LOW-TEMPERATURE OXYGEN ORDERING AND CARRIER CONCENTRATION IN YBA2CU3O6+X

Detailed magnetic susceptibility measurements were performed on YBa2Cu 3O6+x(0.36 < x < 0.94) particles embedded in a PrBa2Cu3Oy matrix. The magnetic penetration depth lambda and carrier concentration n were eva luated as functions of oxygen stoichiometry. The plateaus observed in the n(x) and T-c(x) curves in the range 0.45 less than or equal to x l ess than or equal to 0.70 are attributed to the formation of the Ortho -II structure, in which oxygen-filled and unfilled chains alternate. T he carrier concentration is found to rise linearly as x increases from 0.70 to 0.93, whereas T-c passes through a maximum at x approximate t o 0.89. The oxygen ordering during room-temperature holding reduces la mbda and extends the plateaus. The nonlinear variation of T-c with 1/l ambda(2) similar to n suggests that the carrier concentration in the C uO2 planes is not the only parameter determining the T-c of YBa2Cu3O6x.