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

The atomic structure of high-temperature superconductor YBa2Cu4O8 single crystals with Tc≅70 K has been refined from the X-ray diffraction data (sp. gr. Ammm, a=3.8400(7), b=3.8704(7), c=27.225(3) A, Rw=0.83%, R=0.60%). The deviation of thermal vibrations of heavy atoms from the harmonic law has been established. The analysis of the probability-density distribution function shows that the Cu(1) atoms are considerably displaced from the centers of oxygen squares forming the bands, whereas Cu(2) atoms, from the centers of oxygen squares forming the nets toward the middle points of the sides of the respective squares. In other words, the copper atoms are displaced along the bisectors of the angles formed by strong Cu-O bonds