E.M. Kopnin

High-pressure synthesis and heat treatments of the HgBa2Ca4Cu5O12+δ and HgBa2Ca5Cu6O14+δ phases

Abstract The synthesis of the n = 5 and n = 6 members of the HgBa2Can−1CunO2n+2+δ family are reported. Bulk samples containing either Hg-1245 or Hg-1256 as major phases were synthesized using the high-pressure high-temperature technique at 920°C and 18 kbar. The as-synthesized phases ( a = 3.8547(3) A , c = 22.156(5) A for n = 5 and a = 3.8516(1) A , c = 25.232(6) A for n = 6 ) exhibited Tc with onsets of about 100 K. They were characterized by X-ray powder diffraction, transmission electron microscopy and AC susceptibility measurements. Heat treatments increased the Tc onset to 110 K for n = 5 and to 107 K for n = 6. These phases with high n are more inert to thermal annealing under different atmospheres than the members with smaller n. Only strong oxidizing {P(O2) ≥ 500 bar} or reducing {Ar/400 °C} condition can induce significant Tc variations.

Suppression of superconductivity in Hg-1223 and Hg-1234 by partial replacement of Hg by carbon

Abstract Hg based 1223 and 1234 phases were prepared by a high-pressure high-temperature technique using precursors obtained by the decomposition of BaO 2 , CaCO 3 and CuO in oxygen flow at 925°C. These phases were found to exhibit much lower T c s (64 K for 1223 phase, and no superconductivity for the 1234 phase) than the corresponding Hg based samples prepared by different precursors and this even after heat treatments in oxygen flow. The structural refinement of this 1223 phase carried out by using synchrotron X-ray powder data revealed a large deficiency on the Hg site; the occupancy factor for this site was found to be 0.69(1). This may be due to the replacement of some of the Hg cations by C. By IR spectroscopy the characteristic frequencies corresponding to the vibrations of the CO 3 −2 group were found in these samples. Large amounts of intergrowth defects were defected by electron microscopy and no superstructure spots were found in the diffraction patterns. The replacement of Hg by C in the structures of the Hg based superconductors induces a decrease of the concentration of the extra oxygen in the (Hg, C) layer bonded to the Hg atoms. Since it is this oxygen which oxidizes the (CuO 2 ) layers, its decrease causes a reduction of these layers, which is responsible for the decrease of the superconducting transition temperatures.

Synthesis, structure, and resistivity properties of K1 − xBaxNbO3 (0.2 ≤ x ≤ 0.5) and K0.5Sr0.5NbO3

The K{sub 1{minus}x}Ba{sub x}NbO{sub 3} (0.2{<=}x{<=}0.5), BaNbO{sub 3} and K{sub 0.5}Sr{sub 0.5}NbO{sub 3} compounds have been synthesized by high-pressure high-temperature technique in a Belt-type apparatus. The synthesis of the K-containing phases carried out in evacuated sealed silica tubes led to the preparation of polyphasic samples. The phases obtained have an ideal perovskite-like structure. The a parameter of Ba-containing compounds increases concomitantly with increasing Ba concentration. Studies of the K-containing phases by X-ray powder diffraction did not reveal any distortion from cubic symmetry. Electron Dispersive Spectroscopy (EDS)analysis confirmed the cation nominal stoichiometry. Resistivity measurements showed semiconducting-like type of conductivity for the K{sub 1{minus}x}Ba{sub x}NbO{sub 3} (0.2{<=} x {<=} 0.5) and K{sub 0.5}Sr{sub 0.5}NbO{sub 3} samples, but the relative increase of {rho} with the decrease of temperature became smaller for the phases with lower Nb valence, and moreover K{sub 0.5}Sr{sub 0.5}NbO{sub 3} exhibited a slight decrease of the resistivity in the 100--4.2 K temperature range. On the contrary, BaNbO{sub 3} after high-pressure treatment showed a metallic-like behavior down to 4.2 K.

Structure and superconductivity of the HgBa2Ca3Cu4O10+δ phase

Abstract A bulk superconducting HgBa 2 Ca 3 Cu 4 O 10+δ phase (Hg-1234) was obtained by a high-pressure high-temperature technique in a polyphasic sample containing ≈ 15% of impurities. The lattice parameters of the as-synthesized phase were found to be a = 3.8524(4) A , c = 18.942(5) A , with a T c, onset of 116 K. Subsequent heat treatments showed that only strong reducing or oxidizing conditions allow one to obtain a major variation of lattice parameters and of T c, onset , while heat treatments in 1 bar oxygen do not produce any significant change. The structural model refined by X-ray Rietveld powder diffraction showed a large deficiency of the Hg site [0.79(1)], relatively high thermal parameters for Hg and Ba, and an unusual short HgO ap distance. All these features can be explained by the presence of carbon replacing Hg in the as-prepared Hg-1234 phase.

Synthesis and resistivity properties of Sr1-xLaxPbO3-δ and SrPb1-xBixO3-δ

Abstract The solid solutions with the composition SrPb1 − xBixO3 − δ and Sr1 − xLaxPbO3 − δ were synthesized. It was found that the solubility limits do not exceed the values x = 0.2 and x = 0.1, respectively. It was shown by X-ray powder diffraction that the initial crystal symmetry (orthorhombic, space group Pnma) remains unchanged with the substitution, whereas unit cell parameters increase monotonically. The undoped SrPbO3 − δ exhibit positive and negative temperature coefficient of resistivity at high and low temperatures, respectively. La-doping leads to a drastical decrease of the absolute value of the resistivity and for the doped samples with x ≥ 0.05 dp/dT is positive in the entire temperature range. Bi-doping leads, first to a similar decrease of resistivity, but at higher substitutions, variable range hopping conduction is observed. The results were interpreted hi terms of quantum interference and a subsequent transition to localized electronic states,

Synthesis, neutron diffraction study and cation substitutions in Srn−1Cun+1O2n (n = 3, 5)

Abstract The n = 3 and n = 5 members of the homologous series Sr n −1 Cu n +1 O 2 n have been synthesized by a high-pressure, high-temperature technique and characterized by X-ray powder diffraction and electron microscopy as well as EDS microanalysis. The solubility range of Sr 4− x Nd x Cu 6 O 10 solid solution was found to be ≤ 0.1. The crystal structure of Sr 4 Cu 6 O 10 was refined from neutron powder diffraction data.

Effect of Pb doping on the structure of HgBa2CuO4+δ single crystals

Abstract X-ray structure analyses were performed with HgBa2CuO4+δ and Hg1−xPbxBa2CuO4+δ single crystals. The onset of the superconducting transition of the crystals ranges from 78–92 K. Hg-1201 crystallizes with space group P4/mmm and has lattice pafameters a = 3.8907(2) A and c = 9.556(1) A . The structure was refined to R = 0.023 (wR2 = 0.036) in the ideal 1201 type with an excess oxygen content of 6(2)% and an occupancy of 97% at the Hg site. Pb-doping decreases the lattice parameters to a = 3.8836(2) A and c = 9.553(1) A , and the refinement converged at R = 0.042 (wR2 = 0.088). Pb atoms are shifted by 0.5 A to (0.14, 0, 0) and the excess oxygen atom O(3) by 0.45 A to (0.38, 1 2 , 0). It has not a well defined position in the lattice, but is distributed randomly in a ring-like configuration around the ( 1 2 , 1 2 , 0) site, supporting a Pb clustering model. Rather high anisotropic temperature factors U11 and U22 of Hg and apical O(1) atoms are observed, which are also present in higher members of the Hg(Pb)-12(n − 1)n series. They are discussed in terms of corrugated OHgO bonds. Comparison with other structure refinements of 1201 shows a correlation of the transition temperature with the position of the Ba atom, which seems to be a good indicator for the hole doping evel in 1201.

Synthesis and properties of NbM2RCu2O8 and TaM2RCu2O8 phases (MBA, Sr: RPr, Nd, Sm, Eu and Gd)

Abstract NbM 2 RCu 2 O 8 and TaM 2 RCu 2 O 8 (M Ba, Sr; R Pr, Nd, Sm, Eu and Gd) phases were synthesized. Electron diffraction study of TaSr 2 EuCu 2 O 8 showed the presence of a weak superstructure ( a = 82a 07 ), like that found for a number of similar phases. Rietveld refinement of this structure based on X-ray powder diffraction data was carried out in the space group P 4/ mmm ( R 1 = 0.038, R p = 0.037) since no superstructure reflections were observed. Various types of cation replacement were performed in order to increase the copper valence. Monophasic Nb(Ta) 1 , Ti,Sr 2 EuCu 2 O 8 samples were synthesized at P ( O 2 ) = 1 bar and T = 1050°C up to x = 0.1 (Nb) and 0.3 (Ta), but no superconductivity was detected down to 12 K. High pressure syntheses were carried out to obtain Nb(Ta)Sr 2 Eu 1 , Ca, Cu 2 O 8 phases, but they were found to have a narrow homogeneity range that provides no opportunity of inducing superconductivity in such a way.

New complex oxides in RBaThCuO systems (R = LaGd)

Abstract The new complex oxides R2Ba1.33Th0.67Cu3O8+δ (R = La, Pr) and La0.67Ba1.33R2−xThxCu3O8+δ (R = NdGd, x = 0.5, 0.67) were synthesized. These compounds have a similar structure to the Ce-223 high-Tc oxides. The oxygen content (or δ) was varied over a wide range after annealing at high oxygen pressure (up to 90 atm). Unit cell parameters and δ values for all the new phases were determined. The resistivities of these semiconducting compounds obey the law ϱ = ϱoT−α. A single crystal of Nd2Ba1.5Th0.5Cu3−xAlxO8+δ (x = 0.3, δ = 0.8) was obtained and its structure was determined by X-ray single-crystal experiment (tetragonal system, space group I4 mmm , a = 3.9057(5) A, c = 28.569(4) A, Z = 2).

High-pressure synthesis of the members with n = 1to 6 of the new superconducting family HgBa2Can−1CunO2n+2+δ

Abstract The synthesis of the members n=1to 6 of the new mercury-bearing cuprate series has been carried out using high-pressure high temperature synthesis in a belt type apparatus at 18–40 Kbar and 850–950°C. At the present time, this method appears to be the only route to prepare the members with n=3 to 6 or greater as bulk nearly-pure samples. The synthesis procedure involves two steps : the preparation of a precursor Ba 2 Ca n−1 Cu n O x , and then the sintering of HgO with the precursor at the requested values of P,T,t.