GROWTH OF BULK YBA2CU3O7-DELTA SINGLE-CRYSTALS AND THEIR PROPERTIES

The effects of the charge composition, temperature-time conditions, va lue and direction of temperature gradients in a melt on the crystalliz ation of bulk YBa2Cu3O7-delta single crystals have been studied. The m orphology, chemical composition, structural, magnetic and resistive pr operties of these crystals have been characterized. The width of a met astable zone of YBCO is found to be about 0.8 K. At overcooling less t han 0.8 K a bulk crystal of polyhedral type grows on a seed. At overco oling greater than 0.8 K a spontaneous crystallization takes place and crystals in the form of thin plates are produced. Orientation figures are defined for as-grown YBCO faces by a chemical etching method. The distribution of the main impurity (Al) is shown to be homogeneous in the volume of the growing crystal (the homogeneity level is 3%), its c oncentration varying from 0.05 to 0.11 at/Fu in different runs. The ab sence of the block structure in (001) planes of the YBCO single crysta l is established using x-ray topography. The contrast unevenness on th e (100) topogram indicates that a certain amount of the block structur e occurs on (100) planes, comparable with the resolution of the method of 0.02-degrees. The lattice parameters and the M(T) and rho(T) depen dences are measured before and after the thermal treatment of crystals .