Synthesis, structure and properties of Ca-doped La[sub 2]CuO[sub 4]-based single-layer high-T[sub c] superconducting copper oxides

Includes bibliographical references (pages [80]-84).In this thesis, the synthesis, structure and properties of Ca-substituted La[sub 2]CuO[sub 4]-based single layer high-T[sub c] superconducting copper oxides will be presented. Homogeneous single phase samples of La[sub 2-x]Ca[sub x]CuO[sub 4] and La[sub 1.85-y]Nd[sub y]Ca[sub 0.15]CuO[sub 4] were synthesized through nitride method and high oxygen pressure technique. The calcium solid-solubility limit in La[sub 2-x]Ca[sub x]CuO[sub 4] has been extended from x~0.10 for air-synthesized material to x=0.2 using synthesis at an oxygen pressure of 600 atm and lllO °C . Under the same synthesis conditions, the neodymium solid-solution in La[sub 1.85-y]Nd[sub y]Ca[sub 0.15]CuO[sub 4] has been extended to y=1.0. The tetragonal-to-orthorhombic transition for Ca-substituted materials occurs at higher temperatures, or larger doping levels, than for the isostructural Srsubstituted material. The superconducting transition temperature increases with doping to 34K at x=0.15 and is suppressed for x>0.15, similar to the Sr-substituted material. Comparison of structures and superconducting properties for Ca- and Sr-substituted materials along with Nd-doped compounds indicates that (1) both orthorhombic and tetragonal structures are superconducting; (2) at a fixed doping level, the optimum superconducting properties (the highest T[sub c]) are found for flat and square CuO[sub 2] planes; and (3) T[sub c] is suppressed by the distortion of the CuO[sub 2] planes or by overdoping, which results in a maximum of T[sub c] found at x=0.15.M.S. (Master of Science