The formation of superconducting oxide phases in the (Bi,Pb)−Sr−Ca−Cu−O System

The crystallization mechanism of superconducting phases in the (Bi,Pb)?Sr?Ca?Cu?O system was determined on the basis of the results of DTA, DTG and TG studies, supplemented by X-ray examination of ceramic powders obtained by the sol-gel method. It has been demonstrated that the factor determining the formation of superconducting phases: Bi₂Sr₂CaCu₂O_x (low-T _c ) and Bi₂Sr₂Ca₂Cu₃O_x (high-T _c ) is the kinetics of reaction of calcium and strontium carbonates with molten CuBi₂O₄. As a result of the reaction of the bimetallic compound CuBi₂O₄ with SrCO₃ in the liquid phase the compound Bi₂Sr₂CuO₆ is formed. This compound, reacting with calcium and copper oxides, yields superconducting phases: the low-T _c and the high-T _c phase. It has been also observed that an increase in the volume fraction of high-T _c phase in powder subjected to thermal treatment takes place probably due to the repeated disproportionation of low-T _c phase and its repeated synthesis from Bi₂Sr₂CuO₆, CuO and CaO.     

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.     

First Observation of an Inverse Ruddlesden‐Popper Series: (A3n+1ONn−1)Bin+1 with A = Sr,Ba and n = 1, 3

Single crystals ((Ba_0.78(1)Sr_0.22)₄O)Bi₂ and ((Ba_0.62(1)Sr_0.38)₁₀N₂O)Bi₄ were successfully prepared from melt beads of Ba, Sr, and Bi in nitrogen atmosphere with oxygen impurities. The phases can be prepared in single phase from the appropriate mixtures of alkaline‐earth metal, bismuth, and bismuth oxide upon heating in pure nitrogen atmosphere. ((Ba_0.78(1)Sr_0.22)₄O)Bi₂ crystallizes in the K₂NiF₄ structure type (space group I4/mmm, No. 139, a = 522.34(5) pm, c = 1844.0(2) pm, Z = 2, R_gt(F) = 0.039) with layers of vertex‐sharing octahedra ((Ba,Sr)_4/2Ba₂O). ((Ba_0.62(1)Sr_0.38)₁₀N₂O)Bi₄ crystallizes as an isotype of Sr₄Ti₃O₁₀ (space group I4/mmm, No. 139, a = 531.3(1) pm, c = 3983.2(4) pm, Z = 2, R_gt(F) = 0.050) containing slabs of three layers of vertex‐sharing octahedra further connected via corners. These compounds are interpreted in terms of members of an inverse Ruddlesden‐Popper series with the general formula n (A₃ON_n?1)Bi · ABi or (A_3n+1ON_n?1)Bi_n+1, respectively, with n = 1, 3. Partial order of the alkaline‐earth metal ions is analyzed.     

Time and temperature-based study for the production of high T C phase by sol-gel technique in Pb-bscco system

The high T_C superconducting phase Bi₂Sr₂Ca₂Cu₃Ox (2223) in the Pb-BSCCO system has been produced by EDTA-gel processing using nitrate solutions. The precursor has heated in two stages, at 300 and 800°C each for 2 h, to avoid the burning of the important species involved in the final product. The effects of time (6 to 48 h) and temperature (845 and 855°C) on the formation of the 2223 phase have been studied by sintering the samples in air. Thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and a vibrating sample magnetometer (VSM) have been employed to investigate the powder produced at different stages of decomposition, oxidation and formation of sintered materials from the powders. The volume-fraction of the 2223 phase at 845°C increases with time, the maximum value of the 2223 phase was obtained at 120 h. It has been observed that the formation of the high T_C phase is remarkably enhanced at the temperature of the endothermic peak of the DTA curve. The best result has been obtained in the sample sintered for 24 h at the temperature 855°C (endothermic peak). This also indicated that at 855°C, the large volume-fraction of 2223 phase with T_C 113 K grew in short time and as the sintering time increased, it decomposed into the Bi₂Sr₂CaCu₂Ox (2212) phase and other phases.     

Substitution Effects in Zintl Phases: Synthesis and Crystal Structure of the Novel Phases Ae3Sn4−xBi1+x (x ≤ 1; Ae = Sr,Ba) Containing Shubnikov‐Type Nets $^{2}_{\infty}\rm [Sn_{4-x}Bi_{x}]$

The compounds Ae₃Sn_4?xBi_1+x (Ae = Sr, Ba) with x < 1 have been synthesized by solid‐state reactions in welded Nb tubes at high temperature. Their structures were determined by single crystal X‐ray diffraction studies to be tetragonal; space group I4/mcm (No. 140); Z = 4, with a = 8.968(1) Å, c = 12.859(1) Å for Sr₃Sn_3.36Bi_1.64(3) ( 1 ) and a = 9.248(2), c = 13.323(3) Å for Ba₃Sn_3.16Bi_1.84(3) ( 2 ). The structure consists of two interpenetrating networks formed by a 3D Ae_6/2Bi substructure (anti‐ReO₃ type) forming the host, and layers of interconnected four‐member units [Sn_4?xBi_x] with “butterfly”‐like shape as the guest. According to the Zintl‐Klemm concept, the compounds are slightly electron deficient and will be charge balanced for x = 1. The electronic structures of Ae₃Sn_4?xBi_1+x calculated by the TB‐LMTO‐ASA method indicate that the compounds correspond to ideal semiconducting Zintl phases with a narrow band gap for x = 1 (zero‐gap semiconductor). The origin of the slight deviation from the optimal electron count for a valance compound is discussed.     

Potassium barium bismuth oxide

KBa₄Bi₃O crystallizes in the centrosymmetric tetragonal space group I4/mcm. In this compound, bismuth is present as two anionic species, i.e. Bi₂^4? dumbbells [Bi—Bi 3.113 (3) Å] and isolated Bi^3?. Atom Bi1 (Bi^3?) lies inside a bicapped square antiprism (2 × K and 8 × Ba). Atom Bi2, which forms the Bi₂^4? dumbbell, sits inside a bicapped distorted trigonal prism (2 × K and 6 × Ba). O atoms occupy tetra­hedral voids between Ba atoms.     

On the characterization of BiMO2NO3 (M=Pb, Ca, Sr, Ba) materials related with the Sillén X1 structure

The compounds BiMO₂NO₃, with M=Pb, Ca, Sr, and Ba, were obtained as single-phase products from solid-state reactions in an atmosphere of nitrous gases. The oxide nitrates with Pb and Ca crystallize in the tetragonal space group I4/mmm with two formula units per unit cell; the oxide nitrates with Sr and Ba crystallize in the orthorhombic space group Cmmm with four formula units per unit cell. Lattice parameters at room temperature are a=397.199(4), c=1482.57(2) pm for M=Pb; a=396.337(5), c=1412.83(3) pm for M=Ca; a=1448.76(3), b=567.62(1), c=582.40(1) pm for M=Sr and a=1536.50(8), b=571.67(3), c=597.55(3) pm for M=Ba. The structures, which were refined by powder X-ray diffraction, consist of alternating [BiMO₂]⁺ and [NO₃]⁻ layers stacked along the direction of the long axis. IR and thermogravimetric data are also given. The various M²⁺ cations in BiMO₂NO₃ are compatible with each other; therefore and because of their layer-type structure, these compounds are interesting precursors for oxide materials, e.g., the HTSC compounds (Bi,Pb)₂Sr₂Ca_n−1Cu_nO_x.     

Sodium “stuffed” Ba2−xSrxFe4O8 ferrites: new cationic conductors

Sodium insertion in the tetrahedral layer structure of the ferrites Ba_2−xSr_xFe₄O₈ was performed by solid state reaction at 1220 K in air. Superstoichiometric oxides with the actual formula (Ba_2−xSr_x)_1−y/4Na_yFe₄O₈—y0.56; 0.60Ba/Sr1.67—were characterized by X-ray and neutron powder diffraction. The hexagonal unit-cell volume shows an increasing dependence on the sodium insertion when the Ba/Sr ratio reaches the largest values. The marked expansion of the c parameter is the likely signature of the location of the inserted sodium cations within the interlayer space. One-half of the sodium cations partly sits on the Sr(Ba) sites in octahedral coordination and the other half occupies extra octahedral and tetrahedral sites. ac conductivity measurements point to a cationic conductivity whose thermally activated regime—E_a 0.7 eV—evidenced from 570 K, is unsensitive to the sodium content. The bottleneck of the 2D sodium mobility regards the crossing of the oxygen triangular faces shared by the different polyhedra within the interlayer space.     

Crystal structure refinements of the three-layer Aurivillius ceramics Bi2Sr2−xAxNb2TiO12 (A=Ca,Ba, x=0,0.5,1) using combined X-ray and neutron powder diffraction

Three-layer Aurivillius ceramics Bi₂SrCaNb₂TiO₁₂, Bi₂Sr_1.5Ca_0.5Nb₂TiO₁₂, Bi₂Sr₂Nb₂TiO₁₂, Bi₂Sr_1.5Ba_0.5Nb₂TiO₁₂, and Bi₂SrBaNb₂TiO₁₂ were formed via solid-state synthesis and their structures characterized by combined Rietveld analysis of powder X-ray and neutron diffraction data. Static disorder was observed in the form of mixed cation occupancies between the Bi and the Sr, Ca, or Ba on the A sites in the perovskite block, as well as between the Nb and Ti sites. The degree of site mixing between the Bi site in the (Bi₂O₂)²⁺ layer and the perovskite-block A site increased with increasing average A site cation radius (ACR). Bi₂SrBaNb₂TiO₁₂ displayed the greatest degree of Bi-A site static disorder. Bond valence sum (BVS) calculations showed an increase in A site BVS with average A site cation radius. All compositions except Bi₂SrCaNb₂TiO₁₂ had overbonded A sites and the A site BVS increased nearly linearly with lattice parameter and ACR. A preference was observed for Ca²⁺ to remain on the A site while Ba²⁺ preferred to disorder to the Bi site, indicating that the cation site mixing occurs to reduce strain between the (Bi₂O₂)²⁺ layer and the perovskite block in the structure. Unusually large Ti site BVS and thermal parameter for the equatorial oxygen in the TiO₆ octahedra were observed in structural models that included full oxygen occupancy. However, excellent structure models and more reasonable BVS values were obtained by assuming oxygen vacancies in the TiO₆ octahedra. AC impedance spectroscopy performed on all samples indicate that the total electrical conductivity is on the order of at 900°C.     

Impact of Pb substitution on the formation of high <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconducting phase in BSCCO system derived through sol-gel process

Sol-gel process was employed to synthesize the Pb-BSCCO system having general composition Bi_2−xPb_xSr₂Ca₂Cu₃O_10−δ, where x=0.2, 0.4 and 0.8. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), dilatometry and vibrating sample magnetometer (VSM) were employed to study the materials produced at different stages. Two-stage heating firstly at 300 and then 800°C was adopted in order to avoid the burning of the materials and formation of carbonates. The carbonate formation was avoided by heating the materials firstly at 300°C for 2 h and without intermediate cooling moved to the furnace having temperature 800°C and hold for 2 h. The sintering behaviour of samples was studied by dilatometry and the results revealed that the sample having x=0.4 was stabled up to a temperature of 700°C while samples having x=0.2 and 0.8 to a temperature of 625°C. The maximum shrinkage was observed at 850°C in all the samples. On the basis of dilatometry results, the samples were sintered at 845°C for 60 h to observe the superconducting phases. The highest volume fraction of high superconducting phase (2223) was noticed in the sample containing x=0.4 having onset T _c=110 K.     

Mössbauer spectroscopic analysis of Bi1−xSrxFeO3−δ perovskites

Bi_1?xSr_xFeO_3?δ perovskites synthesised by solid-state reaction in ambient atmosphere or in a closed vessel were analysed by X-ray diffraction and Mössbauer spectroscopy. The evolution of the valence state of iron with both Bi/Sr ratio and oxygen content has been more particularly discussed. The samples are single phase and homogeneous for x ≤ 0.5 (Bi-rich) and x > 0.8 (Sr-rich). For intermediate Sr contents, the samples are less homogeneous and tend to contain both Bi-rich and Sr-rich phases. The appearance of Sr-rich phases for x > 0.5 corresponds to the appearance of Fe⁴⁺, to compensate for the lack of positive charges due to the replacement of Bi³⁺ by Sr²⁺.     

Electronic structure and properties of aurivillius phases

The electronic structure of compounds from the family of Aurivillius phases of the general formula Bi₂O₂[A_n−1B_nO_3n+1], where n is the number of perovskite layers, was calculated by the ab initio LMTO-ASA method. For compounds with B=Nb, Ti; A=Ca, Sr, Ba, Bi, and n=1, 2, variations of the electronic structure and properties depending on the number of perovskite units and on the varieties of A and B cations were studied. Effects of vacancy formation in the Bi₂O₂ layers and metal-oxygen planes are considered. The instability of Bi₂NbO₆ is explained, and favorable positions for oxygen replacement by fluorine are found. The possibility of superconductivity in these compounds is considered. Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 3, pp. 471–478, May–June, 1996.     

Synthesis and characterization of high temperature superconductors

The high temperature superconductors of the system Bi-Pb-Sr-Ca-Cu-O show promising properties and large scale applications can be considered. The (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀₊ (Bi,Pb(2223)) phase is attractive because of its high superconducting temperature (110 K) and high current transport capabilities.Simultaneous DTA/TG has proved an excellent tool for synthesizing, characterizing these high temperature superconductors, confirming phase diagrams and finding the optimal annealing temperature for Bi,Pb(2223)/Ag tapes in order to obtain high critical current densities around 30 kA cm^–2 at 77 K and 0T. The identification of the main DTA peaks of measurement of different phase mixtures made it possible to observe the formation of an intermediate phase Bi,Pb(2212) during the formation of Bi,Pb(2223).     

On hydrolysis of Ba–Bi–O and K–Ba–Bi–O oxide systems

Fundamentally different behavior of Ba–Bi–O (Ba : Bi = 11 : 4, 1 : 1, 2: 3, and 1 : 5 mol/mol) and K_nBa_mBi_m+nO_y (m = n = 1, 2,...; exhibiting superconducting properties with T_c = 28–32 K) oxides and BaO₂ in hydrolysis reactions has been revealed by means of potentiometry and chemical analysis. Products of the oxides treatment with water do not contain H₂O₂, evidencing the absence of peroxide ions in their structure. The perovskite-type barium-bismuth(III) oxides are completely hydrolyzed into Ba(OH)₂ and Bi₂O₃ at room temperature.     

New Phase in the Bi–Sr–Ca–Cu–O System

Cooling a melt of a Bi–Sr–Ca–Cu–O system (Bi:Sr:Ca:Cu = 4:3:3:4 or 2:2:2:4) from 1000°C-1050°C yielded crystals of a new red-colored nonsuperconducting phase, accompanying the superconducting 2212 and 2201 phases. Based on the EPR spectra, it was concluded that copper is univalent in this compound. The new phase has a composition Bi_2.2Sr_1.6Ca_1.3Cu₂Ox. The X-ray diffraction pattern has been indexed, and the unit cell parameters of the phase have been determined: space group P2/m, a = 12.93, b = 4.55, c = 10.94 ; = 102.72°.     

Cation disorder and phase transitions in the four-layer ferroelectric Aurivillius phases ABi4Ti4O15 (A=Ca, Sr, Ba, Pb)

Crystal structures of a series of bi-layered compounds ABi₄Ti₄O₁₅ (A=Ca, Sr, Ba, Pb) have been investigated using a combination of synchrotron X-ray and neutron powder diffraction data. All four oxides adopt an orthorhombic structure at room temperature and the structures have been refined in space group A2₁am. This orthorhombic structure is a consequence of a combination of rotation of the TiO₆, resulting from the less than optimal size of the A-type cation, and displacement of the Ti atoms towards the Bi₂O₂ layers. There is partial disorder of the Bi and A-type cations over two of the three available sites, which increases in the order Ca<Sr and Pb<Ba.     

Determination of thermal parameters of glasses from the system Bix(As2S3)100?x based on DSC curves

Thermal properties of glasses from the system Bi _x (As₂S₃)_100−x were studied by differential scanning calorimetry of a representative series of samples with x = 0.5, 2, 4, 6, 8, and 10 at.% Bi by determining the characteristic temperatures (T _g, T _onset, T _c, T _m) and enthalpies (H _c, H _m) of the processes taking place in the samples during their thermal treatment. Analysis of DSC recordings for the samples at the same heating rate allowed characterization of the phase transition temperature T _g as a function of the content of doping atoms in accordance with the criteria of chemical bonds formation in amorphous materials. Samples with 4 and 6 at.% Bi were thermally treated at different heating rates with the aim of determining, among the others, the parameters of their thermal stability. The assessment was done based on three different criteria. A higher tendency toward crystallization was observed with the glasses having a higher Bi content. Also, a trend of T _g shifting toward higher values, observed with increase in the heating rate, is in concordance with the Lasocka equation.     

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.     

Elektrochemische Synthese von Perowskiten im System K/Ba/Pr/Bi/O

Electrochemical Synthesis of Perovskites in the System K/Ba/Pr/Bi/O An easy procedure for the synthesis of well crystalline samples of (K,Ba)(Pr,Bi)O₃ is provided by anodic oxidation of melts consisting of Ba(OH)₂ / KOH / Pr(NO₃)₃ / Bi(NO₃)₃, at comparatively low temperatures of about 220 °C. We have explored the influence of different parameters like temperature, potential of the working electrode, and composition of the electrolyte. Chemical and thermal analyses were performed. Products obtained at different experimental conditions revealed different Ba/K and Pr/Bi ratios with a large homogeneity range. X‐Ray powder diffraction and single crystal structure analyses of K_xBa_1?xPr_yBi_1?yO₃ proved these compounds to be cubic perovskites. Barium and potassium ions are disordered occupying the A‐sites while praseodym and bismuth ions share the B‐sites or are ordered, as indicated by a doubling of the lattice parameter. The composition x and y can independently be altered. XPS analysis and physical properties are reported and discussed.     

Etude des systemes CaHPO4−MHPO4−H2O (M=Sr,Ba) A 50°C

The systems CaHPO₄−MHPO₄−H₂O (M=Sr, Ba) were studied at 50°C. ForM=Sr, the series of single phases, Ca_1−xSr_xHPO₄ for 0.95<X<0.75 and Ca_xSr_1−xHPO₄ for 0.4<X<1 have been prepared. These solid solution were caracterized by their infrared spectra and their crystallographic unit cell parameters. ForM=Ba a new phase Ca₂Ba(HPO₄)₃ has been determined. It was characterized by DRX, IR, ATD and chemical analyses.

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Zusammenfassung Bei 50°C wurde das System CaHPO₄−MHPO₄−H₂O (mitM=Sr, Ba) untersucht. FürM=Sr wurden Serien von Einzelphasen erhalten: Ca_1−xSr_xHPO₄ für 0.95<X< 0.75 und Ca_xSr_1−xHPO₄ für 0.4<X<1. Diese Mischkristalle wurden anhand ihrer Infrarotspektren und ihrer kristallographischen Elementarzellenparameter charakterisiert. FürM=Ba wurde die neue Phase Ca₂Ba(HPO₄)₃ ermittelt. Sie wurde mittels DRX, IR, ATD und chemischer Analyse charakterisiert.