HIGH-PRESSURE SYNTHESIS AND CHARACTERIZATION OF SOME CA-DOPED R2BA4CU7O15-DELTA (R = Y, SM, DY) CUPRATE SUPERCONDUCTORS

Three series of (R1-xCax)(2)Ba4Cu7O15-delta (Ca-doped R247; R = Y, Sm, Dy) cuprate superconductors have been synthesized at, 990 degrees C a nd under 26 - 30 bar of oxygen pressure. The effect of Ca2+ doping oil the variation of oxygen content, structural chemistry and superconduc tivity for Ca-doped R247 phases was investigated. The upper limit of s ubstitution of Ca2+ for R in R247 was determined to be 0.10, 0.10, and 0.04, for phases with R = Y, Dy, and Sm, respectively, as indicated b y x-ray diffraction (XRD) data. The lattice parameters of Ca-doped R24 7 phases were found to vary insignificantly or decrease: slightly upon Ca doping and these observations were attributed to the possibility o f dual substitution of Ca2+ for both R3+ and Ba2+ sites. However, the corresponding orthorhombicity was found to increase or vary insignific antly. Upon Ca2+ doping the superconducting transition temperature (T- c) of R247 phases was found to decrease or increase from 93.8 K, 83.4 K and 64.5 K to 90.3 K (x = 0.10), 86.8 K (x = 0.10), and 72.2 K (x = 0.04) for phases with R = Y, Dy! Sm, respectively. Oxygen content or h ole concentration (n(h)) of Ca-doped R247 phases was found to decrease with increasing Ca2+ content, as indicated by the iodometric titratio n data. The variation of T-c, sites of substitution and oxygen composi tions for the title Ca-doped R247 phases will be discussed.