Oxygen non-stoichiometry, redox thermodynamics, and structure of LaFe1 − x Co x O3 − δ

The oxygen non-stoichiometry and redox thermodynamic properties of the LaFe_{1 ? x} Co _x O_{3 ? δ} system (x?=?0.25 and 0.75) are studied. At low temperatures, the LaCoO₃ and LaFeO₃ systems show partial solid solubility. At 1,273 K (in air), both compounds are single phases and are orthorhombic and rhombohedral for x?=?0.25 and 0.75, respectively. Thermogravimetry has been used to measure the oxygen non-stoichiometry versus oxygen partial pressure at three temperatures, 1,223, 1,273, and 1,323 K. Redox thermodynamic quantities are extracted directly from the oxygen non-stoichiometry curves. The extracted enthalpies of oxidation do not vary significantly with stoichiometry, and for x?=?0.25 and 0.75, they are ?640?±?60 and ?440?±?60 kJ (mol O₂)^?1, respectively. Ideal solid solution thermodynamic models are used to analyze the redox mechanisms.     

High-Pressure Synthesis of SeCu 1− x Zn x O 3 Perovskites

The SeCu 1 m x Zn x O 3 solid solution, with a distorted perovskite-type structure, has been synthesized under high pressures and temperatures. X-ray diffraction analysis indicates that the zinc ions occupy the copper sites, a solid solution being formed. It seems that high-pressure stabilises a small cation such as Se 4+ in the A site of the perovskite structure ABO 3 although the material is better described as formed by selenite anions \rm{SeO}_{3}^{-} and Cu 2+ /Zn 2+ cations.     

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     

Magnetic order in Sr x Ca1−x Cu0.99 57Fe0.01O2

Sr _x Ca_1–x Cu_0.99 ⁵⁷Fe_0.01O₂ was studied forx=0.13, 0.15, and 0.17. Mössbauer spectroscopy and magnetization measurements indicate magnetic ordering characteristic of spin glass systems withT _f70K forx=0.15 and 0.13.     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Twins of YBa2Cu3O7−x and origin of four orientation variants

Abstract

Twins are often visible in YBa₂Cu₃O_7?x samples by X-ray diffraction and electron imaging or diffraction. The origin of four different orientation variants are related here to the 4/mmm to mmm transformation.     

Features of the twin structure of YBa2Cu3O7−x epitaxial films

X-ray diffraction is used to investigate YBa₂Cu₃O_7?x (YBCO) films on NdGaO₃(110) and a ${{(100)CeO_2 } \mathord{\left/ {\vphantom {{(100)CeO_2 } {(1\bar 102)}}} \right. \kern-0em} {(1\bar 102)}}Al_2 O_3 $ heterostructure. Symmetric, asymmetric, and axial geometries for θ and θ/2θ scans are used to obtain diffraction spectra from different crystallographic planes. The orientational and quantitative twinning characteristics of the films are determined. While the crystallographic parameters of these two types of film (the films are c-axis oriented with c=11.67 Å) are similar, there are differences in the twin structure. In particular, the features of the NdGaO₃ structure lead to the appearance of an angle differing from 90° (90.20°) between the possible (110) and θ/2θ twin planes in a YBCO film and a different number of twin components in each system of twins. It is concluded from an analysis of the broadening of reflections, which are sensitive to twinning, that there is not twinning in a 60% film of YBCO on Al₂O₃ with a CeO₂ buffer layer.     

Seebeck-effect in the mixed state of Y−Ba−Cu−O

We report on measurements of the Seebeck-effect, the Nernst-effect, and the magnetoresistance in the mixed state of ac-axis oriented expitaxial film of Y–Ba–Cu–O. In contrast to conventional superconductors we find a large Seebeck-coefficientS, which is comparable in magnitude to the Nernst-effect. The broadening of the super-conducting transitions of magnetoresistance and Seebeck-effect are rather similar with respect to (1) the temperature dependence, (2) the dependence on the direction between magnetic field and crystal axis and (3) the dependence on the direction between magnetic field and driving forces. The large Seebeck-effect has to be attributed to dissipation due to normal quasiparticle-excitations, since the vortex-contribution to the Seebeck-effect is by far too small to account for the observed magnitude ofS. It is argued that such a quasiparticle contribution to the dissipation is large in the high-T _c superconductors because of the small coherence lengths and thus the small vortex cores. Another possibility is that granularity leads to dissipation proportional to the normal state transport properties. The Seebeck-voltage depends on all dissipative processes other than vortex motion, whereas the Nernst-effect depends only on the vortex motion. Therefore by measurements of thermomagnetic effects the various dissipative properties may be separated.     

The crystallization of Fe1−x P x , 0.09≦x≦0.26

The crystallization behaviour of about 60 samples of electrodeposited Fe_1–x P _x , 0.09<x<0.26, was investigated with DSC and X-ray diffraction. Several subsequenting transformations during heating at 10 K/min were identified. The corresponding transformation temperatures, reaction heats and reaction rates were measured. If the as-deposited samples contain metal-rich crystallites (microcrystalline and partly amorphous material) crystallization starts with primary precipitation of -iron followed by polymorphous crystallization of Fe₃P. In amorphous samples these two processes become less important compared with the eutectoid reaction the higher the phosphorus content. In a small concentration range around 19 at% P this transformation is the only one. Above this range the primary crystallization of Fe₃P becomes more and more dominant.     

Magnetic and electrical properties of weakly doped manganese-deficient La1 − x Ca x Mn1 − z O3 manganites

Samples of La_{1 ? x} Ca _x Mn_{1 ? z} O_{3 + δ} (x = 0.05?0.15) with deficient manganese and excess oxygen δ do not pass into a metallic state and have low spin ordering temperatures T _C at acceptor Mn⁴⁺ concentrations near the percolation threshold. These results are explained by carrier localization in clusters near cation vacancies. A break in the carrier transport chain Mn-O-Mn in the form of absent manganese favors cluster formation and decreases the double exchange energy and T _C of the samples. Closeness to the percolation threshold results in strong (more than four orders of magnitude) changes in the electrical resistivity in a magnetic field. The changes in the cluster sizes with the temperature and the magnetic field that are determined from the magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a dielectric paramagnetic and an antiferromagnetic matrices.     

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.     

Spin dynamics in Gd x Y1−x Ba2Cu3O6.85 investigated by ESR of Gd spin probes

Electron spin resonance investigations of Gd³⁺ ions in YBa₂Cu₃O_7?δ give distinct indications for an exchange coupling of these paramagnetic ions to the doped charge carriers and allow the corresponding interaction parameters, as well as the mutual Gd-Gd exchange coupling, to be extracted.     

Specific heat of Eu x Sr1−x Te

In this paper, we report on measurements of the specific heatC of single-crystalline Eu _x Sr_1–x Te at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the Néel temperatureT _N=9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent =–0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close toT _N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay ofC is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: becomes negative and decreases continuously towards –1 atxx _c. This concentration dependence of was previously observed in the diluted ferromagnetic system Eu _x Sr_1–x S. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentrationx lower than the critical concentrationx _c reveal spin-glass behavior in the specific heat. In addition, the dependence ofT _N on magnetic fields is discussed. The data yield a normalized magnetic phase boundaryB _c(T)/B_c(T=0) vs.T _N(B)/T_N(B=0) which is independent of concentration.     

Local electronic structure of Cu2O,CuO and YBa2Cu3O7−δ

Cu 3d and O 2p electronic states of Cu₂O, CuO, and the highT _c compound YBa₂Cu₃O_7– have been probed by means of high resolution x-ray emission spectroscopy (XES). The CuL and OK XES bands are compared in detail with recently reported x-ray photoelectron and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements and densities of states obtained by local density functional (LDF) theory. The XES data show that the hybridization between Cu 3d and O 2p states is completely modified in CuO and YBa₂Cu₃O_7–, whered-d correlation energy is large, as compared to LDF predictions. Such is not the case for Cu₂O where agreement between theory and experiment is good.The Cu 3d states are found to be highly localized in YBa₂Cu₃O_7– (though less so than in CuO). The O 2p states lie at lower binding energies than in the simpler oxides and are mainly situated above the Cu 3d states. The respective positions of the centre of gravity of the OK emission bands on an x-ray energy scale indicate that the oxygen sites are less well screened by the O 2p states in the highT _c compound. This provides indirect evidence for the presence ofd-like states at the oxygen sites.     

Optical Properties of Films of Cu x Ag1−x InSe2 Solid Solutions Obtained by Laser Evaporation

The structure and optical properties of the films of ternary compounds AgInSe₂, CuInSe₂ and Cu _x Ag_1–x InSe₂ solid solutions obtained by pulsed laser evaporation are investigated. It has been established that the films like bulk crystals have the structure of chalcopyrite. By the transmission and reflection spectra near the edge of natural absorption the refractive index and coefficient of optical absorption are calculated and the energies of interband transitions and the crystalline and spinorbit splitting are determined.     

Density of states of substoichiometric TiN1−x

The electronic states of substoichiometric TiN_1–x are studied by the CPA method and both the partial and total densities of states (DOS) are calculated. The results reveal that with increasing vacancy concentrationx the Fermi levelE _F moves to higher energies and the DOS atE _F,g(E _F), increases linearly. The calculated L_{II, III} X-ray spectra of Ti compare well with experiment.Work supported by the Research Institute for Powder Metallurgy, Sumperk, Czechoslovakia.The author is indebted to Dr. B. Velický, CSc for suggestion of the method to solve the Soven's equation. The financial assistance of the Research Institute for Powder Metallurgy, umperk, Czechoslovakia is gratefully acknowledged.     

LaAg x In1−x

The structures of LaAg _x In_1–x alloys withx=0.75, 0.89 are determined by neutron diffraction on powder samples. The space group isI4/mmm (D _4h ^/17 ). The lattice constants splitting, the order parameter and the mean square vibrational amplitudes of the atoms are given in the temperature range from 20 KT300 K.     

Effect of process parameters on the properties of plasma-sprayed superconducting Y1Ba2Cu3O7−x coatings

A study on the effect of powder, spray and coating parameters on superconducting properties of plasma-sprayed Y₁Ba₂Cu₃O_7–x coatings has been carried out. Optimised spray parameters to yield strong, adherent and dense coatings on many substrates including AISI 304 SS and alumina have been obtained. Coatings show superconductivity after oxygen annealing at 950°C for an hour. Coatings thinner than 100 m do not show zero resistivity down to 77 K due to weak links between granular superconductors. Coatings prepared with fine powder have superior superconducting properties, viz. higherT _c andJ _c values than coarse powder coatings. The choice of substrate material strongly affects theJ _c values.