EFFECT OF STRUCTURAL-CHANGES ON THE TRANSITION-TEMPERATURES IN Y2BA4CU7O14+X SINGLE-CRYSTALS

A structural investigation of 14 Y2BaCu7O14+x single crystals with transition temperatures 0<T(c)<93 K, grown by recrystallization ceramic (with BaCO3 or barium metal as precursor) and by a high pressure flux-growth method, has been performed. X-ray structure refinements yielded changes in the single chain 123-unit only. With increasing T(c) a shortening of the Cu(2)-O(1) apical bondlength is observed, together with a reduction of buckling in the Ba(1)-O(1) plane. High-T(c) > 90 K crystals have the ideal 247 structure, with full occupancies of all sites and well aligned single chains. Lower T(c) 247 crystals show a smaller electron density not only at the O(8), O(9) sites (oxygen nonstoichiometry), but also at the O(1) and Cu(1) site (Cu nonstoichiometry). These deficiencies are explained by the incorporation of aluminium, carbonate or vacancies at the single chain Cu(1) position. The resulting defects cause a misorientation of the single chains in the 123-unit, whereas the 124-unit is not affected by any distortions within the whole T(c) range. The results clearly support the explanation that Cu(1)-defects (and the resulting misorientation of single chains) are responsible for the decrease of T(c). Electron density maps of some 247 crystals show a splitting of the apical oxygen O(1) or the plane copper Cu(2) site, indicating the possible coexistence of different apical bondlengths, requested by recent polaron models.