Surface decomposition of superconducting YBa 2 Cu 3 O 7

High resolution electron microscopy is performed on a number of samples of YBa 2 Cu 3 O 7 before and after heating at several temperatures between 20 and 450 °C, and in three different atmospheres: oxygen, air, and vacuum. By comparison studies of the same crystals it is shown that in all atmospheres the surface decomposes rapidly at temperatures above approximately 150 °C. Below 150 °C the rate of decomposition is much lower; crystals exposed to air for one week show several planar defects indicating the onset of the decomposition. The decomposition reaction proceeds by the insertion of extra CuO planes into the original structure resulting in (CuO) 2 double layers between BaO planes. This reaction induces large lattice strains at the exposed surfaces, sometimes resulting in fragmentation and spalling.