Defects and transport in SrFe1−xCoxO3−δ

The non-stoichiometry and chemical diffusion coefficient of SrFe_1−xCo_xO_3-δ have been measured by steady state and transient thermogravimetry in the temperature range 750–1200 °C at different oxygen partial pressures. At high oxygen partial pressures, the chemical diffusion coefficient was in the range 1·10⁻⁴ – 7·10⁻⁴ cm²/s. This, combined with high concentration of disordered vacancies make these materials perhaps the fastest solid oxygen ion diffusers known at high temperatures and high oxygen partial pressures. However, due to the high concentration of defects in SrFe_1−xCo_xO_3-δ the compound transforms from a cubic (disordered) perovskite to a brownmillerite type of structure under reduced oxygen partial pressures below approx. 900 °C. Due to this phase transition, the mobility of oxygen vacancies in SrFe_1−xCo_xO_3-δ decreases up to about an order of magnitude at 850 °C. We also observe an ordering effect at 1000 °C, although smaller in size, and this is suggested to be due to short range ordering of four-coordinated polyhedra of Fe. For possible use as oxygen separation membranes, phase stability against sulphur and carbon containing atmospheres is also discussed with respect to the formation of carbonates and sulphates. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Synthesis and ionic conduction of C3A-type nasicon Na3+3x−yY1−xSi3−yPyO9

In the solid solutions of Na_3+3x−yY_1−xSi_3−yP_yO₉, new fast Na⁺ -conductors were obtained. The ionic conductivity, σ, and the activation energy, E, of the most conductive material, Na_4.425Y_0.375Si_2.55P_0.45O₉ were 5 × 10⁻³ S/cm at 300°C and 10 kcal/mol, respectively. Those values were strongly dependent upon the composition. With an increase of Na concentration, [Na], σ increased and E decreased. Accommodation of higher [Na] in the structure was madefeasible by decreasing [Y] and increasing [P]. Based on the crystal structure assumed for Na_3+3x−yY_1−xSi_3−yP_yO₉ and on the above results, a conduction model was presented. The structural consideration revealed that a conduction path is formed along the 〈1 1 1 〉 direction, where Y³⁺ ions locate in the way of Na⁺ ions.     

Low temperature electrical transport in La1−xNdxNiO3−δ

We have investigated low temperature electrical transport in La_1−xNd_xNiO_3−δ perovskite oxide samples. Samples were prepared by a sol–gel method and were characterized by X-ray diffraction and chemical methods. High precision electrical resistivity, magnetoresistance (MR) and electron tunneling conductance measurements were performed. Crystal structure investigations showed a phase transition from rhombohedral to orthorhombic phase at x=0.4. In the orthorhombic phase Ni–O–Ni bond angle was found to depend very sensitively on the value of x; as the value of x increases Ni–O–Ni bond angle decreases resulting in the tilting of NiO₆ octahedra. A Correlation between the Ni–O–Ni bond angle and electrical transport has been observed. The analysis of the electrical resistivity data showed the presence of disorder driven quantum correction effects, namely e–e interactions and weak localization, in the system. A dip in the tunneling density of states and negative MR also suggest the presence of e–e interaction and weak localization effects in the system.     

Optical properties of Li1−xNb1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of LiNbO₃ is reported. Li_1?xNb_1?xEu_2xO₃ solid solutions exist over a very limited range of europium concentrations (x ≤ 0.01). The solid solutions were characterized by X-ray powder diffraction and density measurements. Within the range of compositions of the solid solutions, the optical properties (emission and excitation spectra) of the Eu³⁺ ions have been characterized. The obtained optical data indicate that two equally abundant europium luminescent species exist in the stoichiometric speciments; these correspond to the Eu³⁺ ions occupying the Li⁺ and Nb⁵⁺ sites, in agreement with the replacement mechanism which is inferred from density measurements. Other Eu³⁺ luminescent species which appear to be closely related with lithium deficiency were found to exist in nonstoichiometric samples, in addition to those which are present in the stoichiometric materials.     

3 S 1−3 D 1 mixing and E1 radiative transitions in charmonium

The radiative transitions ψ′→γ+χ _J and χ _J →γ+ψ are studied in the quark model with the ψ and ψ′ interpreted conventionally as³ S ₁ \(c\bar c\) states and the χ _J as³ P _J \(c\bar c\) states. In particular, the effect of³ S ₁?³ D ₁ mixing on the decay widths is calculated. The photon angular distributions in these decays are also studied and are found to be a more sensitive test of³ D ₁ admixtures in the Ψ states. The³ D ₁ content of the Ψ is found to be large but the estimate is very sensitive to the photon angular distributions. More precise measurements could help to explain the large ψ″ leptonic width.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

q-analog ofA m−1 ⊕A n−1 ⊂A mn−1

A natural embeddingA _m–1 A _n–1 A _mn–1 for the corresponding quantum algebras is constructed through the appropriate comultiplication on the generators of each of theA _m–1 andA _n–1 algebras. The above embedding is proved in theq-boson realization by means of the isomorphism between theA _q ^– (mn ~ ⁿ A _q ^– (m) ~ ^m A _q ^– (n)algebras.     

Crystallographic and optical properties of Li1−xTa1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of Li_1?xTa_1?xEu_2xO₃ solid solutions are reported. The solid solutions exist over a very limited range of europium concentrations (x ≤ 0.02). They were characterized by X-ray powder diffraction and density measurements. The liquid nitrogen fluorescence and excitation spectra of Eu³⁺ in the solid solutions were measured in the frequency range 12,200–45,450 cm^?1. The emission mainly arises from the ⁵ D ₀ level to Stark components of the five lowest ⁷ F _j (j = 0, 1, 2, 3, 4) states. It was inferred from the analyses of the optical and density data that the Eu³⁺ ions may lie on both Li and Ta sites, but not on the intrinsic vacant lattice sites.     

Structural and superconducting properties of Eu1−xPrxBaSrCu3O7−δ

We have investigated the structural and superconducting properties of Eu_1−xPr_xBaSrCu₃O_7−δ by X-ray diffraction analysis, AC and DC magnetization and DC resistivity measurements. The parent compound crystallizes in the orthorhombic symmetry which transforms to tetragonal symmetry at x=0.2. Superconducting transition temperature T_c decreases monotonically with increasing x which approaches zero at x=0.6. The enhancement in critical concentration x_cr observed in Eu_1−xPr_xBa₂Cu₃O_7−δ on replacing 50 at.% Ba by Sr is attributed to the resulting local structural changes.     

Phase transitions in La1−x CaxMnO3−x/2 manganites

The crystal structure parameters and magnetic and electrical properties of La_1?x Ca_xMnO_3?x/2 reduced manganites with 0≤x≤0.5 are established. These investigations contribute to the understanding of magnetic interactions in manganites without Mn⁴⁺ ions. It is found that these manganites show a long-range antiferromagnetic order up to x=0.09 and transform into spin glasses at 0.09<x≤0.35. The compositions in the range 0.35<x≤0.5 show a strong increase in the spontaneous magnetization and critical point associated with the appearance of spontaneous magnetization and can therefore be viewed as inhomogenious ferromagnets. The magnetic and crystal structure peculiarities of La_0.5Ca_0.5MnO_2.75 are established by the neutron diffraction method. The strongly reduced samples show a large magnetoresistance below the point where the spontaneous magnetization develops. The magnetic phase diagram of La_1?x Ca_xMnO_3?x/2 is established by magnetization measurements. The magnetic behavior is interpreted assuming that the Mn³⁺-O-Mn³⁺ magnetic interaction is anisotropic (positive-negative) in the orbitally ordered phase and isotropic (positive) in the orbitally disordered phase. Introduction of the oxygen vacancies changes the magnetic interaction sign from positive to negative, thereby leading to a spin glass state in strongly reduced compounds. The results obtained reveal unusual features of strongly reduced manganites such as a large ferromagnetic component, a high magnetic ordering temperature, and a large magnetoresistance despite the absence of Mn³⁺-Mn⁴⁺ pairs. In order to explain these results, the oxygen vacancies are supposed to be ordered.     

Spin polarons at high temperatures in nonstoichiometric La1 − x Mn1 − y O3 manganites

The magnetic properties of the La_{1 ? x} Mn_{1 ? y} O₃ family of nonstoichiometric lanthanum manganites are studied in the 80 K < T < 640 K temperature range. The Curie temperature varies nonmonotonically with the growth in the density of Mn⁴⁺ ions. In the existence range for the paramagnetic phase, magnetic polarons appear in both the orthorhombic and rhombohedral phases. In the range of paramagnetism, the temperature dependence of the magnetic susceptibility is nonlinear and can be fitted by the Curie law with the temperature-dependent Curie constant.     

Magnetic behavior of the alloys CeCuyGa4−y and Ce1−xLaxCuGa3

The results of magnetic susceptibility (x), electrical resistivity (ρ) and heat-capacity (C) measurements of the alloys CeCu_yGa_4−y (y = 0.5, 1.0, 1.25 and 1.5) and Ce_1−xLa_xCuGa₃ (x = 0.2 and 0.9), crystallizing in the BaAl₄-type tetragonal structure, are reported. The Kondo effect tends to dominate with increasing y. All the data for the y = 0.5 alloy are consistent with the onset of ferromagnetic ordering at 6 K. For y = 1.0 and 1.25, though ρ gradually drops below 6 K, the C data do not show the existence of magnetic ordering above 2 K; presumably, for y = 1.0, 1.25 and 1.5, long-range magnetic ordering sets in below 2 K. Above all, C tends to increase with decreasing temperature before the onset of long range magnetic ordering and the values of C/T per Ce mol are considerably diminished for x = 0.9. It is proposed that this C/T enhancement lies in the magnetic precursor effects and not on the heavy-fermion behaviour.     

Laser-induced chemical etching of ceramic PbTi1−xZrxO3

The laser-induced etching of ceramic PbTi_1–xZr_xO₃ in a hydrogen atmosphere and in air has been investigated. Visible Ar⁺ and Kr⁺ laser radiation was employed in most of the experiments. In H₂ atmosphere, regular patterning of the ceramic is possible. Average etch rates reach up to about 250 m/s.     

Ultrasonic and X-ray studies on BaPb1−xBixO3

By means of ultrasonic measurements for BaPb_1−xBi_xO₃ (x = 0.15 ∼ 0.75), it is shown that softened transverse optical phonon modes (ω_TO(O)′s ∼ 150 K in the temperature scale) appear in only the samples in the metallic region (x ≲ 0.30). X-ray measurements show that all the present samples do not exhibit any macroscopic crystal transformation between 25 K and 300 K, so that the appearance of the soft phonon modes must be closely related to the presence of the conduction electrons.     

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.