新系列超导相TlBaCan-1CunO2n+2.5(n=2,3)的晶体结构和超导电性

本文用粉末烧结法合成了Tl-Ba-Ca-Cu-O体系TlBa₂Ca_n-1Cu_nO_2n+2.5新系列超导相,用粉末衍射法测定了n=2和3的晶体结构,属简单四方单胞,空间群为D_4h¹-P4/mmm,点阵常数a相同,而c值不同,随n值而增大。a=3.847?,c分别为12.73?和15.89?,每单胞含一个化合式单位,阳离子沿四方晶系的(0,0,z)和(1/2,1/2,z)位置交替排列,排列的顺序相同,n=3比n=2依次多增加Ca和Cu及相应的氧离子。TlBa₂Ca_nCu_nO_2n+2.5,n=3时,零电阻超导转变温度T_c(0)=112K;n=2时,T_c(0)=101K,与Tl₂Ba₂Ca_n-1Cu_nO_2n+4型体心四方晶胞超导相不同,TlBa₂Ca_n-1Ca_nO_2n+2.5结构中缺氧类钙钛矿型结构单元Ba₂Ca_n-1Cu_nO_2n+1]仅被TI-O单层所隔开,当n相同时,Tl-O双隔层的体心四方单胞比Tl-O单层的简单四方单胞的超导转变温度高10K左右,这说明Tl-Ba-Ca-Cu-O体系中超导相的转变温度不仅与类钙钛矿结构单元的CuO₄]和CuO₅]的数目有关,而且与Tl-O的隔层数目有关。 关键词：

THE CRYSTAL STRUCTURES AND SUPERCONDUCTIVITY OF A NEW SERIES OF SUPERCONDUCTING PHASES

We synthesized new superconducting phases in TlBa₂Ca_n-1Cu_nO_2n+2.5 by solid state reaction. The crystal structures of TlBa₂CaCu₂O_6.5 and TlBa₂Ca₂Cu₃O_8.5 were determined by means of X-ray powder diffraction method. The two compounds belong to primary lattice with space group D_4h¹-P4/mmm. The lattice constants of TlBa₂CaCu₂O_6.5 are α= 3.847 ?, c = 12.73 ?. The lattice constants of TlBa₂Ca₂Cu₃O_8.5 are α = 3.847?, c= 15.89?. Each cell contains one chemical formula unit. The cations are distributed alternately at positions of (0,0,z) and (1/2, 1/2, z) along Z axis. TlBa₂CaCu₂O_6.5 shows zero resistance at 101.4K. TlBa₂Ca₂Cu₃O_8.5) shows zero resistance at 112K. The oxygen deficient pseudo-perovskite unit Ba₂Ca_n-1Cu_nO_2n+1 is separated by single Tl-O layer in the crystal structure of TlBa₂Ca_n-1Cu_nO_2n+2.5, while the oxygen deficient pseudo-perovskite unit Ba₂Ca_n-1Cu_nO_2n is separated by bilayer of Tl-O in the crystal structure of Tl₂Ba₂Ca_n-1Cu_nO_2n+2. At the same n value, T_c of Tl₂Ba₂Ca_n-1Cu_nO_2n+4 is about 10K higher than T_c of TlBa₂Ca_n-1Cu_nO_2n+2.5. This indicates that T_c depends on the number of Tl-O layer as well as the number of CuO₅] and CuO₄] in oxygen deficient pseudo-perovskite unit Ba₂Ca_n-1Cu_nO_2n.
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