Location of calcium in Ca substituted YBa₂Cu₄O₈, and impurity phases in off-stoichiometry compositions

Abstract

The location of Ca in calcium-substituted YBa₂C_u4O₈ (124) has been determined by identifying the phases formed during the reaction of several single-phase Ca substituted YBa₂Cu₃O₇ (123) with CuO in high-pressure oxygen. Starting compositions which are Ba poor or Y poor give the same superconducting phase but different impurity phases: (Ba,Ca)₂Cu₃O₆(236) or (Ca,Y)₄Cu₅O₁₀ (4510). The measured concentrations of these phases tally with the material balance if Ca substitutes at Y sites in 124. This Y site occupations is preferred by Ca even in Ba poor compositions which could provide ample vacant positions at Ba sites and not at Y sites. Although Ca is soluble up to 0.3 atom per formula unit in 123, the excess over 0.1 is expelled during formation of 124, indicating that the thermodynamic limit of solubility has been reached. When (Y_0.9Ca_0.1)Ba₂Cu₄O₈ is heated up to 800° C, oxygen is retained as in pure 124. Thus 123 intergrowths are ruled out as a cause of the T_c enhancement to 90 K in Ca substituted 124.