Crystal structure and characteristics of atomic thermal vibrations in the high-temperature superconductor YBa2Cu4O8

The atomic structure of high-temperature superconductor YBa2Cu4O8 Single cr ystals with T-c approximate to 70 K has been refined from the X-ray diffrac tion data (sp. gr. Ammm, a = 3.8400(7), b = 3.8704(7), c = 27.225(3) Angstr om, R-w = 0.83%, R = 0.60%). The deviation of thermal vibrations of heavy a toms from the harmonic law has been established. The analysis of the probab ility-density distribution function shows that the Cu(1) atoms are consider ably displaced from the centers of oxygen squares forming the bands, wherea s Cu(2) atoms, from the centers of oxygen squares forming the nets toward t he middle points of the sides of the respective squares. In other words, th e copper atoms are displaced along the bisectors of the angles formed by st rong Cu-O bonds.