INFLUENCE OF DISORDER ON THE VORTEX-LATTICE MELTING IN TWINNED OXYGEN-DEFICIENT ORTHO-II YBA2CU3OY (Y-APPROXIMATE-TO-6.6)

We investigated with ac susceptibility the mixed state of twinned, deo xygenated single crystals and highly c-axis-oriented melt-textured sam ples of YBa2Cu3O6-x with oxygen stoichometry corresponding to that of the orthorhombic II phase. In the single crystal we found, below H ap proximate to 700 Oe and over almost four orders of magnitude in freque ncy, a frequency-independent ''irreversibility line'' (IL). Complement ry de magnetization measurements seem to disregard an IL arising from geometric effects when H\\c, and they suggest a possible meltinglike t ransition of the flux-line lattice below H. The reduced value of H* i s associated with the increased anisotropy induced by the oxygen deple tion, and an upper limit of the anisotropy parameter is estimated as g amma(max) approximate to 147. However, in the twinned melt-textured sa mple having a major grade of disorder due to the presence of micrometr ic nonsuperconducting precipitates of Y2BaCuO5, the IL is frequency de pendent for all applied magnetic fields, suggesting a cancellation of H in the sufficiently disordered specimen.