UNUSUAL CRYSTAL-STRUCTURE OF NONSUPERCONDUCTING Y1BA2CU3O7-X FILMS ONBUFFERED SILICON SUBSTRATES

Superconducting common c-axis oriented YBCO films as well as non-super conducting films epitaxially grown at significantly reduced substrate temperatures were deposited by laser ablation on CeO2/YSZ buffered sil icon substrates. The crystal structure of the non-superconducting film s measured by XRD is collapsed from the original orthorhombic one, whe reas the chemical composition detected using RES and EDX remains ident ical, Especially the oxygen content in the non-superconducting materia l is as high as in the common 90 K superconductor. Thus, the change in the electrical behaviour is only due to a structural modification. Th e crystallographic model of a simple cubic YBCO perovskite unit cell d oes not represent completely our experimental results measured on the non-superconducting YBCO. A modified model based on detailed TEM and X RD measurements and known crystal defects of orthorhombic YBCO is prop osed. (C) 1997 Elsevier Science B.V.