SUBSTITUTION FOR COPPER IN YBA2CU3O9-DELTA BY 3D-TRANSITION AND PRETRANSITION METALS

The range of solid solubility, as well as structural and superconducting properties, of YBa2(Cu1-zM(z))3O9-delta-phases (M = Li, Mg, Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Zn) have been carefully re-examined for a series of phase-pure samples having high resolution in composition as obtained using citrate gel precursors, mixed at the atomic level. The upper composition limits of the solid solution regimes are Z(lim) = 0.00(3) for the elements Sc to Mn, 0.04(1) for Li and Mg, and 0.22(1), 0.30(5), 0.08(1) and 0.09(1) for Fe, Co, Ni and Zn, respectively. The phases in equilibrium with YBa2(Cu1-zM(z))3O9-delta-at the limiting compositions are specified for the different systems, referring to standardized preparation conditions. For oxygen-saturated samples the formal Cu valency (as measured by iodometry) remains constant, and, when permitted, the substituents adopt higher oxidation states, e.g. Fe(III), Co(III), Ni(> II). For such samples with M = Fe and Co, the oxygen content hence may exceed seven. A crossover from orthorhombic to tetragonal symmetry, as seen by powder X-ray diffraction (at 295 K), is studied in detail and occurs at Z(O.T) = 0.030(2) and 0.025(2) for M = Fe and Co, respectively. This behaviour is significantly different from the findings for the substitution of La for Ba in YBa2Cu3O9-delta, where the symmetry change is attributed to the linkage in the copper-oxygen chains where the additional oxygens enter. However, when heating orthorhombic samples (M = Fe or Co) with composition close to Z(O.T), an increased orthorhombic distortion is induced by thermal removal of oxygen (onset around 570 K), before the symmetry eventually becomes tetragonal at ca. 920 K. All the substituents supress T(c), and the suppression is correlated with the degree of tetragonal deformation of the unit cell in terms of the deformation parameter D(T) = [2c/3(a + b)] - 1. No correlation with the orthorhombic distortion parameter D(o) = (b/a) - 1 is found.