Showing papers by "Colin Greaves published in 1993"
TL;DR: In this paper, the incorporation of sulphate and phosphate groups into the YBa 2 Cu 3 O 7−δ structure, to form materials such as YSr 2 Cu 2.
Abstract: Following observations that carbonate anions can be substituted into perovskite structures containing Cu to yield superconducting phases, the possibility that other oxy-anions may behave similarly has been explored. Here we report the incorporation of sulphate and phosphate groups into the YBa 2 Cu 3 O 7−δ structure, to form materials such as YSr 2 Cu 2.79 (SO 4 ) 0.21 O 6.16 . The partial replacement of Y 3+ ions by Sr 2+ or Ca 2+ ions injects holes into the CuO 2 layers and results in a change from antiferromagnetic to superconducting behaviour with T c (onset) up to ≈60 K. These materials contain not only new charge reservoir layers but also novel superconducting CuO 2 sheets with both 4-coordinate Cu (as in Nd 2 CuO 4 ) and 5-coordinate Cu (as in YBa 2 Cu 3 O 7 ).
71 citations
TL;DR: In this article, the authors describe the superstructures which consist of a regular alternation of the two types of symmetry related MO-chains leading to period doubling along the direction normal to the chains.
62 citations
TL;DR: In this paper, the effects of partially replacing Cu by borate anions in the system YSr 2 − x Ba x Cu 3− z (BO 3 ) z O y (YSr 2− x Ba X Cu 3 − z B z O 7−δ ) have been examined.
Abstract: The effects of partially replacing Cu by borate anions in the system YSr 2− x Ba x Cu 3− z (BO 3 ) z O y (YSr 2− x Ba x Cu 3− z B z O 7−δ ) have been examined. The optimum substitution level to give single-phase samples was found to be z≈0.5. Borate-substituted samples of this composition were found to be non-superconducting as prepared, but superconductivity could be induced by the partial replacement of Y by Ca (for samples with 0.5≤×≤1.5), or by high oxygen pressure treatment (for 1.0≤×≤1.5). The highest T c onset recorded for a single-phase sample was 55 K for Y 0.85 Ca 0.15 Ba 1.5 Cu 2.5 B 0.5 O 7 annealed in high pressure oxygen. The unit cell symmetry changes from orthorhombic for the Ba-free phase ( x =0)to tetragonal at higher Ba contents, which appears to be associated with a randomisation of the orientations of the borate groups.
35 citations
TL;DR: In this article, the effects of partial substitution of Ba for Sr on the superconducting transition in the sulphate containing materials [Y0.74Ca0.16Sr0.12]Sr2Cu2,78(SO4)0.22O6.
Abstract: The effects of partial substitution of Ba for Sr on the superconducting transition in the sulphate containing materials [Y0.74Ca0.16Sr0.1]Sr2Cu2.78(SO4)0.22O6.12 and [Y0.84Sr0.16]Sr2Cu2,78(SO4)0.22O6.12 have been investigated. As the Ba content was increased, the superconducting transitions became sharper and Tc onset increased to reach a maximum of 78 K. The cell symmetry was also observed to change from orthorhombic to tetragonal, which is thought to be associated with randomization of the orientations of the sulphate groups in the structure.
26 citations
TL;DR: In this paper, it was shown that the phase YSr 2 Cu 2.8 Cr 0.2 O 7 can be made superconducting by substitutions of either Ca at the Y site or Ba at the Sr site.
Abstract: It is shown that the phase YSr 2 Cu 2.8 Cr 0.2 O 7 can be made superconducting by substitutions of either Ca at the Y site or Ba at the Sr site. The variation of T c for Y 1− y Ca y Sr 2−χ Ba χ Cu 2.8 Cr 0.2 O 7 has been determined for a range of χ and y , and the superconducting properties have been correlated with structural changes determined from time-of-flight neutron powder diffraction data. The hole densities on the superconducting CuO 2 layers, deduced from structural data and the oxygen content, suggest that for this system the critical hole density for superconductivity, and optimum hole density, increase significantly as the Sr:Ba ratio is raised. The relevance of this to previous discussions of the variation of T c with composition in Y(Ba,Sr) 2 Cu 3 O 7−δ is highlighted.
20 citations
TL;DR: In this paper, an incommensurate modulation is attributed to the ordering of b-oriented SO 4-rich chains in the Cu(1)-S-O-layer, described in terms of an SO 4 -concentration wave.
Abstract: YBCO-based materials containing SO 4 -tetrahedra centered on the Cu(1)-sites of the CuO-chain plane have been examined by means of electron diffraction and high resolution electron microscopy. An incommensurate modulation is observed and attributed to the ordering of b -oriented SO 4 -rich chains in the Cu(1)-S-O-layer, described in terms of an SO 4 -concentration wave.
19 citations
TL;DR: In this paper, the synthesis of Na containing Nasicon-type phases of the form NaxM′(II)xM″(III)2-x(SO4)3-y(SeO4)y (M′ = Mg, Zn, Cd, Mn; M″ = Al, Cr, Fe, In, Yb; 0 ≤ x ≤ 1.75, 0 ≤ y ≤ 3) is reported.
15 citations
TL;DR: In this article, a neutron powder diffraction investigation has been carried out of the magnetic structure of Fe 60 Ga 17 As 23 at 1.5 K. The crystal structure is isotypic with the hexagonal Ni 2 In type structure.
7 citations
TL;DR: In this paper, partial substitution of oxy-anions for the Cu in the ‘chain’ sites in the system [Y/Ce]2Sr2-xBaxCu3O9-y has been examined, and the successful substitution of borate, phosphate and sulphate groups at these sites has been achieved.
Abstract: The partial substitution of oxy-anions for the Cu in the ‘chain’ sites in the system [Y/Ce]2Sr2–xBaxCu3O9–y has been examined, and the successful substitution of borate, phosphate and sulphate groups at these sites has been achieved. The highest substitution level (80% replacement of the ‘chain’ Cu sites) occurred for borate ions and was associated with a significant reduction in the unit-cell c-axis length. For phosphate and sulfate groups, however, the maximum substitution level corresponded to only 25% of the ‘chain’ Cu sites. None of the samples prepared showed any evidence for superconductivity.
6 citations
TL;DR: The phase YSr[sub 2]Cu[sub2]CoO[sub 7] (orthorhombic, Ima2) as mentioned in this paper has a component of disorder associated with the oxygen sublattice, and substitutions of small amounts of Ba for Sr, or Cu for Co, have been shown by powder diffraction methods to increase the level of disorder.
5 citations
TL;DR: A superstructure of corner-linked oxygen tetrahedra coordinating the A1O ions is shown in the AlO layer as mentioned in this paper, and the weakness of corrrelations between adjacent AlO layers, and the presence of two orientational variants, may explain why this superstructure was overlooked by neutron diffraction investigations.
Abstract: Neutron-diffraction experiments have shown that the title compound A1-2212 has an average tetragonal structure closely related to the quasi-tetragonal YBa 2 Cu 3 O 7 structure, but with the Y layer replaced means of electron diffraction and high-resolution electron microscopy have refined the existing model. A superstructure of corner-linked oxygen tetrahedra coordinating the A1O ions is shown in the AlO layer. The weakness of corrrelations between adjacent AlO layers, and the presence of two orientational variants, may explain why this superstructure was overlooked by neutron-diffraction investigations.
TL;DR: The (Nd,Ce)2Sr2Cu2GaOx (M=Ga,Ta) as discussed by the authors is an orthorhombic supercell, which consists of two types of zigzag chains of tetrahedra, which are distinguished by the direction of atomic displacements.
Abstract: The (Nd,Ce)2Sr2Cu2MOx (M=Ga,Ta) are structurally related to YBCO with yttrium replaced by a fluorite-type block of composition (Nd,Ce)2O2 , and barium by strontium. Neutron powder diffraction studies on these materials have been performed to compare structural order within the layers containing the M atoms. For M=Ga, long-range ordering of the corner-linked GaO4 tetrahedra along the (110) direction of the simple perovskite cell results in an orthorhombic supercell. These zigzag chains of tetrahedra are of two types which are distinguished by the direction of atomic displacements in the chains; equal quantities of both types occur in (Nd,Ce)2Sr2Cu2GaOx. When the tetrahedral Ga ions are replaced by octahedral Ta, the oxygen positions in the basal plane are fully occupied, and the tetrahedral chains are converted into octahedral layers to give a smaller tetragonal unit cell. Cooperative rotation of the TaO6 octahedra within these layers has been confirmed.
04 Apr 1993-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: The incorporation of strontium and the variation of oxygen content in compounds of the type Eu 2 SrCu 2 O 6 can alter the cationic oxidation states and influence the conductivity of the materials.
Abstract: The incorporation of strontium and the variation of oxygen content in compounds of the type Eu 2 SrCu 2 O 6 can alter the cationic oxidation states and influence the conductivity of the materials. The value of δ in the semiconducting compound Eu 1.3 Sr 1.7 Cu 2 O 6−δ has been estimated by thermogravimetric analysis as 0.35. 151 Eu Mossbauer spectroscopy shows that europium is present as Eu 3+ . Europium L III -edge and copper K-edge EXAFS are consistent with the ordered anionic vacancies creating a one-dimensional tunnelled oxygen sublattice and with the partial occupation by strontium of some of the lanthanide sites.