Superconducting Properties of Calcium Substitution in Barium Site of Porous YBa2Cu3O7 Ceramics

The influence of calcium substitution at the barium site of porous Y(Ba1-xCax)2Cu3Oδ (x= 0.00, 0.10, 0.20 and 0.30) samples prepared via solid-state reaction method have been investigated. The structure, morphology, critical temperature and critical current were determined by x-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and four-point probe method, respectively. Generally, the curves of normalized resistance for all samples displayed normal metallic behavior above Tc onset. The Tc zero was between 84 K and 71 K with increase in Ca concentration corresponding to a small change of carrier concentration. The critical current density, Jc decreases with increase in Ca concentration. The highest Jc of 2.657 A/cm2 at 50 K was obtained in Ca-free porous YBCO which is higher than that of Ca-free non porous YBCO. Further substitution of Ca at Ba site decreased Jc monotonously. The increase of Ca concentration decreased the volume of unit cell but the crystallographic structure remains in the orthorhombic form where a≠b≠c. The grains are highly compacted and randomly distributed while the grain size decreased as the Ca concentration increased.

Die Synthese und Kristallstruktur von Ba2Mn(B3O6)2 und Ba2Co(B3O6)2 / Synthesis and Crystal Structure of Ba2Mn(B3O6)2 and Ba2Co(B3O6)2

Single crystals of Ba2Mn(B3O6)2 (I) and Ba2Co(B3O6)2 (II) were prepared by using a B2O3 flux technique, Ba2Mn(B3O6)2 in an argon atmosphere. X-ray investigations on single crystals of both compounds yield the space group C3i2R3̄ (No. 148) with 3 formula units in the cell of dimensions (I) a = 711.01(4), c = 1694.90(3) pm and (II) a = 707.2(2), c = 1673.8(1) pm. The crystal structure is built by nearly planar (B3O6)3- rings and contains one ninefold coordinated Barium site. Mn(I) and Co(II) are octahedrally coordinated by oxygen. The compounds are isotypic to B2Ca(B3O6)2.

The Nature of Superconductivity in Ba1-XKXBiO3

Superconductivity in potassium-doped BaBiO3 was first observed by Mattheiss et al. in 1988 and the structure of the superconducting material was determined by Cava et al. The material crystallizes in a simple perovskite structure with potassium substituted on the barium site as shown in Figure 1. Unlike the layered copper-oxide materials, this compound is simple cubic and thus isotropic in the superconducting state and shows no magnetism. Band theory calculations show that the electronic structure is simple, with conduction bands composed of Bi(sp)O(p) hybridized orbitals and, unlike the copper-oxide materials, there are no d electrons at the Fermi surface or involved in the superconductivity. Photoemission and inverse-photoemission agree well with the theoretical calculations and show metallic behavior with a low density of states at the Fermi level consisting of Bi-O sp hybrids. This material is structurally and electronically simple compared to the copper-oxide compounds and, therefore, should be much easier to investigate experimentally and understand theoretically. This article discusses recent results concerning the structural properties of the material and describes several experiments that give information on the superconducting state.