the aim of the work is to study the structural parameters of phase 1-2-3 by precision X-ray diffraction method. The refinement of composition and structural parameters to the divergence factor R = 0.008 was carried out. The composition of the substance can be expressed by the formula . Higher values of the mean-square amplitudes of atoms vibrations and vibration anisotropy were obtained for Си1 and О1 located in the basis plane z = 0. Comparison with data obtained at low temperatures suggests that the high mobility of Си1 and O1 is related to static displacements of atoms from their crystallographic positions. There is a correlation between the deviation from the oxygen stehiometry, the lattice period c and the value of the superconducting transition temperature Tc. An increase in the oxygen content is accompanied by an increase in Tc. Dependences of isotropic mean-square amplitudes of atoms-components vibrations of compounds on the oxygen content are plotted. For the O1 atom in the tetragonal phase, a marked growth of mean-square amplitudes of vibrations with increasing oxygen index is observed, which may be associated with an increase in the instability of the structure. It can be concluded that the analysis of precision diffraction studies for yttrium-barium-copper oxides allows us to identify the structural features responsible for the instability of this class of materials.