare earth moments order at low temperatures (0.025–2.2 K) [4–8] and the ordered antiferromagnetic state coexists with the superconducting state. We have investigated the antiferromagnetic ordering of these compounds by neutron diffraction both on powder and single crystal samples obtained from several laboratories [8–10]. Magnetic structures of all these compounds consist of antiferromagnetic (001) planes stacked ferro- or antiferromagnetically. There have been some controversies as to the stacking of antiferromagnetic (001) planes in GdBa₂Cu₃O_7–.     

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.     

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.     

Concentration-phase transitions in solid solutions Li x Na1 − x Ta0.1Nb0.9O3, Li0.12Na0.88Ta y Nb1 − y O3, and NaTa y Nb1 − y O3 and their manifestations in Raman spectra

Using the Raman-spectroscopy method, we have studied concentration-phase transitions in the solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃, Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, and NaTa _y Nb_{1 ? y} O₃ (x = 0?0.16, y = 0–1). It has been revealed that, for the solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃ and Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, the concentration-phase transition is a transition between the ferroelectric and antiferroelectric states. It is accompanied by the disappearance from the spectrum of a line that corresponds to stretching bridge vibrations of oxygen atoms along the polar axis, which is forbidden by selection rules in the centrosymmetric phase, and by splitting into two components of a line that corresponds to librational vibrations of oxygen octahedra as a whole, which can be caused by doubling of the unit cell in the antiferroelectric phase. Manifestation and variation of intensities of lines with frequencies 860–873 and 900–905 cm^?1 upon variation of the concentration of tantalum for solid solutions Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, and NaTa _y Nb_{1 ? y} O₃ is caused by the formation of polar clusters in the medium, which is centrosymmetric in general due to disordering in the sublattice of niobium and tantalum.     

Preparation and characterization of Al and La co-doped (Ce1−x−y Al x LayO2-(x+y)/2) ceria

Effect of co-doping of lanthanum and aluminum on ionic conductivity of CeO₂ for its use as a solid electrolyte in intermediate temperature solid oxide fuel cells has been studied. A few compositions in the system Ce_1???x???y Al _x La_yO_{2???(x?+?y)/2} (CAL) with x?=?0.05, 0.025, and 0.00 and y?=?0.00, 0.025, and 0.05 such that x?+?y?=?0.05 are prepared by auto-combustion method. X-ray diffraction patterns show that all the synthesized samples are ceria-based solid solutions. Microstructures of thermally etched samples have been studied by scanning electron microscope. To determine the contribution of grains, σ _g and grain boundaries, σ _gb to the total conductivity, σ _t impedance is measured in the frequency range 1–1 MHz at different steady temperatures in the range 250–500 °C. It has been found that conductivity of the co-doped composition is more than singly doped with Al but less than singly doped lanthanum composition.     

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.     

Elastic properties of (Pb y Sn1−y )2P2S6 solid solutions

Elastic properties of (Pb _y Sn_1?y )₂P₂S₆ solid solutions were studied using Brillouin scattering technique. Different scattering geometries were used for sound velocities determination that make it possible to find all components of the stiffness tensor. The concentration dependencies of volume compressibility, the Grüneisen parameter and Debye temperature were investigated. The results obtained were used to analyze chemical bonding with substitution of tin by lead at room temperature in the crystals under consideration.     

High-temperature background of internal friction in (Co45Fe45Zr10) x (Al2O3)100 − x , Co x (CaF2)100 − x , and Co x (PZT)100 − x nanocomposites

The elastic (G) and inelastic (Q ^?1) properties of (Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 ? x} , Co _x (CaF₂)_{100 ? x} , and Co _x (PZT)_{100 ? x} (x = 23–76 at %) nanocomposites obtained by ion-beam sputtering are studied in the temperature range 300–900 K. A significant rise in the Q Q ^?1 (T) curve is observed at temperatures above 650 K, which is attributed to thermally activated migration of point defects under the conditions of confined geometry.     

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.     

Critical behavior of La0.67−y (Sr, Ba, Ca)0.33+y Mn1−x Sn x O3 (x = 0.01, 0.02, y = 0, 0.07) perovskites

The magnetic critical behavior of the manganese perovskite series $ {\text{La}}_{{0.67 - y}} {\left( {{\text{Sr,}}\,\,{\text{Ba,}}\,\,{\text{Ca}}} \right)}_{{0.33 + y}} {\text{Mn}}_{{1 - x}} {\text{Sn}}_{x} {\text{O}}_{3} The magnetic critical behavior of the manganese perovskite series (x = 0.01, 0.02, y = 0, 0.07) is studied by means of dc magnetic measurements and ¹¹⁹Sn M?ssbauer spectroscopy. The structure can be described by a rhombohedral unit cell (space group R–3C) for the samples where the A-site is occupied by La and Sr or La and Ba ions and orthorhombic unit cell (space group Pnma) for the samples where the A-site is occupied by La and Ca ions. Arrott and scaling plots show that the samples, where the A-site is occupied by La and Sr or La and Ba ions, follow the behavior of a conventional second-order ferromagnetic transition. In contrast, the samples that contain La and Ca ions in the A-site show anomalous behavior around Curie point. M?ssbauer measurements show two magnetic phases below T _c. One of them exhibits stronger exchange interactions with more rapid electron transfer between Mn³⁺/Mn⁴⁺, compared to the other.     

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.     

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca_{1 ? x ? y} Y _x Gd _y F_{2 + x + y} single crystals has revealed spectra that are not typical of gadolinium-doped CaF₂ crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd³⁺ ions localized in yttrium clusters. Weak spectra of tetragonal Gd³⁺ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd³⁺ ion, have been also detected. These centers are attributed to isolated Gd³⁺ ions localized near octahedral rare-earth clusters or their associations.     

Phase diagram and domain structure of the Rb2 x Tl2(1− x )Cd2(SO4)3 solid solutions

From measurements of temperature dependences of the birefringence and from observations of the domain structure the x, T–phase diagram for the Rb_{2 x} Tl_{2(1? x )}Cd₂(SO₄)₃ solid solutions have been obtained. Ferroelastic domain structure and movement of phase boundaries have been studied in the course of phase transitions in Rb_{2 x} Tl_{2(1? x )}Cd₂(SO₄)₃ crystals. Possible reasons for the appearance of mechanically non-compatible (‘forbidden’) ferroelastic domain walls are given.