K. Donnelly

Hydrogen absorption and desorption in Nd2Fe17 and Sm2Fe17

The uptake of hydrogen by Nd2 Fe17 and Sm2Fe17 has been monitored in a thermopiezic analyser as a function of temperature at an initial pressure of 1 bar (105 Pa). The first stage of hydrogen absorption around 250° C yields R2Fe17Hy (R = Nd, Sm) withy ≅ 2.2; this compound retains the Th2Zn17 structure of the starting alloy but the cell volume is increased by about 3%. The Curie temperature increases from 57 to 175° C for R = Nd and from 115 to 253° C for R = Sm. A second stage of hydrogen absorption at about 600° C corresponds to disproportionation of the alloy into α-Fe + RH2−ɛ.

Superconducting EuBa2Cu3O7. A Mössbauer study

EuBa2Cu3O7 shows a sharp transition to zero resistance atTsc=85(1) K. Europium is entirely trivalent. The151Eu Mössbauer recoilless fraction in the range 4–300 K is well-fitted by a Debye model withTsc = 85(1) K. There is no anomaly around 240 K, nor atTsc, and therefore no significant change in the vibrational amplitude of the rare-earth ion. Another sample doped with 1%57Fe, shows two ferric quadrupole doublets associated with Fe3+ on Cu1 and Cu2 sites. There is no sign of any magnetic order down to 4.2 K.

Structure and morphology of EuBa2Cu3Oχ for 6⩽χ⩽7: Mössbauer spectroscopy and EXAFS of samples containing iron impurities

Abstract The ternary oxide EuBa 2 Cu 3 O χ has been studied as a function of oxygen content using thermopiezic analysis, X-ray diffraction and scanning electron microscopy. The same measurements, in addition to 57 Fe Mossbauer spectroscopy and EXAFS, have been carried out on samples where 1 or 2% of the copper is replaced by iron. The orthorhombic-tetragonal transition occurs on heating to 500°C in vacuum when χ =6.6. There is little change in the ceramic microstructure prior to the onset of vitrification at about 900°C. Mossbauer spectra are consistently fitted using four quadrupole doublets in the orthorhombic phase and two doublets in the tetragonal phase. The assignments of these doublets to iron in different oxidation states on Cu1 and Cu2 sites is based on EXAFS data and on the Mossbauer hyperfine splitting. Iron has a strong preference of Cu1 sites and the data suggest an Fe 4+ charge state. Iron on Cu2 sites is Fe 3+ .

Hydrogen absorption in single-crystal Bi2Sr2CaCu2O8 and its effect on superconductivity☆

Abstract In Bi 2 Sr 2 CaCu 2 O 8 , hydrogen absorption at 1 bar occurs in three stages starting at approximately 275°C, 500°C and 600°C. Only the first stage occurs without decomposition of the compound or a change in structure; there is an increase in cell volume of 1.1 A 3 (H atom) −1 up to approximately 2 H atoms per formula unit. The diamagnetic a.c. susceptibility is reduced by more than a factor of two with the absorption of 1.1 H atoms per formula unit, but the onset of superconductivity at 87 K is depressed by no more than 3 K. A possible explanation is that hydrogen accepts electrons from the 6p(Bi) pocket and resides in the (BiO) 2 double layers; physical decrepitation then greatly increases the surface to volume ratio thereby increasing the volume fraction lying within the penetration depth.

Production and characterisation of single crystal, superconducting Bi2Sr2CaCu2Ox

Abstract The morphology of the tetragonal phase of approximate composition Bi 2 Sr 2 CaCu 2 O x depends on melt temperature. Crystals have a superconducting transition at 90 K. The oxygen desorption characteristic, measured by thermopiezic analysis from vacuum, shows progressive oxygen loss up to 800°C and then a sharp irreversible oxygen loss in the region of the melting point. Oxygen deficient samples show a decrease in cell volume of 3.5(5) % per oxygen lost per formula unit.

Oxygen desorption effects on the superconducting properties of Bi1.8Pb0.2Sr2.2Ca1.9Cu3.0Ox

Single-phase Bi1.8Pb0.2Sr2.2Ca1.9Cu3.0Ox superconducting ceramics with Tc=107(1) K have been studied by thermopiezic analysis and thermogravimetry. Samples lose very little oxygen ( Delta x approximately=-0.1) on heating in vacuum up to the melting point at 885 degrees C. There is no observable change in lattice parameters on quenching from 850 degrees C in vacuum. On heating in hydrogen, samples appear to lose oxygen from a skin around each crystallite with the core remaining relatively unaffected. The skin is amorphous and non-superconducting and alters the weak-link superconducting nature of the ceramic to one of isolated superconducting islands. Superconductivity can be restored throughout the sample by heating in oxygen.

Hall effect in Bi‐Sr‐Ca‐Cu‐O thin films

Resistivity and Hall effect are measured in the a‐b plane for thin films of Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 superconductors prepared by laser ablation deposition. The Hall coefficient shows a maximum at 150 K for both compounds which could be associated with bipolaron formation.

The effect of hydrogen on Bi1.8Pb0.2Sr2.2Ca1.9Cu3.0Ox : a neutron diffraction study

Abstract Neutron diffraction has been used to study the effect of H2 and D2 on the high temperature superconductor Bi1.8Pb0.2Sr2.2Ca1.9Cu3.0Ox at temperatures up to 850 K. Hydrogen acts to remove oxygen inhomogeneously from the structure and as deoxygenation proceeds there is a maximum in the unit cell volume. Deoxygenation is also accompanied by the appearance of a diffuse background in the diffraction patterns for both D2 and H2 treatments. This background, which is associated with the formation of an amorphous surface layer on each crystallite increases monotonically as oxygen is removed.

Superconducting YBa2Cu3O7 prepared by arc melting and laser melting

Abstract Melting orthorhombic YBa 2 Cu 3 O 7 either in an arc furnace or with7ms laser pulses of wavelength 1.06 μ (7 × 10 3 W.cm −2 ) leads to similar, complex mixtures of oxides. In both cases the pure orthorhombic superconducting phase can be restored by an oxygen anneal. The laser melt depth (15 μ) is discussed in terms of the thermal parameters of the Y Ba Cu oxide.