Oxygen thermodynamics and ion conductivity in the solid solution La1−xSrxCoO3−δ at large strontium content

The equilibrium oxygen content was measured in the model system and important oxygen permeable material La_1−xSr_xCoO_3−δ, where x=0.6, in the temperature range 650–900 °C and oxygen partial pressure range between 10⁻⁵ and 1 atm. The data were utilized to obtain changes in the partial entropy and enthalpy of oxygen in the solid as a function of the oxygen content. It is shown that the initially cubic perovskite undergoes to a phase transition to a tetragonal structure at δ >0.3. The oxygen permeation of L_0.4Sr_0.6CoO_3−δ at 700–900 °C is found to be controlled by bulk solid state processes. The activation energy equals about 0.8 eV at high oxygen pressure and small oxygen nonstoichiometry. Increasing oxygen deficiency results in a rapid increase in the activation energy. In combination with thermodynamic data, these changes can be explained as resulting from the intrinsic spatial inhomogeneouty in oxygen vacancy distribution which varies both with temperature and oxygen nonstoichiometry. It is shown that, when the oxygen deficiency increases at constant temperature, the oxygen vacancies form locally ordered microdomains (clusters), which eventually results in a transition of the cubic perovskite structure to the tetragonal structure. The oxygen ion conductivity depends strongly on the development of the ordering. Paper presented at the 6th Euroconference on Solid state Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Nonstoichiometry of the perovskite-type solid solution La0.9Ca0.1Cr1−yAlyO3−δ

Nonstoichiometry of the perovskite-type solid solutions La_0.9Ca_0.1Cr_1−yAl_yO_3−δ was studied by high-temperature gravimetry under controlled P(O₂) atmospheres of 1–10^− 23 bar at 1073–1273 K. The observed data were described by a regular solution-like model for the randomly distributed defects of V_O¨, Cr_Cr., Ca_La,, and Al_Cr^X. With the increase in y, V_O¨ formation becomes much easier. For y > 0.8, some fraction of Ca_La, becomes surrounded only by Al_Cr^X and V_O¨ remains around such Ca_La, up to high P(O₂) to reduce the maximum oxygen content below 3.000.     

Europium-doped nanocrystalline Y2O3−La2O3 solid solutions with bixbyite structure

Nanocrystals of Y₂O₃ and La₂O₃ solid solutions were synthesized with a cubic bixbyite structure containing La₂O₃ content of up to 50 mol%. This is comparatively higher than that in bulk materials of the same structure, where La₂O₃ content of only 20 mol% can be obtained. A set of europium-doped (Y_1−xLa_x)₂O₃ (x=0.1, 0.2, 0.3, 0.4) cubic bixbyite solid solutions with crystallites of approximately 10 nm in size was prepared using the polymer complex solution method. Structural analysis was performed using X-ray diffraction measurements, Rietveld full profile refinement, and from Eu³⁺ luminescence emission. The energy levels of the Eu³⁺ ion, second order crystal field parameters, and crystal field strength were obtained for all compositions of solid solutions. We show that the crystal field parameters linearly depend on unit cell parameter and that these dependencies may be considered as part of an overall dependence for the entire sesquioxide family.     

An EXAFS study of the local structure of the PbxSn1−x S solid solution

The local lead-atom environment in the Pb_xSn_1?x S solid solutions with cubic and orthorhombic structure was studied by EXAFS spectroscopy. The shortest Pb-S distance in samples with orthorhombic structure was found to be smaller by ≈0.2 Å than that in cubic-lattice samples, which is a sign of stereochemical activity of the two paired 6s ² electrons of a Pb atom. The metal atom arrangement reveals strong short-range order, which results in the formation of-Pb-Sn-Pb-Sn-... zigzag chains aligned with the c axis (in the Pbnm system) in orthorhombic samples. It was shown that the onset of such short-range order in Pb_0.5Sn_0.5S can initiate the formation of superstructures belonging to the C _2v ² or C _2v ⁷ space groups.     

The NASICON solid solution Li1−x La x /3Zr2(PO4)3: optimization of the sintering process and ionic conductivity measurements

The Li_1−x La _{x /3}Zr₂(PO₄)₃ NASICON-type compounds (0 ≤ x ≤ 1) have been synthesized in powder form by a sol-gel method and sintered for ionic conductivity measurements. In order to improve the compactness of the ceramic without decomposition of the compound, several sintering processes have been tested for one member of the solid solution (x = 0.6): the use of sintering aids (ZnO, B₂O₃, TiO₂ and LiNO₃), a ball-milling of the synthesized powder, a flash heating, high isostatic pressure, and spark plasma sintering. Finally, a satisfactory compactness of 85% is obtained compared to the referenced value (63%) obtained by uniaxial and isostatic pressing. The ionic conductivity study was performed by impedance spectroscopy. It shows that, despite the formation of vacancies, the substitution Li⁺→ 1/3 La³⁺ + 2/3 □ has unfortunately no influence on the conduction for 0 ≤ x ≤ 0.7 since the ionic conductivity remains identical to the LiZr₂(PO₄)₃ one. For higher x values, the ionic conductivity strongly decreases.     

Dielectric studies of phase transitions in the ferroelectric CdTiO3 and the Sr1−x CdxTiO3 solid solution

Temperature dependences of the permittivity and of the dielectric hysteresis loops in ceramic samples of nominally pure CdTiO₃ and a Sr_1?x Cd_xTiO₃ solid solution were studied. At 76.5±0.5 K, CdTiO₃ was established to undergo a ferroelectric phase transition close to the tricritical point. The temperature dependence of spontaneous polarization of CdTiO₃ is described within the Landau theory of phase transitions with the critical order parameter exponent ≈0.25. The phase diagram of the Sr_1?x Cd_xTiO₃ solid solution was drawn in (T, x) coordinates, and the critical concentration x _c =0.002, above which an induced polar state sets in the solid solution, was determined.     

Ferromagnetic phase transitions in the magnetically heterogeneous solid solution system EuxLa1−xS

The ferromagnetic phase transitions of the solid solution system Eu_xLa_1-xS with Eu-concentrations x=0.85, 0.65 and 0.50 are analyzed by measurements of the initial permeability, the specific heat and the magnetization. For the sample with x=0.85 the spontaneous magnetization develops continuously between two temperatures T_c1 and T_c2. For the sample with x=0.65 a well defined magnetic ordering temperature exists. One observes strongly curved magnetization isotherms when plotting the magnetization data in form of modified Arrott plots. The M(H, T) data above the curved region show usual ferromagnetic scaling with the critical exponents β=0.5 and δ=4.7. These exponents fulfill the scaling relations with the exponents γ=2.1 and α≈-1 derived for the initial susceptibility and the magnetic specific heat. The sample with the concentration x=0.50 turns out to be not truely ferromagnetic. It is a spin glass with strong ferromagnetic short range order.     

Thermal conductivity of single crystals of the Ca1 − x Y x F2 + x solid solution

The thermal conductivity of single crystals of the solid solution of yttrium fluoride in calcium fluoride Ca_{1 ? x} Y _x F_{2 + x} with the fluorite structure (x ≤ 0.20) and the Ca_0.27Y_0.73F_2.73 phase with the tisonite structure has been studied by the absolute steady-state longitudinal heat flow method in the temperature range 50–300 K. It has been established that the thermal conductivity drops sharply with increasing yttrium trifluoride concentration, especially in the low-temperature region.     

Superionic conductivity of the heterovalent solid solutions R 1−x M x F 3−x (R=REE, M=Ca, Ba) with tysonite-type structure

The fluorine-ion conductivity of anion-deficient solid solutions R _1−x Ca_xF_3−x and R _1−x Ba_xF_3−x having the tysonite (LaF₃) structure was investigated by the impedance spectroscopy method. R _1−x Ca_xF_3−x (R=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho) and R _1−x Ba_xF_3−x (R=La, Pr, Nd) single crystals were grown from the melt by the Bridgman-Stockbarger method. The electrophysical measurements were performed in the frequency range 5−5×10⁵ and temperature range 300–700 K. The temperature dependences of the electrical conductivity for the crystals studied is determined by the migration of fluorine anions along various structural positions. It is shown that, from the standpoint of increasing the conductivity of tysonite matrices RF₃ (R=La, Pr, Nd), doping by CaF₂ and BaF₂ is less promising than SrF₂. Fiz. Tverd. Tela (St. Petersburg) 41, 638–640 (April 1999)     

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.     

Electronic structure and optical response in GaxAl1−xN solid alloys

A new method for the calculation of the electronic band structure for the Ga_xAl_1−xN solid alloys is proposed. We present the solid alloy structure as a superposition of two sublattices with the appropriate weighting factors. Both the ab initio norm-conserving pseudo-potential (PP) and LCAO methods give essential disagreement with the experimental data. Only the appropriate correction of the norm-conserving PP method by the orthogonalized LCAO wave functions essentially stabilizes the convergence procedure. To check the reliability of the proposed method we have carried out experimental investigations of the optical functions for two representatives of the investigated wurtzite solid alloys: Ga_xAl_1−xN (x = 0.85 and 0.65). Calculations were performed as a function of the energy cutoff as well as of the Slater screening parameter α. The optical functions at zero frequency give overestimated values compared with the experimental data. The best agreement with experiment can be achieved by using the orthogonalization procedure between the norm-conserving PP and LCAO wave functions. The latter ones help in understanding the solid alloy structure modulations in the compounds mentioned. The possibility is shown of achieving good agreement between the experimental and theoretical data.     

Ionic conductivity properties and 19F NMR investigation of Pb1−xCrxF2+x solid solutions with fluorite-type structure

The F⁻ ion transport and diffusion properties inside the solid solution Pb_1−xCr_xF_2+x (0≤x≤0.12) with fluorite-type structure have been determined by impedance spectroscopy and ¹⁹F NMR investigation. The various electrical parameters of Pb_1−xCr_xF_2+x are compared with those of Pb_1−xAl_xF_2+x (0≤x≤0.12) whose structure derives also from the fluorite-type. The Cr³⁺ substitutional cation is of a slightly larger size than the Al³⁺ cation but both cations offer the same usual coordination (CN=6) in ternary fluorides. Comparison of ¹⁹F NMR spectra of some compositions Pb_1−xM_x′F_2+x (M′=Al, Cr) at different temperatures has been realized in order to discover whether the clustering process proposed in Pb_1−xAl_xF_2+x can be extended to Pb_1−xCr_xF_2+x.     

Electronic transport in the novel SOFC anode material La1−xSrxCr0.5Mn0.5O3±δ

Electronic conductivity in the potential SOFC anode material La_1−xSr_xCr_0.5Mn_0.5O_3±δ has been investigated in the range 0.2 < x < 0.3. log(σT) vs. 1/T plots indicate conduction via thermally activated polaron hopping. At 900 °C, conductivity in air increases with Sr²⁺ via an increase in [B_B] holes (B—transition metal). X-ray absorption spectroscopy (XAS) studies indicate that compensation for A-site Sr substitution and oxygen vacancy formation is via the Mn cation only; Cr maintains a 3+ oxidation state and 6-fold oxygen coordination. Electronic transport occurs by percolation between Mn cations in a disordered B-site sub-lattice. Conductivity decreases with p(O₂), which is indicative of p-type conduction behaviour, but the relationship cannot be explained by a simple redox equilibrium involving Mn³⁺, Mn⁴⁺ and oxygen, possibly due to co-existence of Mn²⁺, Mn³⁺ and Mn⁴⁺ via disproportionation as with La_1−xSr_xMnO_3±δ.     

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.     

Substrate effect on the electronic structure in GaxIn1−xAsySb1−y/GaSb and GaxIn1−xAsySb1−y/InAs

We present a theoretical study of the substrate effect on electronic structure in cubic Ga_xIn_1−xAs_ySb_1−y lattice-matched to GaSb and InAs. Our calculations are based on the empirical pseudopotential formalism within the virtual crystal approximation where the effect of disorder is taken into account. We show that the electronic band structure of the quaternary alloy Ga_xIn_1−xAs_ySb_1−y is altered by the change of substrate for the entire range of alloy compositions x. Moreover, we find that at Ga concentrations (x≤0.8), the ionicity is less important when Ga_xIn_1−xAs_ySb_1−y is lattice-matched to GaSb instead of InAs. The information will be useful for the choice of substrate in the composition range 0–1 and hence for the determination of the lattice-matching conditions for Ga_xIn_1−xAs_ySb_1−y quaternary materials.     

Unusual behaviour of conductivity and superconductivity in solid Sn−Xe mixtures

We have measured the normal statedc conductivity , the superconducting transition temperatureT _c and the critical superconducting currentI _c0 of frozen Sn–Xe mixtures in dependence of the Sn atomic fractionc. A new, high-T _c phase is observed forc between 0.58±0.02 and 0.74±0.02. This phase has aT _c-value of about 5.7 K, it shows a strong decrease in and inI _c0 and transforms into disordered Sn during annealing between 15 K and 50 K.It is a pleasure for us to dedicate this paper to Prof. Dr. S. Methfessel on the occasion of his 60^th birthday.This work was supported by the Deutsche Forschungs-gemeinschaft     

Temperature evolution of the crystal structure of Bi1 − x Pr x FeO3 solid solutions

The crystal structure of solid solutions in the Bi_{1 ? x} Pr _x FeO₃ system near the structural transition between the rhombohedral and orthorhombic phases (0.125 ≤ x ≤ 0.15) has been studied. The structural phase transitions induced by changes in the concentration of praseodymium ions and in the temperature have been investigated using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. It has been established that the sequence of phase transformations in the crystal structure of Bi_{1 ? x} Pr _x FeO₃ solid solutions with variations in the temperature differs significantly from the evolution of the crystal structure of the BiFeO₃ compounds with the substitution of other rare-earth elements for bismuth ions. The regions of the existence of the single-phase structural state and regions of the coexistence of the structural phases have been determined in the investigation of the crystal structure of the Bi_{1 ? x} Pr _x FeO₃ solid solutions. A three-phase structural state has been revealed for the solid solution with x = 0.125 at temperatures near 400°C. The specific features of the structural phase transitions of the compounds in the vicinity of the morphotropic phase boundary have been determined by analyzing the obtained results. It has been found that the solid solutions based on bismuth ferrite demonstrate a significant improvement in their physical properties.     

Lattice distortion in the solid solutions of UAsxSe1−x

The solid solutions of UAs_xSe_1−x for x = 0.10−0.40 were studied by X-ray diffraction at the temperature region 130–200 K. A rhomboedrical distortion of the crystal structure below T_c was observed. A dependence of an intersitial angle and coefficient of spontaneous magnetostriction on the temperature or the composition of samples was determined.