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The atomic structure of YBa 2 Cu 3 O y fine crystalline HTSC samples with various average crystal lite sizes ranging from 0.4 to 2 μm and an oxygen concentration y close to the optimal value for super conductivity (y ≈ 6.93) is investigated by the neutron diffraction technique. We have found some effects asso ciated with the redistribution of cations and oxygen atoms and with variations in the positions of atomic layers in the unit cell, which are not observed in macrocrystalline samples. In all probability, these effects appear due to nonequilibrium conditions of synthesis required for obtaining this compound in the fine crystalline state. The results have made it possible to explain the peculiar physical properties of fine crystalline YBa 2 Cu 3 O y samples (in particular, the coexistence of high superconducting transition temperatures T c and noticeably lower values of magnetization in strong magnetic fields for T < T c ). It is shown that a nanoscale structural inhomogeneity exists in fine crystalline YBa 2 Cu 3 O y samples with the optimal oxygen content and changes the fundamental superconducting parameters, viz., the magnetic field penetration depth and the coherence length.

The effect of structural inhomogeneity on the superconducting gap near d-wave for optimally doped high-Tc superconductors YBa2Cu3O6.92 sites is considered. For this purpose, the heat capacity was investigated in the temperature range T = 2-10 K and in magnetic fields H = 0-9 T for a series of fine-crystalline samples with different degrees of controlled structural disordering. The information on the features of superconducting gap near d-wave sites in structurally disordered samples is obtained. It is shown that the d-wave sites inherent in an ideal crystal structure under the influence of structural disorder are modified with the formation of small Fermi arcs in the vicinity of nodal points at T << Tc, not covered by a superconducting gap. In this case, the gap itself remains at T ≤ Tc in other directions of the Brillouin zone and coexists with the Fermi arcs. This transformation is accompanied by an increase in the steepness of the nodal slope, a decrease of the Volovik effect, and the generation of a linear term γ (0) T of the metallic type in the temperature dependence of heat capacity, which should not be present in superconductors with an ideal crystal structure and the nature of which has not yet been established.

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