HREM study of structural changes at or near the surface of ErBa₂Cu₄O₈ upon heating in air at 100-250°C

ErBa₂Cu₄O₈ has been studied with High Resolution Electron Microscopy to determine whether ErBa₂Cu₅O₉ can be formed locally by the intercalation of Cu and O, in samples heated in the temperature range 100-250°C. Prior to the formation of planar defects, a 2a,b,c superstructure is formed upon heating, occurring only near the surface. The 2a period is present only in the (CuO)₂ double layer. Upon heating, several types of planar defects are formed: (1) a set of defects showing an increase in the c lattice parameter of 0.15-0.30 nm; the defects seem to be obtained by insertion of a more or less amorphous layer between the CuO layers of the (CuO)₂ double layers; (2) a new group of structures in which extra layers are intercalated between the CuO layers, each layer having a width of 0.28(2) nm. The latter group of structures and the 2a,b,c superstructure can be explained with the presence of CO₃ groups in the lattice. Continuous heating above 150°C leads to amorphization, progressing from the surface. No evidence is obtained which would indicate that ErBa₂Cu₅O₉ can be formed under these conditions.