Structure of the new "1201" lead cuprate superconductor

The structure of the newly discovered "1201" lead cuprate superconductor (Pb, Cu) (Sr, La)2CuO5-δ with Tc=27.5 K at onset and a shielding fraction 38% at 5 K and 10 Oe, has been determined using neutron powder diffraction. The structure is similar to the other 1201 materials TlBa1.2La0.8CuO5 and Tl0.5Pb0.5Sr2CuO5 (where the former superconductors with a Tc of K and the latter is not superconducting), belonging to the space group P4/mmm. The doping scheme in (Pb, Cu)(Sr, La)2CuO5-δ combines the doping scheme used in these two compounds, in that both the TI and Sr sites are doped. The starting stoichiometry, the refined scale factors for the impurity phases and the refined site occupancies for oxygen suggests that the stoichiometry (relative to Cu) of the superconducting phase is Pb0.60Cu0.40Sr1.08La0.92Cu04.96. Calculation of the average hole concentration in these compounds from charge summation is difficult with these compounds because the TI/PbO layers provide polarizable charge reservoirs that can participate in substantial covalent bonding and because of the probable mixed-valent nature of Tl and Pb. Nevertheless, bond valence sums calculated for the Cu ions in the CuO2 layers for the three 1201 cuprates do provide a correlation with the values of Tc or the absence of superconductivity. © 1991

Structure of the new "1201" lead cuprate superconductor

The structure of the newly discovered "1201" lead cuprate superconductor (Pb, Cu) (Sr, La) CuO with T =27.5 K at onset and a shielding fraction 38% at 5 K and 10 Oe, has been determined using neutron powder diffraction. The structure is similar to the other 1201 materials TlBa La CuO and Tl Pb Sr CuO (where the former superconductors with a T of K and the latter is not superconducting), belonging to the space group P4/mmm. The doping scheme in (Pb, Cu)(Sr, La) CuO combines the doping scheme used in these two compounds, in that both the TI and Sr sites are doped. The starting stoichiometry, the refined scale factors for the impurity phases and the refined site occupancies for oxygen suggests that the stoichiometry (relative to Cu) of the superconducting phase is Pb Cu Sr La Cu0 . Calculation of the average hole concentration in these compounds from charge summation is difficult with these compounds because the TI/PbO layers provide polarizable charge reservoirs that can participate in substantial covalent bonding and because of the probable mixed-valent nature of Tl and Pb. Nevertheless, bond valence sums calculated for the Cu ions in the CuO layers for the three 1201 cuprates do provide a correlation with the values of T or the absence of superconductivity. © 1991. 2 5-δ c 1.2 0.8 5 0.5 0.5 2 5 c 2 5-δ 0.60 0.40 1.08 0.92 4.96 2

Loading Path and Control Mode Effects During Thermomechanical Cycling of Polycrystalline Shape Memory NiTi

Loading path dependencies and control mode effects in polycrystalline shape memory NiTi were investigated using in situ neutron and synchrotron X-ray diffraction performed during mechanical cycling and thermal cycling at constant strain. Strain-controlled, isothermal, reverse loading (to ± 4%) and stress-controlled, isothermal, cyclic loading (to ± 400 MPa for up to ten cycles) at room temperature demonstrated that the preferred martensite variants selected correlated directly with the macroscopic uniaxial strain and did not correlate with the compressive or tensile state of stress. During cyclic loading (up to ten cycles), no significant cycle-to-cycle evolution of the variant microstructure corresponding to a given strain was observed, despite changes in the slope of the stress–strain response with each cycle. Additionally, thermal cycling (to above and below the phase transformation) under constant strain (up to 2% tensile strain) showed that the martensite variant microstructure correlated directly with strain and did not evolve following thermal cycling, despite relaxation of stress in both martensite and austenite phases. Results are presented in the context of variant reorientation and detwinning processes in martensitic NiTi, the fundamental thermoelastic nature of such processes and the ability of the variant microstructure to accommodate irreversible deformation processes