C. Chaillout

Structure of the 100 K Superconductor Ba2YCu3O7 between (5 ÷ 300) K by Neutron Powder Diffraction

Recently Siegrist et al. proposed a structure for a high-Tc superconductor Ba2YCu3O(9-∂) based on an orthorhombic (a = ap, b = ap, c = 3ap) perovskitelike model containing Ba and Y cations ordered over the A-sites of the ABO3 structure. This ordering is responsible for the tripling of the c-axis. Half of the oxygen vacancies (at z = 1/2) are ordered, while the other half (at z = 0) are disordered over two sites. Using profile refinement of high-resolution neutron powder data at six different temperatures between 5 K and 300 K, we have refined the structure of a pure and well-characterised powder sample with onset of superconductivity at 100 K. At all temperatures we confirmed the previous model except that in our structure, all oxygen vacancies are ordered. Two-thirds of the copper cations have a pyramidal coordination and one-third has a square coordination. In our structure all squares are parallel to the (b, c)-plane, while in the one reported by Siegrist et al. the squares are disorderedly parallel to either the (a, c)- or (b, c)-plane. The difference between the two models is probably due to the fact that the single crystal used in the earlier work was highly twinned. Empirical calculations of the copper valences show that the Cu+++ cations are almost equally distributed over the two sites. No structural change has been detected at the transition.

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.

The crystal structure of superconducting La2CuO4.032 by neutron diffraction

Abstract The crystal structure of superconducting (37.5 K) La2CuO4.032 has been refined from single-crystal neutron-diffraction data at room temperature and 15 K. At both temperatures it exhibits the orthorhombic symmetry Cmca. The extra oxygen O4 atoms occupy the special positions ( 1 4 y 1 4 with y=0.243 at 15 K). They are located between two successive LaO layers and are surrounded by distorted cubes built up of two interpenetrated tetrahedra, one comprising four La atoms and the other four O1 atoms (the apical oxygen atoms of the CuO6octahedra). The O4 insertion causes a 0.75 A displacement of 4.8% of the O1 atoms towards new O3 positions (x=0.030(5), y=0.182(2), z=0.100(5)). From the refined values of the occupancy factors at 15 K, it is deduced that for each extra O4 three O1 are displaced to O3 with one short O3–O4 distance of 1.64(3) A. This value indicates the formation of a strong O-O covalent bond of peroxide type with a formal 2-valence. Since the La and Cu sublattices have been found to be fully occupied and the doping does not change the oxygen charge, the La, Cu and O sublattices have the formal valences 6+, 2+, and 8−, respectively. However, the increase in La coordination and the consequent La-O distance readjustment indicate, when compared to the undoped compound structure, that a charge transfer occurs in La2CuO4.032, with the excess positive charge going either to the La or to the O sublattice. In the latter case it would correspond to the formation of holes in the O 2p band.

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.

Two-phase structural refinement of La2CuO4.032 at 15 K

Abstract A two-phase refinement of a crystal for the compound La2CuO4+δ (δ=0.032), based on the neutron diffraction data collected at 15 K with the D9 diffractometer at ILL, using ǂ=0.48 A , has been carried out. One phase (30%) consists of stoichiometric La2CuO4 domains, while the other (70%) of oxygen-rich La2CuO4.048 domains. The percentages of each phase, which have been refined together with the other parameters (scale factor, positional parameters, thermal factors and occupancy factors of the oxygen atoms), agree very well with the value determined from χAC measurements. The La2CuO4.048 structure is essentially the same as the average structure reported in ref. [1], the only difference being the oxygen content which is found to be δ= δ 0.70 . The extra oxygen, O(4), is found to be in between two LaO layers in a similar position as the oxygen atoms located between two Nd layers in the N2CuO4 structure. The insertion of extra oxygen causes the displacement of some of the oxygen O(1) towards the O(3) positions. Different models are proposed for the distortion induced by this insertion according to the experimental value, 3.3 [6], found for the ratio of the amount of O(3) to that of O(4). If this ratio is assumed to be 3 and the O(3) atoms are localized about the insertion, then the formation of a short O(4)-O(3) bond would occur. The models not requiring the formation of the short bond correspond to ratios of 2 or 4. In the latter case there would be four displaced O(1), but due to the rigidity of the oxygen octahedra only two O(3) would be bonded to O(4), the other two being the apically opposite oxygen atoms of the same octahedra.

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.

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 .

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.

A new HTSC family: the copper analogs of the single-layer Hg or Tl copper oxide superconductors

Abstract A new family of superconducting oxides has been obtained at high pressures in multiphasic samples within the Ba-Ca-Cu-O system. It corresponds to the substitution by copper of the Tl or Hg atoms in the so-called single-layer superconductors. At least four members have been detected, corresponding to n =3−6, in CuBa 2 Ca n -1 Cu n O 2 n +2+ x . T c values of up to 120 K have been measured.

A family of non-stoichiometric phases based on Ba2YCu3O7−δ (0≤δ≤1)

Abstract In the light of recent electron diffraction evidence a reappraisal is made of the problem of non-stoichiometry by oxygen deficiency in high-temperature superconducting Ba 2 YCu 3 O 7−δ . On the basis of the ordered states of the oxygen vacancies detected by that technique, a family of phases is proposed as a better description of oxygen stoichiometry and compositional variations in that material. Five phases appear to exist within the usually considered range 0≤δ≤1. All of these phases, which can be considered as consecutive members of a homologous series differing in oxygen content along the CuO rows running along the b axis, are themselves non-stoichiometric and appear to correspond to well differentiated regions in the characteristic T c versus composition phase diagram found by several authors.