Comparison of superconductivity and structure for YBa2Cu3Oy with potassiumand sodium doping

The nominal composition of YBa2-xMxCu3Oy (M = K, Na) cuprates with x less t han or equal to 0.30 were synthesized by the standard solid-state reaction technique. X-ray diffraction (XRD) and the resistivity measurements were us ed to characterize the structure and the superconductivity of the doped cup rates. There was no impurity phase detected within the whole doping range. The structure of the main phase ("123") has the orthorhombic with P-mmm sym metry. With increasing the content of dopants, the lattice constants and so me other structural parameters are almost unchanged for M = K, whereas they changed for M = Na. The refined contents of dopants are consistent with th at of the nominal ones. The zero resistance temperature T-c0 decreases shar ply with the increase of the content of potassium in potassium-doped sample s as x less than or equal to 0.20. For sodium-doped YBa2-zNaxCu3Oy cuprates , T-c0 varies very little. The difference in superconductivity depression m ay result from the shift oxygen, which transfers conducting carriers from C u-O chains to Cu-O sheets or the structural stress effect.