ISOMORPHIC SUBSTITUTIONS IN CATION SUBLATTICES OF ORTHORHOMBIC Y-123 CUPRATES AND MICROTWINNING .1. 2 STRUCTURAL STATES OF THE PSEUDOTETRAGONAL ORTHORHOMBIC Y0.89BA2.00CU2.86AL6.00-CRYSTAL WITH DELTA=0.00(8) AND 0.18(7)(DELTA SINGLE)

A structure of Y-123-cuprate single crystal of the composition Y0.89Ba 2.00Cu2.86Al0.06O6.00 + delta has been studied by the methods of X-ray structure analysis (XRSA) and transmission electron microscopy (TEM) (here delta = 0 and delta = 0.18 correspond respectively to structural states I and II of the same crystal stored for a year in air at room temperature). The TEM study showed that the tetragonal structure estab lished by the XRSA methods [sp. gr. P4/mmm; unit-cell parameters for I are a' = 4a = 4 x 3.863(3), c = 11.782(3) A and for II are a 2a = 2 x 3.871(3), c = 11.810(3) Angstrom] is a statistical symmetry and is pr ovided by the presence of orthorhombic microdomains, which have mutual ly orthogonal [110] and [1(1) over bar0$] boundaries. The orthorhombic ity is associated with the formation of the chains of Cu(1)O-4-squares along the [010] direction alternating with the chains of Cu(1)O-2-dum bbells in the 1:3 (for state I) and 1:1 (for state II ratios) in the [ 100] direction. Structures I and II are refined within the approximati on of the tetragonal symmetry without allowance for a multiple increas e in a to the reliability factor values R = 0.035 and 0.037, respectiv ely. The probability of location of O(1) atoms at a z = 0 height in th e Cu(1) plane in structure I numerically coincides with the probabilit y of vacancies for apical O(2) atoms. This leads to the oxygen content indicated by subscript 6, which is quite unusual for orthorhombic pha ses. The mechanisms of twinning are also discussed.