LOCATION OF CALCIUM IN CA SUBSTITUTED YBA2CU4O8, AND IMPURITY PHASES IN OFF-STOICHIOMETRY COMPOSITIONS

The location of Ca in calcium-substituted YBa2Cu4O8 (124) has been det ermined by identifying the phases formed during the reaction of severa l single-phase Ca substituted YBa2Cu3O7 (123) with CuO in high-pressur e oxygen. Starting compositions which are Ba poor or Y poor give die s ame superconducting phase but different impurity phases: (Ba,Ca)(2)Cu3 O6 (236) or (Ca,Y)(4)Cu5O10 (4510). The measured concentrations of the se phases tally with the material balance if Ca substitutes at the Y s ites in 124. This Y site occupation is preferred by Ca even in Ba poor compositions which could provide ample vacant positions at Ba sites a nd not at Y sites. Although Ca is soluble up to 0.3 atom per formula u nit in 123, the excess over 0.1 is expelled during formation of 124, i ndicating that the thermodynamic limit of solubility has been reached. When (Y0.9Ca0.1)Ba2Cu4O8 is heated up to 800 degrees C, oxygen is ret ained as in pure 124. Thus 123 intergrowths are ruled out as a cause o f the T-c enhancement to 90 K in Ca substituted 124.