The Synthesis and Characterization of Pb- and/or La-Doped TlBa2CaCu2Ox Cuprate Superconductors
|摘要:||本研究分別在900℃常壓氧氣氛下合成(Tl1-yPby)(Ba2xLax)CaCu2O7-δ(0≦x≦1.0; 0≦y≦0.4; 簡稱Tl-1212)與在920℃合成摻雜鉀之Tl(Ba0.5Sr1.5)Ca2Cu3Oz等系列高溫超導體，以探討鉛、鑭與鉀離子之取代對於上述超導體之晶體結構化學、臨界溫度(Tc)、電阻率、磁化率以及氧含量變化之效應。X光繞射數據顯示：當Pb4+取代量為0或0.2時，可合成出純Tl-1212相。但當Pb4+取代量為0.4時，則發現有BaPbO3與Tl-1212兩相共存。此外，晶格常數a軸長度隨著La3+取代量增加而增加，而c軸長與單位晶格體積則減少。
另一方面，當Pb4+取代量分別為0、0.2、0.4時，則隨著La3+取代量之增加，臨界溫度先增後減，其最高臨界溫度分別為：87K(x = 0.3)、88K (x = 0.2)、83K (x = 0.3); 當所摻加鑭超過取代上限之後，Tc會降低並且導致樣品趨向半導性與絕緣性，終至造成超導性之消失。另外，摻雜鉛離子Tl-1212樣品之氧含量則因鉈缺陷之存在而較無規律性。
This research is attempted to investigate the effect of Pb, La, and K cation substitution on the structural and superconducting properties (i.e., Tc, electrical resistrvity and magnetic susceptbility) of (Tl1-yPby)(Ba21-xLax2)CaCu2O7-δ(0≦x≦1.0; 0≦y≦0.4; abbr. as Pb-and La-doped Tl-1212) and (Tl1-xKx)(Ba0.5Sr1.5)Ca2Cu3Oz(K-doped Tl-1212) high-Tc cuprate superconductors that were synthesized at 900℃ and 920℃, respectively, under ambient oxygen atmosphere. Pure Tl-1212 phases with Pb content of 0 and 0.2 can be prepared, whereas BaPbO3 and Tl-1212 were found to coexist when the substitution content of Pb is 0.4, as indicated by x-ray diffraction data. Furthermore, cell parameters and c of Pb-doped Tl-1212 phases were found to incease and decrease with increasing La3+ content, respectively. On the other hand, Tc of Pb-doped Tl-1212 phases was found to first increase and then decrese when substituted by La3+ for phases with Pb content of 0, 0.2, 0.4, respectively. The optimal Tc was determined to be 87K, 88K, and 83K for phases with la3+ content of 0.3, 0.2, and 0.1, respectively. In the substitution range of higher La3+ content Tc of Tl-1212 was then found to decrease with increasing La3+ content and La-doped Tl-1212 eventually exhibit semiconducting and insulating behaviors. In addition, the oxygen composition of Tl-1212 did not exhibit systematics which was attributed to the uncontrollable deficiency of Tl, as indicated by oxygen content analysis. The crystal structure and Tc of K-doped Tl(Ba0.5Sr1.5)Ca2Cu3Oz phases were found to be unchanged upon K+ doping, while superconductor volume fraction of Tl-1223 phases was found to decrease upon increasing K+ content. These observations may be rationalized by the proposal that added K+ cation was considered not entering the crystal latice and substituting for Tl3+ ion.
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