powerful vacuum devices
V.P. Marin, N.A. Tomilin, V.S. Belov
Definition of crystal structure YBa2Cu3Ox was carried out on automatic diffractometer CАD-4 (Mo-K radiation, graphite monochromator). Monocrystals YBa2Cu3Ox in the size of 0,1∙0,1∙0,1 mm with the various contents of oxygen (х = 6,4 (I), х = 6,7 (II), х = 6,9 (III), received by a method isothermal recrystallizing annealing were investigated. Stoichiometrical the parity on yttrium, to barium and copper in researched samples has been confirmed with application X-ray spectral the microanalyzer, and the contents of oxygen was estimated on parameters of an elementary cell with his subsequent specification by results of X-ray diffraction researches. Specification of coordinates of atoms was carried out in anisotropic approximation use of software package SDP. Atoms Y, Ba, Cu1, Cu2, O1, O2, O3, O4, O5 settle down in private positions h, f, a, q, q, q, r, e, b for spatial groups Rmmm and d, h, a, g, g, g, n for P4/mmm.
Changes in a crystal structure of phases YBa2Cu3Ox depending on quantity of the oxygen which is included in formulas unit from х =6,4 up to х =7,0, are expressed not only in distortion of elementary cells and minor alteration of internuclear distances, but also, that is more important, in the certain ordering oxygen vacancies. In structure I atoms of oxygen statistically occupy 10 % of a position, on a measure of increase in quantity of oxygen in structure II and III the tendency to ordering oxygen vacancies is observed. In structures II and III filling by oxygen of a position е makes 40 % and 78 %, and positions b – 30 % and 12 % accordingly. Such tendency in ordering oxygen vacancies should affect change of anisotropy of physical properties of the given phases.
Thermoanalytical research of connection (methods calorimetry, DТА, thermogravimetry, dilatometry and chromatography) in a wide range of temperatures and pressure has found out abnormal changes of temperature dependences of a thermal capacity and thermal expansion at the high temperatures, connected with allocation and absorption of oxygen at heating and cooling and an event thus phase transition YBa2Cu3Oх from rhombic (superconducting) in tetragonal (not superconducting) updating