Superconductivity and hole concentration of La3−xCa2xBa3−xCu6Oy compounds

Rietveld analysis and superconductivity of compounds

“…The ONs of all the oxygen atoms in all the oxygen sites were not calculated from the Rietveld analysis due to the low x-ray diffraction intensity of oxygen atoms. They were based on the iodometric result [1] and the following assumption: all the O1 and O2 sites were fully occupied and the rest of the oxygen atoms would then be evenly distributed between the two O3 sites located in the a and b axes. The oxygen stoichiometry determined from the iodometric titration [1] for the x = 0.50 sample was 7.073 per unit cell.…”

“…LCB compounds were prepared as previously described [1]. The intensity of the x-ray powder diffraction data was measured at room temperature by step scanning over an angular range of 20 • < 2θ < 130 • with an increment of 0.02 • , using a Rigaku Rotaflex RTP 500RC powder diffractometer with a Bragg-Brentano geometry and diffracted beam graphite(0002) monochromator Cu Kα radiation.…”

“…It is unusual that La-336 is not superconducting [7] since it, like many p-type superconductors [8], has twodimensional Cu2-O layers [6] and also has p as high as the superconducting Y-123 [7]. Recently, we have found that the superconducting transition temperature is dependent on p sh in the LCB series [1] but not dependent on p. For La-336, the p sh is too small to show superconductivity [1].…”

“…In the La 3−x Ca 2x Ba 3−x Cu 6 O y (LCB) series, single-phase materials have tetragonal triple-perovskite structure [1] like the tetragonal Y-123 phase [2] where the oxygen atoms located in the O3 sites in the basal planes containing the Cu1 sites are evenly distributed in the a and b axes. The average occupancy factor (OF) for each O3 site is close to 0.5.…”

“…Substitution of some of the Ba 2+ and La 3+ ions of La-336 simultaneously with Ca 2+ ions in the LCB series causes an increase of the p sh and the materials become superconductors when x ≥ 0.15 [1]. Why does p sh increase when Ca 2+ ions are introduced into La-336?…”