Metal-organic deposition of YBa2Cu3Ox and Bi2Sr2Ca1Cu2Ox films on various substrates starting from different fluorine-free metallorganic compounds

YBa₂Cu₃O_x (Y-123) and Bi₂Sr₂Ca₁Cu₂O_x (Bi-2212) films on various substrates have been prepared by Metal-Organic Deposition starting from different metallorganic fluorine-free compounds and using a very simple instrumentation. The processing conditions include a rapid pyrolysis step in air and an annealing step in oxygen for Y-123 and in air for Bi-2212. The films obtained have been characterized by X-ray diffraction (XRD) and the formation of a superconducting phase of Y-123 or Bi-2212 was confirmed measuring the critical temperature (T _c) with Ac-susceptibility and resistive measurements. Microstructure and final cationic ratios have been studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).     

Production of bscco bulk high <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors by sol-gel method and their characterization by ftir and XRD techniques

The production of bulk high T _c superconducting phase (2223) by EDTA-gel (ethylenediaminetetraacetic acid) techniques has been investigated. It is shown that close control of pH is necessary for the production of a well-complexed precursor which allows subsequent decomposition in two stages at 300 and 800°C. The problem of carbonate formation was investigated experimentally and solved. Precursors are characterised by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) and the sintering behaviour was monitored by dilatometry. At least three different phases Bi₂Sr₂Ca_x−1Cu_xO_8+y (BSCCO); where x=1, 2, 3 were identified within superconducting pellets using XRD, named as Bi₂Sr₂CuO₇ (2201), Bi₂Sr₂CaCu₂O₉ (2212) and Bi₂Sr₂Ca₃O₁₀ (2223). The superconducting properties of the sintered samples were studied by vibrating sample magnetometer (VSM). Transition to a superconducting state around 80 K appeared in samples (sintered at 845°C) containing the Bi₂Sr₂Ca₁Cu₂O_y (2212) phase. Liquid phase sintering of the samples aided the formation of Bi₂Sr₂Ca₂Cu₃O_x (2223) phase at high temperature (860°C), which showed a superconducting transition temperature of 108 K.     

Umwandlungen glasförmiger BSCCO-Vorstoffe in den kristallinen Zustand

The glassy precursors were fabricated by quenching the melted materials with copper plates. In the case of heating the properties of the quenched samples were changed, because as a result of solid-state reactions formation of cuprate occurs. The amorphous samples were annealed in air and transformed to the crystalline state. This process can be seen by DTA and electric resistivity behaviours. The microscopic observation of the polished surfaces show the growth of the superconducting phase Bi₂Sr₂CaCu₂O_8+x (for composition Bi-2212) and Bi₂Sr₂CaCu₂O_8+x with Bi₂Sr₂Ca₂Cu₃O_10+x (for composition Bi-2223 and Bi-2234), respectively.     

Preferential crystallographic site substitution and oxygen migration in Bi2{Sr, Ca, Ln}3Cu2O8 related to rare earth ion size (Ln = La, Yb)

Substitution of rare earths of various size (i.e., La and Yb) in the superconducting 2212 phase Bi₂Sr₂Ca_1−yLn_yCu₂O_8+x+y/2 (0 ≤ y ≤ 1) shows: (i) the larger La atoms substitute preferentially on the Sr site while the small Yb atoms occupy the Ca site; (ii) the superconductive property disappears when y reaches 0.5; (iii) as the Yb-doped phase approaches y = 0.5 the oxygen layers sandwiched between the Bi layers rearrange; an effect which may be associated with the observed variation of the modulation vector of the superstructure and, subsequently, with the phenomenon of superconductivity. Various phases corresponding to y = 0.1, 0.2, 0.3, 0.4, and 0.5 have been identified as separate through powder and single crystal diffraction, X-ray techniques, and electron microscopy investigations and their superconductivity properties checked by four probe resistivity measurements.     

Sol-Gel Synthesis Route for the Preparation of Y(Ba1−x Sr x )2Cu4O8 Superconducting Oxides

Superconducting oxide ceramics of composition Y(Ba_1–x Sr _x )₂Cu₄O₈ (x = 0.00, 0.10, and 0.20) have been prepared by a simple sol-gel method based on the complexation of metal ions in aqueous medium, by chelating acetate and tartarate ligands. Homogeneous sols were obtained by complexing copper ions with tartaric acid, which prevented the flocculation of copper acetate during the gelation process. Single-phase bulk samples were obtained after firing the mixed-metal polymeric acetate-tartarate precursor to 835°C for 70 h in flowing oxygen atmosphere. Thermal decomposition of the gels was studied by thermogravimetry. Effect of strontium substitution on the properties of the compounds was studied by X-ray powder diffraction, electron microscopy, X-ray photoemission spectroscopy and resistivity measurements. These data indicate that nearly monophasic Y(Ba_1–x Sr _x )₂Cu₄O₈ superconducting samples were obtained for x < 0.30. The Sr-doping in the YBa₂Cu₄O₈ (Y-124) phase shows a pronounced effect on the superconducting properties enhancing the critical temperature from 78 K (for the non-substituted sample) to 88 K (for Y(Ba_1–x Sr _x )₂Cu₄O₈).     

Cation distribution and interatomic interactions in oxides with heterovalent isomorphism: XII. Gd2Sr1? x Ca x Al2O7 solid solutions

The area of existence of Gd₂Sr_1−x Ca _x Al₂O₇ solid solutions at x ≤ 0.5 was determined by the X-ray phase analysis. It was found by full-profile X-ray structural analysis that, in contrast to La₂Sr_1−x Ca _x Al₂O₇ solid solutions, the Ca²⁺ cations occupy not only AO₉ nine-vertex fragments, but also AO₁₂ oxygen cubooctahedra. Full ordering of Sr²⁺ cations in the perovskite layer is observed at the calcium content x 0.5. Original Russian Text ? I.A. Zvereva, A.G. Cherepova, Yu.E. Smirnov, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 4, pp. 557–563. For communication XI, see [1].     

Synthesis and structure of bilayered manganites based on Ca3Mn2O7 with the simultaneous substitution of calcium by lanthanum and strontium

A tetragonal (space group I4/mmm) solid solution La_2-2x (Ca_1-y Sr_y)_{1 + 2x} Mn₂O₇ based on the Raddlesden-Popper phase (n = 2), which is formed by the simultaneous substitution of calcium in Ca₃Mn₂O₇ by strontium and lanthanum, is synthesized by high-temperature annealing of La₂O₃, Mn₂O₃, CaCO₃, and SrCO₃ mixtures (1500°C, air). The concentration area of the solid solution in the scheme is a pentagon, whose corners correspond to the manganites Ca₃Mn₂O₇, Ca_0.75Sr_2.25Mn₂O₇, La_0.2Sr_2.8Mn₂O₇, La_1.6Sr_1.4Mn₂O₇, and LaCa₂Mn₂O₇.     

Thermal analysis and thermal expansion studies on high density YBa2−xLaxCu3O7−δ, 0x0.2

The compositions in the YBa_2−xLa_xCu₃O_7−δ (0x0.2) system were prepared by the solid state reaction, employing a novel high-temperature oxygen sintering route. The modified sintering route yields dense slab like microstructures with large grains. The decomposition (incongruent melting) temperature of the YBa₂Cu₃O_7−δ (Y-123) phase was found to shift to higher temperatures with increasing oxygen partial pressure and lanthanum content. Structure remained orthorhombic up to x=0.2 with a decrease in the orthorhombic strain ((b−a)/b). Iodometric titration indicated a systematic increase in the oxygen content with increasing lanthanum content. Thermo-gravimetric studies in various oxygen partial pressures revealed that the oxygen diffusion in to the YBa₂Cu₃O_7−δ (δ>0.5) lattice is an exothermic event and takes place at temperatures not less than 573 K. High-temperature thermal-expansion measurements in air indicated that the nonlinearity in thermal expansion behaviour was reduced by the substitution of lanthanum.     

A study of the perovskite solid solution series LaxSr1−xRuO3 and LaxCa1−xRuO3 by ruthenium-99 Mössbauer spectroscopy

The magnetic, electronic, and structural properties of the solid solutions La_xSr_1−xRuO₃ and La_xCa_1−xRuO₃ have been studied by ⁹⁹Ru Mössbauer spectroscopy and other techniques. The La_xCa_1−xRuO₃ phases are reported for the first time and have been shown by powder X-ray diffraction measurements to be orthorhombically distorted perovskites. Electrical resistivity measurements on compacted powders show that all the phases are metallic with p 10⁻³, ohm-cm. Progressive substitution of Sr²⁺ by La³⁺ in ferromagnetic SrRuO₃ leads to a rapid collapse of the magnetic hyper-fine splitting at 4.2°K. For x = 0.25 some ruthenium ions still experience a magnetic field but for 0.4 x 0.75 only single, narrow resonance lines are observed, consistent both with the complete removal of the ferromagnetism and with the presence of an averaged ruthenium oxidation state in each phase, i.e., La_x³⁺Sr_1−x²⁺Ru^(4−x)+O₃ rather than La_x³⁺Sr_1−x²⁺Ru_x³⁺Ru_1−x⁴⁺O₃. LaRuO₃ and CaRuO₃ both give essentially single-line spectra at 4.2°K, indicating that the ruthenium ions in these oxides are not involved in long-range antiferromagnetic order but are paramagnetic. The solid solutions La_xCa_1−xRuO₃ (0 < x 0.6) give sharp symmetrical singlets with chemical isomer shifts (relative to the Ru metal) which move progressively from the value characteristic of Ru⁴⁺ (−0.303 mm sec⁻¹) toward the value for Ru³⁺ (−0.557 mm sec⁻¹), consistent with the presence of intermediate ruthenium oxidation states in these phases also.     

Composition dependence of the structural chemistry and magnetism of Ca2.5Sr0.5(Ga,Co)1+xMn2−xO8

Polycrystalline samples of bilayered brownmillerite-like Ca_2.5Sr_0.5GaCo_0.15Mn_1.85O₈ and Ca_2.5Sr_0.5Ga_1.2Mn_1.8O₈ have been prepared and characterised by magnetometry and neutron diffraction over a wide temperature range. The structural chemistry and magnetic properties are compared to those of Ca_2.5Sr_0.5GaMn₂O₈. Ga enrichment has a significant effect on the former but not on the latter, whereas changes in both occur when paramagnetic Co³⁺ cations enter the parent phase on the 4-coordinate sites. The coupling between the environment around the 4-coordinate cations and the transition to an antiferromagnetic ordered state that was observed in Ca_2.5Sr_0.5GaMn₂O₈ is not apparent in the cation-substituted compositions, although both show long-range antiferromagnetic order at low temperatures.     

The synthesis and transport properties of the cuprates La<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>1 + <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>2</Subscript>O<Subscript>6 + δ</Subscript>

The cuprates La_{2 ? x} Sr_{1 + x} Cu₂O_{6 + δ}(x = 0, 0.1, 0.2) are synthesized by solid-state method. Their dc conductivity was studied over the 373 to 1173 K temperature range and 10 to 2.1 × 10⁴ Pa oxygen partial pressure range by using four-probe technique. It is shown that the cuprate conductivity in air is maximal at ～673 K; it is 60 S/cm for La₂SrCu₂O_6.09; 68 S/cm, for La_1.9Sr_1.1Cu₂O_6.18; and 81 S/cm, for La_1.8Sr_1.2Cu₂O_6.10. The thermal expansion coefficient of La₂SrCu₂O_6.09 is determined by thermomechanical method and high-temperature X-ray diffraction method; its value (16 ppm K^?1) shows that the material is compatible with the ceria-based solid electrolytes during the thermal cycling.     

Matrix-Inducing Synthesis of SrxY10?x(SiO4)y(PO4)6?yO2: Eu3+ Micron Crystalline Coral Like Phosphors by Sol-Gel Composition of Hybrid Precursors

In the context, Sr_xY_10−x(SiO₄)_y(PO₄)_6−yO₂ doped with 1 mol%Eu³⁺ (x = 2, y = 6; x = 4, y = 4; x = 5, y = 3; x = 8, y = 0) were synthesized by using 3-aminopropyl-triethoxysilane (APES) as the sources of the silicate network. X-ray diagrams confirm that Sr_xY_10−x(SiO₄) _y(PO₄)_6−yO₂: Eu³⁺ solid solutions are formed as a pure apatitic phase. The SEM picture shows that there exist some novel unexpected coral like morphological structures. The luminescent intensity is the strongest for the host composition of Sr₄Y₆(SiO₄)₄(PO₄)₂O₂ although the effect of the composition on the luminescent intensity is little.     

Compounds and phase compatibilities in the system La2O3---SrO---CuO at 950°C

The subsolidus phase diagram of the system La₂O₃---SrO---CuO at 950°C under 1 bar of pure oxygen has been investigated and a new ternary compound, La_1+xSr_2−xCu₂O_5.5+δ with 0.05 ≤ x ≤ 0.15, was isolated. This compound crystallizes in an orthorhombic unit cell with lattice constants related to the lattice constant of the perovskite cubic unit cell, a_p, by a = 3.80 Å a_p, B = 11.48 Å 3a_p, and c = 20.23 Å 5a_p. The structure is isotypical to that of LnSr₂Cu₂O_5.5+δ with Ln = Sm, Eu, or Gd. Reported data on the crystal chemistry of the equilibrium compounds in the system La₂O₃---SrO---CuO have been summarized and compared with the present data. The structure of all compounds is built up of a La---O rock-salt layer separated by a number of LaCuO₃ perovskite layers. The general formula is (La---O)(LaCuO₃)_n where La can be replaced either partly or completely by Sr. Compounds are found for n = 1, 2, and ∞. The structures of the compounds show different types of oxygen vacancy ordering.     

Characterization of phase transition of Ba2-xSrxIn2O5 by thermal analysis and high temperature X-ray diffraction

The phase transitions of Ba_2-xSr_xIn₂O₅ were investigated with various thermal analyses and high-temperature X-ray diffraction. It was clarified that crystal structure of Ba_2-xSr_xIn₂O₅ with x=0.0~0.4 varies from brownmillerite through distorted perovskite to another distorted perovskite with increase of temperature. The phase transition from brownmillerite to distorted perovskite was revealed to be first order, whereas transition from distorted perovskite to another one was second order. The specimen with x≥0.5 showed only one first order phase transition from brownmillerite to distorted perovskite. The phase diagram of Ba_2-xSr_xIn₂O₅ was established and existence of tricritical point at ~1100°C with x=0.4~0.5 was suggested. This revised version was published online in August 2006 with corrections to the Cover Date.     

Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr₂Nb₂O₇ and calcium niobate Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form C_pm = 248.0 + 0.04350T − 3.948 × 10⁶/T² J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and C_pm = 257.2 + 0.03621T − 4.434 × 10⁶/T² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, S_m°(298.15 K) = 238.5 ± 1.3 J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and S_m°(298.15 K) = 212.4 ± 1.2 J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇, were evaluated from the low-temperature heat capacity measurements.     

Specific heat on Sr<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>7</Subscript> and Sr<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>7</Subscript>

Summary Specific heats on the single crystals of Sr₂Nb₂O₇, Sr₂Ta₂O₇ and (Sr_1-xBa_x)₂Nb₂O₇ were measured in a wide temperature range of 2-600 K. Heat anomalies of a λ-type were observed at the incommensurate phase transition of T_INC (=495 K) on Sr₂Nb₂O₇ and at the super-lattice phase transition of T_SL (=443 K) on Sr₂Ta₂O₇; the transition enthalpies and the transition entropies were estimated. Furthermore, a small heat anomaly was observed at the low temperature ferroelectric phase transition of T_LOW (=95 K) on Sr₂Nb₂O₇. The transition temperature T_LOW decreases with increasing Ba content x and it vanishes for samples of x>2%.     

Ba2Ca2Cu3Ox Precursor Effects on Synthesis of (Hg,Pb)Ba2Ca2Cu3O8+δ Superconductor

The influence of Ba₂Ca₂Cu₃O_x precursor on the synthesis and properties of (Hg,Pb)Ba₂Ca₂Cu₃O_8+δ has been examined. Fine homogeneous Hg-free precursor powder of Ba₂Ca₂Cu₃O_x of desirable phase composition was prepared by sol-gel method using EDTA acid as a complexing agent. A reproducible superconducting sample of Hg_0.8Pb_0.2Ba₂Ca₂Cu₃O_8+δ with fine-grained, dense microstructure, composed predominantly of (Hg,Pb)-1223 phase and with advantageous magnetic properties, was synthesized by high pressure crystallization in mercury environment of well-calcined Ba₂Ca₂Cu₃O_x precursor. This revised version was published online in July 2006 with corrections to the Cover Date.     

Extension of the misfit series to the mercury-based cobaltites

The exploration of the Hg–Sr/Ca–Co–O system using EDS and electron microscopy has shown the possibility to incorporate mercury and excess cobalt in the middle rock salt layer of the misfit cobaltite structure, leading to the new series [Hg_1−xCo_xSr_2−yCa_yO₃]^RS[CoO₂]_b1/b2 with 0.6x0.90, 0y2, and b₁/b₂ ranging from 1.63 to 1.79. These new composite oxides, built up from [CoO₂]_∞ layers of the CdI₂-type stacked with triple [Hg_1−xCo_xSr_2−yCa_yO₃]. rock salt layers are remarkable by their large thermopower S values similarly to the thallium and lead misfit cobaltites. More importantly, the S value is sensitive to the composition of the triple rock salt layer, increasing as the calcium content increases, so that an enhancement of the figure of merit of the pure calcium compound is expected.     

Ein alternativer Weg zur Darstellung von Bi2Sr2CaCu2Ox,YBa2Cu3O7−δ, und YBa2Cu3−xMxO7−δ (M  Ni,Ag und x ≤ 33 Mol%)

An Alternative Method for Preparation of Bi₂Sr₂CaCu₃O_x, YBa₂Cu₃O_7?δ, and YBa₂Cu_3?xM_xO_7?δ (M ? Ni, Ag and x ≤ 33 Mol%) Oxidation of quenched melts of metals in a well defined argon/oxygen atmosphere has been found as an alternative method to prepare hight purity samples of Bi₂Sr₂CaCu₂O_x without Pb-stabilization. A special equipment for the oxidation of powder samples will be described. By our new method it is also possible to prepare YBa₂Cu₃O_7?δ and derivatives of this compound, where Cu is substituted by high amounts of Ni or Ag.     

Subsolidus phase relations and crystal structures of R-Ca-Cu-O (R=Nd, Sm, Gd, Tm) systems

The subsolidus phase relations of R₂O₃-CaO-CuO ternary systems (R=Nd, Sm, Gd, Tm) have been investigated by X-ray powder diffraction. All samples were synthesized at about 950° in air. There exists a ternary compound Ca_14−xR_xCu₂₄O₄₁ (x = 4 for R=Nd, Gd and x = 5 for R = Sm) and a ternary solid solution Ca_2+xR_2−xCu₅O₁₀ (R=Nd, Sm, Gd, Tm) with a wide composition range Δx of about 0.6. The compound Ca_14−xR_xCu₂₄O₄₁ possesses a layered orthorhombic structure and is isostructural to Sr_14−xCa_xCu₂₄O₄₁. The lattice parameters a and c of the compound are basically independent of the ionic radius of R, while the lattice parameter b and unit-cell volume V decrease substantially with the decrease of the ionic radii of R. The Ca_2+xR_2−xCu₅O₁₀ solid solution is isostructural to Ca_2+xY_2−xCu₅O₁₀, the structure of which is based on an orthorhombic “NaCuO₂-type” subcell containing infinite one-dimensional chains of edge-shared square planar cuprate groups crosslinked by the layered cations Ca and R that locate in the inter-chain tunnels.