ISOMORPHIC SUBSTITUTIONS IN CATION SUBLATTICES OF ORTHORHOMBIC Y-123 CUPRATES AND MICROTWINNING .2. Y0.81CA0.11BA2.0CU2.92ZN0.04O6.41 (ORTHORHOMBIC-II PHASE)

Crystals of Y-123-cuprate of the composition Y0.81Ca0.11Ba2.0Cu2.92Zn0 .04O6.41 have been studied by the single-crystal X-ray method (the str ucture type of orthorhombic phase II is R-II = YBa2Cu3O6.5 with a doub le parameter a in comparison with that of the orthorhombic phase I): a = 2 x 3.828(1), b = 3.883(1), and c = 11.689(4) Angstrom, Z = 2; sp. gr. Pmmm, R-aniso = 0.045 for the 681 independent reflection. The defi cit in O atoms is associated with vacancies in the position of the api cal oxygen atom. Along with the previously observed displacements of B a atoms in the [100] direction, the analogous displacements of (Y, Ca) atoms are found. The methods of high-resolution electron microscopy p rovided observation of microtwinning interpreted as twinning by the ax is 2 in contrast to the well known twinning in the m-plane. The depend ence of two types of microtwinning on the oxygen content is also discu ssed.