Cobalt-doped YBCO single crystals and their accurate characterisation

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.