Superconducting single crystals of Tl2Ba2CaCu2O8 and YBa2Cu4O8: Crystal structures in the vicinity of T-c

The structures of superconducting Tl2Ba2CaCu2O8 (T-c similar to 110 K) and YBa2Cu4O8 (T-c similar to 70 K) were investigated by single-crystal X-ray d iffraction techniques in a wide temperature range. Some of the structure pa rameters show anomalous behaviour in the vicinity of the critical temperatu re. The most pronounced structural change is the shift of the apical O atom s in CuO5 pyramids towards the CuO2 planes, indicating charge redistributio n near T-c. This behaviour of the apical O atoms seems to be the common str uctural feature of hole-doped high-T-c superconductors. Another effect of c harge 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.