M. Slaski

Copper oxide superconductors containing sulphate and phosphate groups

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 ).

Superconductivity up to 64 K in the copper oxyfluorides Sr2−xAxCuO2F2+δ(A = Ca, Ba) prepared using NH4F as a fluorinating reagent

Abstract We report here a simple and efficient synthetic route to high-temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2 CuO 3 with NH 4 F at relatively low temperatures produces superconducting Sr 2 CuO 2 F 2+δ ; this simple preparation route eliminates the need for F 2 gas. Importantly, as-synthesised samples from the NH 4 F route appear to have the optimally doped value of the superconducting transition temperature, T c , of 46 K. Moreover, Ba doping for Sr results in an increase in T c , for both F 2 gas and NH 4 F routes, to a maximum value of 64 K for Sr 1.4 Ba 0.6 CuO 2 F 2+δ ; this represents the highest ever recorded value for a material with the confirmed La 2 CuO 4 structure.

An improved route to the synthesis of superconducting copper oxyfluorides Sr2−xAxCuO2F2+δ (ACa, Ba) using transition metal difluorides as fluorinating reagents

Abstract We report here a simple and efficient synthetic route to high temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2− x A x CuO 3 (ACa, 0 ⩽ x ⩽ 2.0; ABa, 0 ⩽ x ⩽ 0.6) with the transition metal difluorides, CuF 2 , ZnF 2 , AgF 2 , NiF 2 , at relatively low temperatures (225–250°C) produces superconducting Sr 2− x A x CuO 2 F 2+δ with a maximum T c of 64 K (for ABa, x = 0.6). This simple preparation route eliminates the need for F 2 gas, and at the same time produces negligible (Sr/A)F 2 impurity, unlike the corresponding fluorination with NH 4 F. Using this route the synthesis of the oxyfluoride, Ba 2 CuO 2 F 2+δ , is also reported for the first time. These studies and the examination of the fluorination of other cuprate systems have shown that this is a powerful and versatile fluorination route.

Fluorination of mercury-based high-temperature superconductors

Abstract There are numerous reports of cationic substitutions on the mercurocuprate very high-temperature superconductors of general formula HgBa 2 Ca n−1 Cu n O 2 n +2+δ . Such investigations have been carried out in an attempt to mimic, within chemical substitution at room pressure, the spectacular effect of external pressure, which has raised the superconducting transition temperature, T c , to record-high values above 160 K. In the present work, we report the post-synthetic incorporation of elemental fluorine as an interstitial anion into the first five congeners of the mercurocuprate family of superconductors. The effect of fluorine incorporation by this route appears to mimic the more usual oxygenation process in giving maximum T c values of 96, 128 and 135 K for the single, double and triple CuO 2 -layer materials, respectively, while the T c of the five CuO 2 -layer compound are increased to an extent only previously observed by high-pressure oxygenation. We comment upon a derived linear correlation of T c with the crystal a -lattice parameter and compare this behaviour against that of high-pressure studies. Interestingly, it appears that intrinsic ‘chemical factors’ dictating the average copper oxidation state have the more marked effect upon T c than the application of external hydrostatic pressure.

Superconductivity in the system (Y, Ca, Sr) (Ba, Sr)2 (Cu3−z(SO4)zOx

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.

Synthesis and structure of the calcium copper oxyfluoride, Ca2CuO2F2+δ

The synthesis of the copper oxyfluoride, Ca2CuO2F2+δ, by reaction of Ca2CuO3 with elemental F2 at low temperatures (240 °C) is reported. Preliminary structure refinement of powder neutron diffraction data suggests that Ca2CuO2F2+δ is structurally related to Nd2CuO4, rather than to the La2CuO4 structure observed for the related copper oxyfluoride, Sr2CuO2F2+δ. The proposed change in structure type is attributed to the decrease in the size of the alkaline-earth cation. Madelung energy calculations support the view that fluorine insertion causes O/F interchange to give CuO2 planes with the fluorine atoms located between the Ca bilayers. Although the structure could potentially support superconductivity, no evidence for superconductivity has so far been found in this phase down to 4 K.

THE MAGNETIC AND SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF SOME PARAMAGNETIC SALTS CONTAINING METALLOCARBABORANE ANIONS

Abstract A series of air-stable salts containing the paramagnetic metallocarboranes [MCb2]− (M = Fe, Ni; Cb2− = 7,8-C2B9H112−) has been prepared and their magnetic properties investigated. The paramagnetic salts [FeIIITp2][FeIIICb2] Tp = HB(C3H3N2)3, [FeIIIC6H4(O)C(Me) = N(CH2)2NHCH22][FeIIICb2], [C5H5NMe][FeIIICb2], [4-MeC5H4NMe][FeIIICb2], [4-PhC5H4NMe][FeIIICb2], [C9H7NMe][FeIIICb2], [FeIIITp2][TCNQ] and [FeIIICp2][FeIIICb2] have been isolated. Magnetic susceptibility measurements show that these salts exhibit typical paramagnetic behaviour with no evidence of cooperative magnetic interactions over the temperature range 6–300 K. The salts [E-Fc-CH = CH-p-C5H4NMe][MIIICb2] (M = Fe, Ni, Co) were also synthesised and their second-order nonlinear optical properties assessed using the Kurtz powder test. Only the cobalt-containing complex gave detectable, though weak, second harmonic generation of 0.01 times urea at 1.907 μm. The salt [C5H5NMe][FeIIICb2] exhibits SHG with an efficiency of 0.5 times urea at 1.907 μm and 0.1 at 1.064 μm. Mass spectrometric and 13C1H NMR studies of the products formed by the degradation of [Co(Cb)2]− by base in the presence of Co2+ ions has provided no evidence for the formation of higher oligomers than the previously reported [Co(Cb)2(C2B8H10)]2− and [CoCb(C2B8H10)2Co]3−. The structures of [FeIIICp2*][NiIIICb2] (Cp* = η5-C5Me5) and [4-MeC5H4NMe][FeIIICb2] have been determined by single crystal X-ray diffraction studies.

Fluorination of the Ruddlesden–Popper type cuprates, Ln2–xA1+xCu2O6–y (Ln=La, Nd; A=Ca, Sr)

The low-temperature (200–350 °C) fluorination of the Ruddlesden–Popper type cuprates, Ln2–xA1+xCu2O6–y(Ln=La, Nd; A=Ca, Sr) using F2 gas, CuF2 , and NH4F is reported. The incorporation of large levels of fluoride ions is observed for each of these fluorinating agents. The general characteristics of each method are discussed, and it is shown that for this system, fluorination mainly occurs by insertion of fluorine for reaction with F2 , substitution of fluorine for oxygen for NH4F, and a mixture of the two processes for CuF2 . For A=Sr, it is assumed that fluorine inserts mainly between the two CuO2 layers, since large expansions of the unit cell along the c direction are observed. No evidence for bulk superconductivity has so far been observed after fluorination.

The AC magnetic response of YBa2Cu3O7 with Y2BaCuO5 inclusions and of single-crystal YBa2Cu3O7. A comparative study

Abstract We have measured the AC magnetic permeability response function μ ′ μ + iμ ″ in melt-process-melt-growth YBa 2 Cu 3 O 7 (Y123) with Y 2 BaCuO 5 (Y211) inclusions (MPMG) and in single-crystal YBa 2 Cu 3 O 7 (SC) in applied magnetic fields up to 8 T, oriented either parallel or perpendicular to the crystalline c -axis. The AC response of the two samples has been mapped out as a function of temperature, AC field amplitude and frequency, DC applied field, and field orientation. The peak in the lossy part of the susceptibility μ″ ( H, T ) is used to probe the flux dynamics in the vicinity of the “irreversibility” line (IL), defined here as the locus of μ″ maxima in the H-T plane. For H ∥ c , the IL ofthe MPMG sample exists at higher temperature and field than the SC sample, indicating that the Y211 insulating inclusions are responsible for additional strong pinning of the vortex structure. For H ⊥ c , the ILs, determined by the peak position of μ″ ( H, T ), are the same for both samples. However the amount of dissipation measured by the temperature width of μ″ ( H, T ), is greater for the MPMG sample for this orientation implying that the Y211 inclusions reduce the vortex pinning in this orientation. The precise natures of these effects due to the Y211 inclusions are still unknown. The frequency dependence and non-linear response differ for the two field orientations. In the range of available parameters and variables we find the system to be in the solid vortex phase for H ⊥ c since there is strong H AC dependence and only weak frequency dependence on the temperature position of the loss peak. For H ∥ c , however, the loss peak depends only weakly on H AC and strongly on the frequency.

Observation of superconductivity (Tc = 50 K) in a new tetragonal alkaline-earth cuprate Sr0.8Ba1.2CuO3+δ, synthesised at ambient pressure

Abstract The ambient-pressure synthesis of a new tetragonal alkaline-earth superconducting cuprate, Sr0.8Ba1.2CuO3+δ, from a cupro-oxycarbonate is reported. Magnetic-susceptibility measurements show the presence of a superconducting transition ∼50 K in a post-annealed sample. The crystal structure, refined from time-of-flight powder neutron-diffraction data was found to have an oxygen-deficient La2CuO4-type tetragonal T structure ( a = 3.8988(3) A and c = 12.815(3) A ) with oxygen vacancies located within the CuO2 planes. Ordering of these oxygen vacancies is responsible for the observation of a superlattice in both neutron- and electron-diffraction measurements. An interpretation of the electron-diffraction patterns suggests that the superlattice in Sr0.8Ba1.2CuO3+δ and also in the isostructural superconductor Sr2CuO3+δ are of an identical nature.