SUPPRESSION OF SUPERCONDUCTING TRANSITION-TEMPERATURE IN ORTHORHOMBIC LA2-XCAXCUO4

The calcium solid-solubility limit in La2-xCaxCuO4 has been extended from x similar to 0.10 for air-synthesized material to x=0.2 using synthesis at an oxygen pressure of 600 atm and 1110 degrees C. The tetragonal-to-orthorhombic transition occurs at higher temperatures, or larger doping levels, than for the isostructural Sr-substituted material. The superconducting transition temperature increases with doping to 34 K at x=0.15 and is suppressed for x>0.15, similar to the Sr-substituted material. Comparison of structural and superconducting properties for Ca- and Sr-substituted materials indicated that: (1) both orthorhombic and tetragonal structures are superconducting; (2) at a fixed doping level, the optimum superconducting properties (the highest T-c) are found for a perfectly flat and square CuO2 plane; (3) T-c is suppressed by the distortions of the CuO2 plane or by overdoping resulting in a maximum of T-c found at x=0.15 under ambient conditions.