掺杂对超导体(Tl_(1-x-y)Pb_xM_y)Sr_2(Ca_(0.8)Ln_(0.2))Cu_2O_(7-δ)(M=Sn^(4+)或Sb^(5+),Ln=Y,La,Nd或Pr)性能的影响

INFLUENCE OF DOPING ON PROPERTIES OF SUPERCONDUCTOR Tl_(1-x-y) Pb_xM_y) Sr_2 (Ca_0.8Ln_0.2) Cu_2O_(7-δ) (M=Sn^(4+) or Sb^(5+), Ln=Y, La, Nd or Pr)

自1988年初Bi-Sr-Ca-Cu-O和Tl-Ba-Ca-Cu-O两个高T_c氧化物超导体问世以后,人们就想了解Tl-Sr-Ca-Cu-O和Bi-Ba-Ca-Cu-O两个体系的超导性能,但后者至今尚未制备成功.Sheng等人报导了Tl-Sr-Ca-Cu-O的超导性能,T_c约为20K,获得的材料的Meissner效应小于1%.Subramanian等人通过Pb部分取代Tl,获得了(Tl_(0.5)Pb_(0.5)

Subramanian et al. found superconductivity at 80 K in Tl_(0.5)Pb_(0.5)Sr_2CaCu_2O_y sa-mple with YBa_2 Cu_3 O_(7-δ)-like structure. However, there is unstable factor in thestructure of the superconductor. Total plus valence is 12. 5, and total minus valenceis 14. The structure was further stabilized by partial substitution of Y, La, Nd orPr for Ca and Sn^(4+) or Sb^(5+) for (Tl, Pb), so that T_c was achieved above 100 K.For example, T_c is about 103 K in a sample with nominal composition (Tl_(0.4) Pb_(0.5)Sn_(0.2)) Sr_2(Ca_(0.8)Y_(0.2)) Cu_2 O_(7-δ). During the preparation of superconductors, the tem-perature of samples was very slowly down after. sintering, or samples were annealedat 500-700℃. The samples absorbed the oxygen during cooling, but no phasechange occured. Appropriate amount of the oxygen made T_c higher.