NEW AES MEASUREMENTS ON CHLORINATED TETRAGONAL YBA2CU3O7-X PHASES

Effects of oxidative chlorination on tetragonal YBa2Cu3O7-x are studie d. After reaction with gaseous chlorine at 415-435 K nonsuperconductin g YBa2Cu3O6.1 becomes superconducting. The formation of BaO2 and CuCl2 . 2H2O is shown by X-ray diffraction analysis. The Cl/element ratio i n the chlorinated-product phase determined by Auger electron spectrosc opy steadily decreases with sputtering time as a result of a topotacti cal solid-ps reaction. The chloride concentration on the grain surface approaches values similar to pure binary metal chlorides. Hence, a pa rtial substitution of chlorine for oxygen in the 123 lattice in this r egion can be excluded. Based on the experimental results a two-step me chanism of the formation of superconducting 123 phase is discussed. Fi rst, oxygen is released by oxidative chlorination especially in the su rface. In a second step the released oxygen is incorporated into unrea cted CuO2 layers forming the orthorhombic structure. Raman spectroscop y and the isotope-exchange behaviour support this mechanism.