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 crystals (Tc=70 K) are performed at eight temperatures in the interval 20–295 K. It is found that a number of structural parameters exhibit anomalous 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 the Cu2 atom of the cuprate plane. The shift of this oxygen toward the Cu2 atom is indicative of a change in the Cu2-O1 chemical bond and of charge transfer to the cuprate plane in the process of the transition of the crystal to the superconducting state.