Doping of Bi-Sr-Ca-Cu-O superconductors with Pb,Sn and Ag

Basic and substituted superconductors in the homologous series Bi₂Sr₂Ca _n−1Cu _n O_2n+4+y , Bi_2−x/2Sr_2−x/2L _x Ca _n−1Cu _n O_2n+4+y and Bi₂Sr_2−x L _x Ca _n−1Cu _n O_2n+4+y (L=Sn, Pb or Ag,x=0−0.4,n=2, 3 or 4) have been synthesized. All the prepared ceramic samples show superconducting behaviour with zero resistance atT _c=70 to 85 K. The compounds withn=3 or 4 showed onset temperature around 115K indicating involvement of a disproportionate solid-state reaction and formation of a two-phase system. The phase involving tin or lead oxides showed similar superconducting properties. Final rapid quenching of samples contributed to preservation of the high temperature equilibria with higher solubility of tin oxide in the quaternary system Bi-Sr-Ca-Cu-O. Silver was not soluble but precipitated in a colloidal form at interfaces between the crystalline grains.     

The Effect of Mn Substitution on Properties of Bi1.6Pb0.4Sr2Ca2?x Mn x Cu3O y Superconductors

The effects of Mn substitution on the physical properties and structural characteristics of Bi_1.6Pb_0.4Sr₂Ca₂Cu_3−x Mn _x Oy (Bi-2223) superconductor system have been studied. For this, the samples of nominal composition Bi_1.6Pb_0.4Sr₂Ca₂Cu_3−x Mn _x Oy (x=0.00, 0.10, 0.15 & 0.20) was prepared by the solid-state reaction method. It has been found that the effects of Mn substitution favor the formation of Bi-2223 phases. The phase identification/gross structural characteristics of synthesized (HTSC) materials explored through powder X-ray diffractometer reveals that all the samples crystallize in orthorhombic structure with lattice parameters (a=5.4918 ?, b=5.4071 ?, and c=37.0608 ?) up to Mn concentration of x=0.20. The critical transition temperature (T _c) measured by standard four probe method has been found to depress from 108 K to 70 K and transport current density (J _c) has been increased from 4.67×10² to 3.52×10³ A cm⁻² as Mn content (x) increases from 0.00 to 0.20. The surface morphology investigated through scanning electron microscope and atomic force microscopy (SEM and AFM) results that voids and grains size increases as the Mn concentration increases besides the nanosphere like structures on the surface of the Mn doped Bi-2223 sample.     

Raman spectra of Bi–Sr–Ca–Cu–O high temperature superconductors

Two high-temperature superconducting BiSrCaCuO compounds were fabricated as Bi_2.1Sr₄Ca₄Cu₃O_10+δ and Bi₂Sr₂Ca₂Cu₃O_10+δ. The X-ray diffraction patterns of both the compounds show that Bi_2.1Sr₄Ca₄Cu₃O_10+δ contains predominantly the high T_c 2223 phase, and Bi₂Sr₂Ca₂Cu₃O_10+δ the low T_c 2212 and high T_c 2223 phases, in equal amounts. The Raman spectra of these samples exhibit broad bands in the 430–512 cm^-1 region, as in the lead-substituted 2212 phase. Change in configuration due to the presence of the 2223 phase in the sample is indicated by the broadening of the 465 cm^-1 line. New bands at 611 and 678 cm^-1 are observed. Peaks in the 430–512 cm^-1 region exhibit a softening when the temperature is reduced to 93 K. This revised version was published online in November 2006 with corrections to the Cover Date.     

Solution-sol-gel processing of superconductors

Bulk materials and thin films of pure and homogeneous YBa₂Cu₃O_7−x and Bi₂Sr₂CaCu₂O_8+x compounds were prepared by a nanocomposite solution-sol-gel (SSG) method. The superconducting oxides of YBa₂Cu₃O_7−x and Bi₂Sr₂CaCu₂O_8+x were prepared at very low temperatures i.e. 750°C and 850°C, respectively by SSG method. Pellets sintered from these nanophasic sol powders showed sharp resistivity drops atT _c ∼ 90°K for YBa₂Cu₃O_7−x andT _c∼67°K for Bi₂Sr₂CaCu₂O_8+x . Thin films were prepared using triphasic sol of Y, Ba, Cu and tetraphasic sol of Bi, Sr, Ca and Cu on MgO and SrTiO₃ substrates. The triphasic sol coated on SrTiO₃ substrates and calcined at 800°C for 12h showed the formation of superconducting phase, YBa₂Cu₃O_7−x with preferred orientation along theC-axis. X-ray diffraction patterns of the Bi₂Sr₂CaCu₂O_8+x films on MgO substrate showed the formation of the superconducting phase with preferential orientation along the C-axis and the microwave absorption data as a function of temperature of this film revealed the onset temperature to be 90°K.     

An X-ray spectral study of the influence on the electronic structure of highT c superconductors of the parameters determining the superconducting transition temperature

CuL _α and OK _α X-ray fluorescence spectra, corresponding to the Cd3d-Cu2p and O2p-O1s transitions, and XANES and EXAFS spectra (CuK, YK, BiL, PbL) have been obtained for the following series of samples: (i) YBa₂Cu₃O_7−δ (0<δ<0·8), (ii) La_2−x Sr _x CuO₄ (0<x<0·3) and (iii) Bi₂(Sr, Ca) _n+1 Cu _n O_2n+4 (n=1,2,3), Bi_1·8Pb_0·3Sr_1·9Ca₂Cu₃O _y . A correlation has been found between the decrease in the superconducting transition temperature (T _c ) with variation in the stoichiometric parametersδ andx in series (i) and (ii), and the shift of the maximum in their CuL _α spectra and the relative increase in the number of occupied states Cu3d in the upper part of the valence band. The changes in the distribution of the Cu3d and O2 _p densities and in the spectral parameters of the samples are discussed.     

Electron paramagnetic resonance studies in the oxides Bi2Sr2Ca1−x Gd x Cu2O y

We have recently reported our results on magnetic susceptibility and microwave absorption studies on Bi₂Sr₂Ca_1−x Gd _x Cu₂O _y and Bi₂Sr₂Ca_1−x Y _x Cu₂O _y . In the present work two important observations of these investigation, viz, the dependence ofT _c on Gdconcentration and the absence of EPR-signals of Gd and Cu, are considered in greater detail. It is suggested that the microwave absorption by fluxon lattice might be playing one of the important and effective pathways for electron spin relaxation resulting in nonobservation of EPR signals.     

Structural and superconducting properties of single-crystal Bi2Sr2Can ? 1CunO2n + 4 + δ (n = 1–3) whiskers grown in gas-filled cavities

Using growth inside gas-filled cavities in KCl-fluxed melts, we have obtained free-standing phasepure Bi₂Sr₂Ca _{n − 1}Cu _n O_{2n + 4 + δ} (n = 1–3) whiskers possessing superconducting properties in the as-grown state. Bi-2212 whiskers have been prepared in a wide range of doping levels, from underdoped (T _c = 76.5 K) to optimally doped (T _c = 84 K). The whiskers possess high structural perfection, as shown by X-ray diffraction. Annealing under optimal conditions raises the superconducting transition temperature of the underdoped Bi-2212 whiskers to T _{c max} = 85.5 K (ΔT _c = 1.8 K), without impairing their structural perfection. Original Russian Text ? M.V. Golubkov, Yu.I. Gorina, G.A. Kalyuzhnaya, V.V. Rodin, N.N. Sentyurina, V.A. Stepanov, S.G. Chernook, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 6, pp. 731–737.     

A new high-T c superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-x Bi x )1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+δ (123 phase) and Bi₄Sr₄Ca₂Cu₄O_16+δ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+δ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+δ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

Interplay of low and highT c phases in Bi-Sr-Ca-Cu-O superconductor

In the Bi-Sr-Ca-Cu-O system stringent conditions of heat-treatment lead to the formation of a mixture of both the low and highT _c phases and obtaining a single-phase material becomes extremely difficult. This study reports preparation of samples with single superconducting transitions at ∼ 75 K and ∼ 108 K; the compositions of which correspond ton=2,3 in the series Bi₂Sr₂Ca _n−1Cu _n O_{4 + 2n} . X-ray diffraction studies show that the lowerT _c material is a relatively pure phase while the higherT _c phase only co-exists with the lowerT _c phase. The most obvious effect of doping the system with lead is to make the reaction take place faster and thereby increase the volume fraction of the 110K phase.     

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Can?1CunOx

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi₂Sr₂Ca _n–1Cu _n O _x (n = l, 2), have been studied through polarization-resolved Raman and¹²⁹I Mössbauer spectroscopy. The polarization dependence of the Raman spectra and the Mössbauer measurement confirmed the dominant species to be triiodide ions, I ₃ ^– , with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO₂ planes upon intercalation, by whichT _c of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi₂Sr₂Ca _n–1Cu _n O _x systems.     

Significant Improvement in Superconductivity by Substituting Pb at Bi-site in Bi2?xPbxSr2CaCu2O8 with x=0.0 to 0.40

Here we report the synthesis and superconducting properties of bulk Bi_2−x Pb _x Sr₂CaCu₂O₈ (x=0.0 to 0.4) compound. Though the superconducting transition temperature (T _c) decreases marginally, the critical current density under magnetic field J _c(H) increases with Pb content. An optimization is observed for x=0.16 with J _c(H) (7.894×10³ A/cm²) that is nearly doubled in comparison to the pristine compound. It seems that controlled substitution of Pb at Bi-site in bulk Bi-2212 (Bi₂Sr₂CaCu₂O₈) system can enhance the superconducting critical parameters. These results are explained on the basis of possible improved inter- and intra-granular properties with Pb substitution in Bi_2−x Pb _x Sr₂CaCu₂O₈ system.     

Superconducting Bi-Sr-Ca-Cu-O films formed diffusion of metallic bismuth on Sr2Ca2Cu4O y substrates

Superconducting Bi-Sr-Ca-Cu-O films have been prepared on Sr₂Ca₂Cu₄O _y substrates by thermal evaporation of metallic Bi layer and heat-treatment at 830°C for a few minutes in air. The zero resistance temperature of Bi₂Sr₂Ca₁Cu₂O _y film of 1–μm in thickness is about 70 K. The surface diffusion process of Bi on the granular structure of Sr₂Ca₂Cu₄O _y was observed by scanning electron microscope and thermogravimetry analysis. The stripe pattern of superconducting film, typically 2μm thick and 0·3 mm wide, is formed by using a mask.     

Electrical conduction properties of Sr-doped Bi<Subscript>4</Subscript>(SiO<Subscript>4</Subscript>)<Subscript>3</Subscript> with the eulytite-type structure

Electrical conduction in 1 mol% Sr-doped Bi₄(SiO₄)₃ with the eulytite-type structure at elevated temperatures was investigated with conductivity measurements. Conductivity of the material under wet condition was higher than that under dry condition, and were 1.2 × 10⁻⁶ – 9.7 × 10⁻⁵ S cm⁻¹ at 500–850 ^°C. From H/D isotope effects and p(O₂)-dependencies of the conductivity, it was found that the Sr-doped Bi₄(SiO₄)₃ exhibited protonic conduction at all the temperatures investigated while contribution of p-type conduction became significant with increasing p(O₂) and/or temperature. Protonic and p-type conductions in the material were discussed in terms of defect equilibria.     

Influence of transition metals substitution on the superconductor Bi4Ca3Sr3Cu4O y

The results of our investigation on the specimens Bi₄Ca₃Sr₃Cu_4−x T _xO _y (T=Fe, Co, Ni,x⩽0.5) synthesized in air are presented. Fe and Co substituents result in the formation of Bi₂Sr₂CuO _y -type of phase, with considerable depression ofT _c of the main phase. However, Ni is completely soluble with Cu in this concentration range without significant depression of superconducting transition temperature (T _c). This difference in the solubility behaviour of Fe and Co on the one hand and Ni on the other is explained taking into account ionic charge and coordination number mismatches.     

Effect of Ba Substitution on the Formation of (Tl,Bi)-1234 Superconducting Phase

(Tl_1−x Bi _x )(Sr_2−y Ba _y )Ca₃Cu₄O _z ((Tl,Bi)-1234) samples (x=0.1–0.3, y=0.4–1.5) were synthesized under ambient pressure by using a two-step solid-state reaction method to investigate the effect of Ba substitution on the formation and superconducting properties of the (Tl,Bi)-1234 phase. X-ray diffraction analysis shows that nearly single-phased samples are obtained for (Tl_0.8Bi_0.2)(Sr_2−y Ba _y )Ca₃Cu₄O _z (0.8≤y≤1.2), suggesting that control of the composition ratio of Sr and Ba in the bridging layer is a key to stabilizing the 1234-type structure. The critical temperature for the nearly single-phased samples remained between 106.9 K and 109.1 K and was not significantly affected by the change in the Ba content.     

Growth of high-temperature superconductor crystals from flux

Crystallization of high-temperature superconductors was studied in La-Sr-Cu-O, Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems. Platelet crystals YBa₂Cu₃O_6.5+x were obtained by spontaneous crystallization from homogeneous nonstoichiometric melts enriched in barium and copper oxide. La_2−x Sr _x CuO₄ was prepared by slow cooling of melts enriched in copper oxide. Bi₂(Sr,Ca) _{n + 1}Cu _n O _y , (n=1; 2) was obtained by melting zone travelling. The crystals show transition to superconducting state atT=93 K, ΔT 0.2–0.5 K (Y, Ba cuprate),T=87 K, ΔT 2K (Bi, Sr, Ca-cuprate). La, Sr-cuprate single crystals obtained by Czochralski method did not show transition to superconducting state. For flux-grown crystalsT _c was 5–26 K depending on the composition, growth and heat treatment. The short characterization of some accessory phases (Ba₃Y₂Cu₃PtO₁₀, Ca_1.75Sr_1.5Cu_0.75PtO₆, BaCuO₂, Ba₄₁Cu₄₄O₈₄Cl₂) is reported.     

Electronic Energy Levels in High-Temperature Superconductors

Parent materials of high-temperature superconductors (HTSC) need to be doped to become superconducting. The optimum doping for maximum critical transition temperature T _c has been analyzed for more than 20 materials. Assuming a uniform doping distribution the distance x between doped unit cells—projected into the CuO₂ plane for cuprates—shows a strong linear correlation to the inverse of T _c in the form (2x)²=m ₁1/T _c with a slope of m ₁=(2.786±0.029)×10⁻¹⁵ m² K. The mercury cuprate homologous series HgBa₂Ca _n−1Cu _n O_2n+2+δ with n=1,2,3 has been used to demonstrate the procedure deriving the doping distance x from the optimum doping value δ.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

A new high-Tc superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-xBix)1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+ (123 phase) and Bi₄Sr₄Ca₂Cu₄O₁₆₊ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Ca n−1Cu n O x

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi₂Sr₂Ca _n?1Cu _n O _x (n = l, 2), have been studied through polarization-resolved Raman and¹²⁹I Mössbauer spectroscopy. The polarization dependence of the Raman spectra and the Mössbauer measurement confirmed the dominant species to be triiodide ions, I ₃ ^? , with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO₂ planes upon intercalation, by whichT _c of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi₂Sr₂Ca _n?1Cu _n O _x systems.