Phase transformations in the system Y2B~CU6+x014+x (0~~) are investigated by high resolution transmission electron microscopy (TEM) and static lattice Monte Carlo computer simulations. Micrographs reveal dislocations and stacking faults associated with the diffusion of copper and oxygen as the stoichiometric parameter x varies locally during a phase transformation, and these defects are accurately reproduced by the calculations in "snapshots" of the simulated structures. In addition, the simulations show the occurrence of the well-known YBa2Cu307, YBa2Cu40g, and Y 2B~Cu7015 phases and predict the formation of Y 2B~Cu9017 and YBa2Cus09 structures at higher oxygen partial pressures. Based on experimental images and Mon~e Carlo results, an atomic mechanism involving the intercalation and removal of extra CuO planes by partial dislocation climb is proposed for changes in the layered YBaCuO crystal structure.

AbslmeL I t is shown that the oxygen-ordered superstructures observed experimentally in YBalCu,O. can be understood in terms of an lsing Hamiltonian containing screened Coulomb repulsions between any two "rygen site% augmented with a short-range attractive covalent interaction between oxygen sites adjoined by a copper atom. Spatially

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A theoretical technique is described, employing Monte Carlo simulation, which allows the investigation of strain effects associated with transformations in solids. The method applies a grand canonical stress ensemble in the study of elasticity during phase evolution in YBa2Cu30, where both oxygen occupancy and atomic positions in the basal plane, as we11 as total sample volume, can vary. Quenching experiments yield both metastable +2ao X +2ao superstructures from short-wavelength modulations and a tweed texture from longperiod (110) modulations of the ordered z=7.0 compound.

Conversion of 124 into 123 + CuO is interesting because it can produce non‐superconducting CuO islands and highly strained local regions, both of which may act as flux pinning centers. Microstructural studies (TEM) show localized regions with high strain fields resulting from the lattice mismatch between 124 and 123 along the c axis. Enhanced Tc (95 K) was found in partly converted samples. The partial pressure of oxygen necessary for conversion from YBa2Cu4O8 (124) to YBa2Cu307 (123) decreases with decreasing temperatures, and below 850°C the boundary between 123 and 124 phase regions is found to fall approximately as log ρ(θ2) = 19.3 ‐22000/T. The phase diagram of the Y‐Ba‐Cu‐O system is given as a function of temperature and partial pressure of oxygen over the range between 500°C and 1000°C and 10‐6< P[O2] < 102 bar.