B.A. Hunter

Structure, doping and superconductivity in HgBa2CaCu2O6+δ (Tc ⩽ 128 K)

Abstract We have studied the defect structure and superconducting propertiess of HgBa2CaCu2O6+δ by neutron powder diffraction, AC susceptibility and DC resistivity. An as-synthesized sample has an onset Tc of 128 K, the highest yet reported for this material. The critical temperature decreases slightly (126 K) after oxygenation, and drops to 92 K after reduction in flowing argon. Neutron diffraction data give evidence that the primary doping mechanism in this material is provided by oxygen atoms in an interstitial position, similar to the case of the one-layer compound HgBa2CuO4+δ. The occupancy of this site varies from 0.08 (1) for the argon-reduced sample to 0.22 (1) for the oxygenated sample. Unlike the case of the one-layer compound, no evidence of an additional defect site was found in these samples. HgBa2CaCu2O6+δ has remarkably long copper-apical oxygen distances and almost perfectly flat CuO2 planes.

Pressure-induced structural changes in superconducting HgBa2Can−1CunO2n+2+δ (n = 1, 2, 3) compounds

Abstract The crystal structures of superconducting HgBa 2 CuO 4+δ and HgBa 2 CaCu 2 O 6+δ have been investigated with a pressure up to 0.6 GPa and HgBa 2 Ca 2 Cu 3 O 8+δ to 9.2 GPa by neutron powder diffraction. The compressibility along the c -axis is nearly the same for the three compounds and up to two times larger than the compressibility along the a -axis. The one-layer compound, HgBa 2 CuO 4+δ , shows the largest a -axis compressibility, while HgBa 2 Ca 2 Cu 3 O 4+δ shows the smallest compressibility. The bond compressibilities of HgBa 2 CuO 4+δ and HgBa 2 CaCu 2 O 6+δ are significantly different from HgBa 2 Ca 2 Cu 3 O 8+δ bond compressibilities. In the one- and two-layer compounds the largest bond compressibility was the Cu-O2 (apical) bond distance, while for the three-layer compound it was the Hg-O2 bond distance.

Structural study of Sr2CuO3+δ by neutron powder diffraction

Abstract Average crystal structures of superconducting Sr 2 CuO3+ δ synthesized under high pressure and nonsuperconducting Sr 2 CuO3+ δ synthesized at ambient pressure from a hydroxometallate precursor were refined from neutron powder diffraction data. A simplified model was used to fit the modulated superstructures. Both compounds have an oxygen-deficient La 2 CuO 4 -type tetragonal T structure with oxygen vacancies located in the CuO 2 planes, not in the Sr 2 O 2 layers. This result raises important questions regarding the nature of superconductivity in Sr 2 CuO3+ δ reported to be a 70 K superconductor.

Defects, defect ordering, structural coherence and superconductivity in the 123 copper oxides

Abstract We discuss the influence of oxygen defects and defect ordering on superconductivity in 123 compounds. Because of the complexity of the high-temperature phase diagram, the synthesis history can significantly influence the properties. Room-temperature annealing experiments show that defect ordering on a short length scale can have a remarkable affect on superconductivity. We propose that structural coherence in the CuO 2 planes, which is present only if there is orthorhombic symmetry on at least a short length scale, is a requirement for superconductivity in these compounds.

Multiple defects in overdoped Tl2Ba2CuO6+δ: effects on structure and superconductivity

Abstract It is shown that the superconducting and structural properties of Tl 2 Ba 2 CuO 6+ δ depend on at least two defects. The Tl/Cu ratio used during synthesis determines the concentration of a defect that is likely to be Cu substitution on the Tl site, based on previously published work that proves the existence of such a defect. The structure exhibits a smaller orthorhombic strain, or tetragonal symmetry, for high concentrations of this defect. T c tends to be lowered by this defect. An oxygen interstitial defect in the TlO double layer has a more pronounced effect on T c , allowing the compound to be overdoped as the concentration of this defect increases. Features in the diffraction data also suggest the existence of a third defect which may involve the filling of oxygen vacancies at an oxygen site in the structure during early stages of oxygenation.

Anomalous behavior of the pressure dependence of lattice constants in Tl2Ba2CuO6+x

Abstract The crystal structure of Tl 2 Ba 2 CuO 6+ x has been investigated as a function of pressure and temperature under hydrostatic conditions using neutron powder diffraction. The crystal structure at 0.609 GPa and 60 K depends systematically on the path in pressure-temperature space by which these conditions are achieved. Applying pressure at room temperature and then cooling produces different changes in the structure than cooling first and then applying pressure. Our results support those of Sieburger and Schilling [1] who reported that the T c at high pressure for this compound depends markedly on whether the sample is pressurized at room temperature or low temperature. We speculate that this unusual behavior may be associated with differences in the mobility of interstitial oxygen defects between room temperature and 60 K and that applying pressure at room temperature changes the state of the sample through such processes as defect ordering, while changing the pressure at low temperature does not.

Suppression of superconducting transition temperature in orthorhombic La2-xCaxCuO4

Abstract The calcium solid-solubility limit in La 2- x Ca x CuO 4 has been extended from x ≈0.10 for air-synthesized material to x =0.2 using synthesis at an oxygen pressure of 600 atm and 1110°C. The tetragonal-to-orthorhombic transition occurs at higher temperatures, or larger doping levels, than for the isostructural Sr-substituted material. The superconducting transition temperature increases with doping to 34 K at x =0.15 and is suppressed for x>0.15, similar to the Sr-substituted material. Comparison of structural and superconducting properties for Ca- and Sr-substituted materials indicated that: (1) both orthorhombic and tetragonal structures are superconducting; (2) at a fixed doping level, the optimum superconducting properties (the highest T c ) are found for a perfectly flat and square CuO 2 plane; (3) T c is suppressed by the distortions of the CuO 2 plane or by overdoping resulting in a maximum of T c found at x =0.15 under ambient conditions.

New 1212 structural analog of YBa2Cu3O7 : the Sr-based superconducting copper oxide CuSr2-xLaxYCu2O7+δ with Tc∼60 K

Abstract We have prepared a new superconducting copper oxide, CuSr2−xLaxYCu2O7+δ (x∼0.4, δ∼0.2) with Tc∼60 K. While the parent compound, x=0, can be prepared only at very high oxygen pressures (P(O2) ≥ 20 000 atm), the tetragonal (a=3.8027, c=11.3806 A ) , La-doped, material can be obtained at relatively moderate pressures, P(O2)=600 atm at 1180°C. Rietveld refinements of neutron powder diffraction data show that La and Sr occupy the “Ba” site but with slightly different positions. The displacement pattern of neighboring oxygen atoms suggets that the La defects pair to form dimers perpendicular to the basal plane.

Pressure effects on Tc and structure in overdoped TlSr2CaCu2O7−δ

Abstract The effects of pressure on Tc and structure in overdoped TlSr2CaCu2O7−δ have been investigated. For superconducting samples dTc/dp is small and negative with d ln Tc/dp varying from −0.016 GPa−1 to −0.006 GPa−1. There is no significant difference in the lattice compressibilities for superconducting and nonsuperconducting metallic samples suggesting that they are not sensitive to differences in defect concentration or doping level. For the apical CuO(2) bond, in contrast, the compressibility is about three times as large as that for the c-axis lattice parameter in the nonsuperconducting simple, while it is nominally equal to the c-axis compressibility for the superconducting sample. This may imply a difference in the amount of carriers transferred into the CuO2 plane under pressure.

High oxygen pressure synthesis of new copper oxide superconductors

The unique coordination of the copper ions in +1, +2, and +3 oxidation states, which are stable in a range of oxygen partial pressure. 10−6<P(O2)< 103 atm, makes possible the formation of a wide variety of distinct structures. By controlling the oxygen pressure during the synthesis and annealing, the distribution of metal and oxygen ions can be modified on an atomic scale to optimize the structural and electronic properties. We present several examples of compounds for which the critical structural elements for superconductivity, i.e., the perfectly ordered CuO2 planes, have been obtained and doped with holes by means of high oxygen pressure,P(O2)>1 atm. These materials, La2CuO4+δ, La2−xCaxCuO4, and YSr2−xLaxCu3O7+δ, offer a unique opportunity to study the relationship between superconductivity and structural, magnetic, and chemical properties.