Superconducting Single Crystals of Tl₂Ba₂CaCu₂O₈ and YBa₂Cu₄O₈: Crystal Structures in the Vicinity of T_c

The structures of superconducting Tl₂Ba₂CaCu₂O₈ (T_c ∼ 110 K) and YBa₂Cu₄O₈ (T_c ∼ 70 K) were investigated by single-crystal X-ray diffraction techniques in a wide temperature range. Some of the structure parameters show anomalous behaviour in the vicinity of the critical temperature. The most pronounced structural change is the shift of the apical O atoms in CuO₅ pyramids towards the CuO₂ planes, indicating charge redistribution near T_c. This behaviour of the apical O atoms seems to be the common structural feature of hole-doped high-T_c superconductors. Another effect of charge redistribution at T_c concerns a very small decrease of in-plane Cu—O distances and/or O—O distances in CuO₅ pyramids. Both effects represent an averaged structure response to charge (hole) redistribution upon the phase transition to the superconducting state.