J.J. Capponi

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.

A study of the Cu-O chains in the high Tc superconductor YBa2Cu3O7 by high resolution neutron powder diffraction

Abstract The structure of orthorhombic YBa2Cu3O7 was refined in the temperature interval 5–320K, to an accuracy higher by a factor of about two compared to previous refinements. No major structural change is observed at the onset of superconductivity but the lattice dimensions and some of the structural parameters show small anomalies near 90K and 240K. All vibrational amplitudes are normal except for those of the 04 atoms across the Cu-O chain, which are large. Good agreement with the data is obtained by assuming that the 04 atoms are located on potential minima away by at least 0.08A from the chain axis at all temperatures. Annealing near 240K to look for a possible order-disorder transition revealed no structural changes.

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.

A note on the symmetry and Bi valence of the superconductor Bi2Sr2Ca1Cu2O8

Abstract The sub-structure of Bi 2 Sr 2 Ca 1 Cu 2 O 8 can best be described on a non-centric orthorhombic A2aa cell, which permits the oxygen in the BiO plane to move off the center of the Bi square to approach to within 2.2 A of a pair of Bi atoms. Each Bi then has two close oxygens within the BiO plane, and a third at 2.12 A connecting to the CuO layer. The new structure permits the apparent Bi valence to approach 3+ with a more reasonable Bi-O co-ordination than for earlier approximate structural models. This model satisfies the high resolution neutron data of Bordet et al., while agreeing in part with the co-ordination proposed on chemical grounds by von Schnering et al.

Oxygen vacancy ordering in Ba2YCu3O7−x around x= 0.5

Abstract Electron diffraction photographs of the title samples show extra diffuse scattering in the reciprocal a * b * plane, which can be attributed to the partial ordering of the oxygen vacancies. A unit cell 2a c x a c x 3a c (a c being the basic perovskite subcell parameter; a c ≅3.8A is proposed together with a structural model.

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.

Structures of superconducting Ba2YCu3O7-ϖ and semiconducting Ba2YCu3O6 between 25°C and 750°C

Abstract The structure and copper valence states of the 100K superconductor, Ba 2 YCu 3 O 7 have recently been determined by neutron powder diffraction between 5K and 300K 1 , and at room temperature 2 . This ′oxygen deficient orthorhombic perovskite structure′, now independently confirmed 3,4 , is well ordered and stochiometric, and contains one Cu +++ and two Cu ++ atoms distributed over two square planar oxygen co-ordinated sites. In the a-b plane CuO 4 squares are linked by their oxygen corners to form infinite sheets, while along the b-axis CuO 4 squares form infinite chains on the second Cu-site. In this paper we show that when Ba 2 YCu 3 O 7 is heated above room temperature, it progressively loses all oxygen from the O4 sites on the b-axis chains, while the remaining oxygen sites remain fully occupied. Near 700°C it becomes tetragonal 5 , with the P4/mmm Ba 2 YCu 3 O 6 structure obtained by X-ray diffraction at room temperature 6 . This material is well ordered, stochiometric and stable when cooled under vacuum. The valence of Cu2 in the a-b sheets remains close to 2, while that of Cu1 in the b-axis chains falls to near 1. The valence of copper in these chains can perhaps fluctuate, since the vibration of O4 out of the chain, which we observed even at 5K 1 , implies a reduction of the valence charge on this copper site. We have observed similar large amplitude oxygen vibration at low temperature in La 2 CuO 4 .

High-temperature oxygen defect equilibrium in superconducting oxide YBa2Cu3O7-x

Abstract The high-temperature superconductor YBa 2 Cu 3 O 7-x has been characterized by thermogravimetry in the 600–1100 K range under oxygen pressures from 10 -3 to 1 atm. The compound reversibly exchanges oxygen in the 0 2 ) vs. 1/T curves are linear with activation energies between 1.5 and 1.65 eV. These values are consistent with a simple interaction model involving the oxygen sites in the basal ab plane of the orthorhombic or tetragonal structure.

Preparation and neutron diffraction of superconducting “tetragonal” and non-superconducting orthorhombic Tl2Ba2Cu1O6

Abstract Many different samples of Tl 2 Ba 2 Cu 1 O 6 have been prepared by different heat treatments at relatively low temperature. The material as first prepared is clearly orthorhombic and non-superconducting, but on annealing and quenching, the samples become almost tetragonal with high T c . Various T c values between 3+ O 2- atoms are missing, creating the electron holes apparently necessary for superconductivity. The main difference between the various samples appears to be that the non-superconducting material is orthorhombic, with a well ordered superstructure, while the superconducting material is pseudo-tetragonal, with disordered oxygen within the TlO plane, as for the higher superconducting members of the Tl 2 Ca n Ba 2 Cu n +1 O 2( n +3 ) series. Superconductivity then appears to depend on the precise structural arrangement, and not just on stoichiometry and the number of electron holes.

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.