BULK AND SURFACE-STRUCTURE OF EPITAXIAL YBA2CU3O7-DELTA THIN-FILMS GROWN ON MG2TIO4 (001) SUBSTRATE LAYERS .1. BULK STRUCTURE INVESTIGATED BY TEM

Epitaxial YBa2Cu3O7-delta thin films have been grown by pulsed laser d eposition onto single crystal Mg2TiO4 (001) substrate layers. The Mg2T iO4 layers have been prepared by a topotaxial solid-state reaction bet ween MgO (001) single crystal surfaces and a TiO2 vapour. The latter w as obtained by electron beam evaporation of anatase powder targets. Th e Mg2TiO4 layers have been shown to be genuinely single crystalline, w ell-oriented and free of defects except cationic antiphase boundaries forming a regular network. The bulk structure of the YBa2Cu3O7-delta f ilms grown under various growth conditions has been investigated by tr ansmission electron microscopy and selected area electron diffraction. Films grown under non-optimum conditions displayed a non-homogeneous bulk structure characterized by film regions of different orientation and by a high density of droplets. Under optimum growth conditions the YBa2Cu3O7-delta films are epitaxially oriented throughout, with the [ 001] axis perpendicular to the film plane and with good in-plane align ment. The number of droplets is low in these films, which are supercon ductive at T(c)(onset) almost-equal-to 88.5 K. These films also showed thickness variations, which can be attributed to the spiral growth hi lls detected by STM and described in part II.