Effect of Sr-for-Ba isovalent substitution on the local structure, hole distribution and magnetic irreversibility of Cu(Ba,Sr)2YbCu2O6.95(2)

The effect of SrII-for-BaII isovalent substitution on the magnetic irreversibility field (Hirr) of Cu(Ba1-ySry)2YbCu2O6.95(2) (Cu-1212) sample series (y=0-0.4) is studied to reveal guiding rules for tailoring the intrinsic Hirr characteristics of high-Tc superconductors. It has been assumed that substitution of the larger alkaline-earth cation, BaII, by the smaller, SrII, might improve the Hirr characteristics as a consequence of the decrease in the thickness of nonsuperconductive blocking block (BB). However, results of the present work show that Sr substitution rather depresses the Hirr characteristics of the Cu-1212-phase superconductors even though the thickness of BB decreases. Both the amount of excess oxygen and the overall positive charge are confirmed to remain constant upon the Sr substitution by wet-chemical and X-ray absorption near-edge structure analyses, respectively. However, from neutron diffraction data analysis, it is found that Sr substitution breaks the conductive CuO chains in BB by shifting part of the excess oxygen atoms from the characteristic b-axis lattice site to the a-axis site. This is believed to decrease the concentration of mobile holes in the BB, as supported by the results of TEP measurements. The lower Hirr(T) lines of the Sr-substituted samples may thus be attributed to the lower concentration of mobile holes in BB