E.V. Antipov

The synthesis and characterization of the HgBa2Ca2Cu3O8+δ and HgBa2Ca3Cu4O10+δ phases

Abstract The third (Hg-1223) and the fourth (Hg-1234) members of the recently-discovered homologous series HgBa2Can−1CunO2n+2+δ have been synthesized by solid state reaction, carried out at 950°C under 50 kbar at different annealing times. These phases have a tetragonal cell with lattice parameters: a = 3.8532 (6) A , c = 15.818(2) A , and a = 3.8540(3) A , c = 19.006 (3) A , respectively. The c parameters are in agreement with the formula c ≌ 9.5 + 3.2 (n − 1). Electron microscopy study showed similar lattice parameters as well as the occurence of different intergrowths and stacking faults. A periodicity of 22 A has also been detected, which may be attributed to the existence of the Hg-1245 phase. EDS analysis data of several grains of Hg-1223 and Hg-1234 are in agreement with the proposed chemical formulae. AC susceptibility measurements show that an increase of the superconducting transition temperature with n in the HgBa2Can−1CunO2n+2+δ series occurs till the third member, after which a saturation seems to be achieved.

Pressure-Induced Enhancement of Tc Above 150 K in Hg-1223

The recently discovered homologous series HgBa2Can-1 Cun O2n+2+δ possesses remarkable properties. A superconducting transition temperature, Tc, as high as 133 kelvin has been measured in a multiphase Hg-Ba-Ca-Cu-O sample and found to be attributable to the Hg-1223 compound. Temperature-dependent electrical resistivity measurements under pressure on a (> 95%) pure Hg-1223 phase are reported. These data show that Tc increases steadily with pressure at a rate of about 1 kelvin per gigapascal up to 15 gigapascals, then more slowly and reaches a Tc = 150 kelvin, with the onset of the transition at 157 kelvin, for 23.5 gigapascals. This large pressure variation (as compared to the small effects observed in similar compounds with the optimal Tc) strongly suggests that higher critical temperatures could be obtained at atmospheric pressure.

Neutron powder diffraction study at room temperature and at 10 K of the crystal structure of the 133 K superconductor HgBa2Ca2Cu3O8+δ

Abstract The crystal structure of the superconductor HgBa 2 Ca 2 Cu 3 O 8+δ has been determined by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a =3.8501(1), c=15.7837(9) A . The structure is described by the sequence of layers: …[(CuO 2 )(Ca)(CuO 2 )(BaO)(HgO δ (BaO)(CuO 2 )(Ca)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba 2 Ca 2 Cu 3 O x at a pressure of 18 kbar and 880°C, has an oxygen composition corresponding to δ =0.41 (2). This extra oxygen is located on the layer of the mercury atoms at the sites 1 2 , 1 2 , 1 2 . The superconducting transition temperature for the as-prepared sample is T c =133 K . No evidence has been found of orthorhombic distortions, nor for substitution of some of the Hg atoms by Cu. The copper and oxygen atoms of the layers (CuO 2 ) are coplanar for the Z =0 (CuO 2 plane and almost perfectly coplanar for the others. No phase transition occurs down to 10 K.

Synthesis and structural characterization of the 127 K HgBa2CaCu2O6.22 superconductor: Tc variations upon different atmosphere annealings

Abstract The synthesis of the HgBa2CaCu2O6+δ (Hg-1212) superconductor under various experimental conditions in sealed silica tubes is reported. Optimization of synthesis conditions allowed the preparation of bulk superconducting Hg-1212 with Tc onset at 127 K and lattice parameters a=3.858 A , c=12.68 A . The neutron powder diffraction analysis yielded δ=0.22(4). As a result of different atmosphere annealings a relationship between the decrease of Tc and the increase of the lattice parameter a is deduced. A 300°C/10 h treatment in 99% N2/1% O2 reduces Tc to 123 K and increases the a parameter to 3.862 A. An additional 10 h reduction in a 100% N2 flow lowers Tc to 98 K while a increases to 3.865 A. Subsequent oxidation at 300°C for 10 h in O2 flow restores the lattice parameters and Tc onset to their original values. The Tc dependence on annealing conditions is shown to be reversible.

Synthesis and neutron powder diffraction study of the superconductor HgBa2CaCu2O6+δ before and after heat treatment

Abstract The crystal structure of the superconductor HgBa2CaCu2O6+δ has been analyzed by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.8526(2), c=12.6367(8) A. The structure is made of the sequence of layers …[(BaO)(CuO2)(Ca)(CuO2)(BaO)(HgOδ)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba2CaCu2Ox at 18 kbar and 880°C, has an oxygen content corresponding to δ=0.35(2) and a transition temperature of T c ⋍ 104 K . Annealing in O2 (300°C for 20 h) reduces the oxygen content to δ=0.28(2). A sample annealed under the same conditions or in N2 (250°C for 12 h) was found to have a Tc of 123 K. Additional annealing in N2 at 400°C for 18 h lowers Tc to 110 K. Our results indicate that the material with δ=0.35 is overdoped. The oxygen above the O6 stoichiometry is located on the Hg layers of the structure, at the positions 1 2 , 1 2 , 0. No evidence has been found of the substitution of mercury by copper. The copper and oxygen atoms of the (CuO2) layers are almost exactly coplanar in this compound.

Synthesis of monophase HgBa2CuO4+δ under controlled partial oxygen pressure

Abstract A reproducible method of synthesis of monophase HgBa 2 CuO 4+δ is described. The synthesis is carried out in sealed silica tubes under controlled partial oxygen pressure. The best results were obtained within the [0.012-0.15] bar partial oxygen pressure range at 800°C. Mixtures of transition metal oxides, Co 3 O 4 /CoO or CuO/Cu 2 O, were used to promote the exact regulation of the partial internal oxygen pressure.

A new way of synthesis and characterization of superconducting oxyfluoride Sr2Cu(O, F)4 + δ

Abstract Superconducting Cu mixed oxyfluoride, Sr2Cu(O, F)4 + δ, was obtained via fluorination of Sr2CuO3 by XeF2 in the 100–250°C temperature range in a closed Ni container. The prepared samples exhibited a lower Tc in comparison with earlier reported values. Different samples in the Srue5f8Cuue5f8Oue5f8F system were prepared by a solid-state reaction at 220–400°C, but no formation of the oxyfluoride was detected even with an addition of xenon difluoride. These data allow one to draw conclusions on the metastability of the oxyfluoride under the conditions used. Electron microscopy and X-ray powder diffraction studies revealed large amounts of badly crystallized SrF2; this could be a reason for the small superconducting volume fraction as well as for the inhomogeneous distribution of the anions.

Atomic structure and defect structure of the superconducting HgBa2Can−1CunO2n+2+δ homologous series

Abstract The perfect as well as the defect structure of several members of the superconducting family HgBa 2 Ca n −1 CuδO 2 n +2+δ have been studied in detail by electron diffraction and high-resolution electron microscopy. Identification of the cation configuration is possible, even in the defect regions, with the help of computer simulations. The fine structure of several defects is analyzed in detail, the most common defect being the intergrowth of slabs corresponding to different n values in the title formula. In general, however, the crystals are of a high perfection, particularly for the lower n members. The occurence of double (HgO) δ layers is seen occasionally, indicating the possible existence of a material with a double mercury layer. Some of these defects are possibly related to recently discovered anomalies at 250 K in these compounds.

The superconducting HgBa2Can-1CunO2n+2+δ homologous series

Abstract Syntheses, heat-treatments, structures and the superconducting properties of the first four members of the homologous series HgBa 2 Ca n -1 Cu n O 2 n +2+δ are presented and discussed. Syntheses under high pressure seem to yield samples which contain less than 5% impurities. The structural characterization has been carried out by X-ray diffraction, electron microscopy and neutron diffraction. Heat-treatments in oxidizing atmospheres usually increase T c . The member with n = 2 does not follow this rule because the as-prepared sample ( T c = 104 K) seems to be overdoped. Heat-treatment in N 2 at 250°C for 10h induces an increase in T c to 123K. Additional heat treatment under N 2 (400°C, 18h) decreases T c to 110K. In Hg-1212 superconductivity exists in underdoped and overdoped regions.

The structures of Nd1+xBa2−xCu3O7−δ (x = 0.05, 0.22, 0.42) phases refined from X-ray single-crystal data

Abstract Crystal structures of Nd 1+ x Ba 2− x Cu 3 O 7−δ phase ( x =0.05, 0.22 and 0.42) were refined from X-ray single-crystal data (tetragonal system, space group P4/mmm, Z =1, R =0.027, 0.027, 0.025 respectively). The main differences between these similar structures deal with the cation and oxygen sites in the (Ba/NdO) layer. Joint occupation of the same site by Ba and Nd ions with essentially different ionic radii results in the splitting of that site along the c -axis and the shift of the oxygen in this layer from the ideal position at the four-fold axis to achieve appropriate coordination for the smaller rare earth cation. These changes may be the structural reason for the suppression of superconductivity in R 1+ x Ba 2− x Cu 3 O 7−δ (R is rare earth) phases.