SYNTHESIS, STRUCTURE, AND SUPERCONDUCTIVITY OF CA2-XNAXCUO2CL2

Synthesis, structure, and superconductivity of a cupric oxychloride Ca 2-xNaxCuO2Cl2 have been studied. A series of polycrystalline samples w ith x up to about 0.20 were prepared at 6 GPa and 1173 K using a high- pressure synthesis technique. The compound contains CuO2 planes as in La2CuO4, but all of the oxygen atoms at the apices of the CuO6 octahed ra found in La2CuO4 are replaced by chlorine. Hole-doped superconducti vity is accomplished by partially substituting Na1+ for Ca2+. A system atic variation of T-c, similar to that of La2-xSrxCuO4, with a maximum value of 28 K has been found as a function of Na content. The results clearly demonstrate that the apical atoms do not play a significant r ole in the electron pairing mechanism leading to high-T-c superconduct ivity.