The crystal structure and superconductivity of new superconducting phase TlSr 2Ca 0.5Sm 0.5Cu 2O 6.75

The crystal structure of superconducting phase TlSr ₂Ca _0.5Sm _0.5Cu ₂O _6.75 with T _c(0) = 37.6K in Tl-Sr-Ca-Sm-Cu-O system has been determined by means of X-ray powder diffraction method. It belongs to primary tetragonal cell with a = 3.817, c = 12.054 Å. Its space group is D¹_{4 h}- P4/ mmm. Each unit cell contains one formula unit. The positions of ions in unit cell are as following: 0.5 Ca and 0.5 Sm cations disorderly occupy 1(d) equivalent point position. The occupation factor for each of them is 0.5. Tl cation occupies equivalent point position 1(a). 2 Cu cations occupy 2(g) with z = 0.365. 2 Sr cations occupy 2(h) with z = 0.210. 7 O anions respectively occupy 1(c), 2(g) ( z = 0.165) and 4(i) ( z = 0.365) equivalent point positions. The crystal structure of TlSr ₂Ca _0.5Sm _0.5Cu ₂O _6.75 is very similar to that of TlBa ₂CaCu ₂O _6.5 in Tl-Ba-Ca-Cu-O system or RBa ₂Cu ₃O ₇ in R-Ba-Ca-O system. In the superconducting phase with oxygen-deficient pseudo-perovskite structure, the similarity of cation-oxygen coordination polyhedron is important for the substitution among the cations.