ANOMALOUS BEHAVIOR OF THE STRUCTURAL PARAMETERS OF YBA2CU4O8 SINGLE-CRYSTALS AT THE TRANSITION TO THE SUPERCONDUCTING STATE

High-precision x-ray crystallographic studies of YBa2Cu4O8 Single crys tals (T-c=70 K) are performed at eight temperatures in the interval 20 -295 K. It is found that a number of structural parameters exhibit ano malous behavior near the superconducting transition of the crystal. A characteristic effect near the phase transition is the displacement of the O1 bridge atom that joins the Cu1 atom of the cuprate chain to th e Cu2 atom of the cuprate plane. The shift of this oxygen toward the C u2 atom is indicative of a change in the Cu2-O1 chemical bond and of c harge transfer to the cuprate plane in the process of the transition o f the crystal to the superconducting state. (C) 1997 American Institut e of Physics. [S0021-3640(97)01119-5].