High-Pressure Synthesis and Characterization of Ca-Doped R2Ba4Cu7O15-δ (R=Y, Dy, Sm) CCuprates
|摘要:||本研究以高溫高壓法，於990℃、26∼30bar氧壓下，合成(R1-xCax)2Ba4Cu7O15-δ(簡稱R247; R=Y, Dy, Sm)等三系列高溫超導體，以探討鈣離子之取代對上述氧化物之晶體結構化學、超導臨界溫度(Tc)、電阻率、磁化率以及氧含量變化之效應。
Three series of (R1-xCax)2Ba4Cu7O15-δ (R247; R=Y, Dy, Sm)cuprate superconducors have been synthesized at 990℃ and under 26-30 bar high oxygen pressure. The effect of Ca2+ doping variation of oxygen content, structural chemistry, and superconductivity was investigated. The substitution limit of Ca2+ for R in R247 was determined to be 0.20, 0.15, and 0.10 for phases with R=Y, Dy, Sm, respectively, as indicated by x-ray diffraction data. In stead of R247, phases such as RBa2Cu3O7-δ and other impurities was found to form when the substitution limit was exceeded. In addition, with increasing Ca2+ content lattice parameters of Ca-doped R247 phases were found to vary insignificantly, whereas the orthorhombicity was found to increase or vary insignificantly. The former observations were attributed to the possibility of dual substitution of Ca2+ for R3+ or Ba2+ sites. On the other hand, superconducting temperature (Tc) of R247 phases was found to decrease or increase from 93.8 K, 83.4 K, 64.5 K to 83.8 K (x=0.20), 83.3 K (x=0.15), and 75.0 K (x=0.10) for phases with R=Y, Dy, Sm, respectively. With increasing Ca2+ content normal state resistivity of Ca-doped R247 was found to increase while volume fraction of superconductor was found to decrease. In general, oxygen contents of Ca- doped R247 phases were found to decrease with increasing Ca2+ content as indicated by iodometric titration. Variation of Tc for the three series of Ca- doped R247 was discovered to be highly dependent upon critical factors such as structural chemistry and hole concentration which was found to be difficult to control in this study.
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