MORPHOLOGY OF BULK RBCO SINGLE CRYSTALS. E.V.Sokol^a, L.P.Kozeeva^b, M.Yu.Kameneva^b; ^aJoint Inst. of Geology, ^bInst. of Inorganic Chemistry, Novosibirsk, 630090, Russia

Understanding of the forms of crystallization of 123 cuprate superconductors is vital for controlling their properties. Morphological analysis of more than 100 well-shaped individuals and crystal aggregates of RBCO (R= Y, Tm, Ho, Lu) obtained by the flux method in alumina crucibles in different conditions showed 3 stages of crystallization. The nucleation of crystals could be: multicentre two-dimensional one forming the crystal base, and the skeleton growth of different relief roughness with tops and (or) edges between {001} and {100} faces of tetragonal prism as generating points. The second stage was layer-to-layer growth of {001} plane. Heterogeneous aggregates of microcrystals from the first stage may be overlapped by {001} layers, producing monocrystal-like specimens. The crystals of clearly thick platy habit usually had shiny {001} facets and sufficiently smooth of {100} ones. With increasing sizes of {100} facets different types of relief roughening were observed: so-called slice- and hooper-like forms and overhanging of {001} facets. Side faces {011} are presented in the habit of the most perfect crystals with smooth {100} faces. Bulk crystals took their final form at the third stage. The basal faces developed to smooth mirror surfaces sometimes with growth spirals. At the same time clear microblock features due to of impurity poisoning of growth surface were observed. Thin platelets with smooth {001} crystallized at this stage too. Summary: {001} and {011} faces are growth forms of crystals of RBCO, {100} faces - passive ones. The real symmetry of as-grown bulk crystals is lower than P4/mmm - the horizontal plane of symmetry (001) is usually absent.