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Abstract:
Single crystals of YBa2Cu3-xCO(x)O7-y have been obtained using a
modified flux growth method involving "soaking" YBCO crystals at
approximately 950-degrees-C in a flux containing excess CuO and CoO for
a long period. Other phases are also formed but for Co/Cu atomic ratios
less than about 1/3 in the melt, the majority phase single crystals are
YBa2Cu3-xCo(x)O7-y. For greater cobalt concentrations in the melt
single crystals of a second majority phase consisting of a new 112
structure of YBa(Cu1-xCO1+x)O5-y are formed instead. Solubility of
cobalt in YBCO based crystals is found to be very high at approximately
950-degrees-C but becomes diminishingly small at approximately
1080-degrees-C. For a growing temperature of approximately
950-degrees-C the concentration of cobalt in the crystals was found to
be linearly related with that present in the melt. The chemical
composition for both cations and oxygen of all the crystals in the
series has been accurately determined by means of energy dispersive
X-ray analysis of small crystallites and wavelength dispersive X-ray
analysis of bulk crystals. X-ray diffraction studies show that all the
compounds have tetragonal symmetry and that for YBa2Cu3-xCO(x)O7-y the
a-axis expands while the c-axis contracts with increasing Co
concentration x. AC susceptibility measurements reveal
superconductivity in annealed crystals with x less than approximately
0.3.
