The crystal structure and superconductivity of the new superconducting phase TlSr₂Ca_0.5Er_0.5Cu₂O_6.75

Abstract

The crystal structure of the superconducting phase TlSr₂Ca_1−xEr_xCu₂O_6.5+x/2 (x=0.5) in the Tl−Sr−Ca−Er−Cu−O system was determined using the X-ray powder diffraction method. It belongs to a primary tetragonal cell with lattice constantsa=3.798,c=12.014 Å, with a space group ofD ¹_4h −P4/mmm. Each unit cell contains one formula unit. The positions of ions in the unit cell are as follows:

0.5 Ca and 0.5 Er cations disorderly occupy 1(d) equivalent point position, each with an occupation factor of 0.5;

• 1. Tl cation occupies the equivalent point position 1 (a);

• 2. Cu cations occupy 2 (g) withz=0.365;

• 2 Sr cations occupy 2(h) withz=0.220;

• 7 O anions respectively occupy 1(c), 2(g), (z=0.165) and 4(i) (z=0.365) equivalent point positions.

The zero-resistance superconducting transition temperatureT _c (0) decreased slightly with increased Er content in the Tl−Sr−Ca−Er−Cu−O system. The crystal structure of TlSr₂Ca_0.5Er_0.5Cu₂O_6.75 is very similar to that of TlBa₂CaCu₂O_6.5 in the Tl−Ba−Ca−Cu−O system or RBa₂Cu₃O₇ in the R−Ba−Cu−O system. In the superconducting phase with an oxygen-deficient perovskite-like structure, the similarity of cation-oxygen coordination polyhedra is important for cation substitution.