Range in Cation Ratio of Crystallized From Liquids

The primary phase field of YBa₂Cu₃O_7-δ (Y123) is roughly triangular (with curvilinear boundaries). Apices of the triangle are the maximum at which liquid, Y123, and Y₂BaCuO₅ (Y211) coexist and those ternary invariant points at which liquid, Y123, Y211, and either CuO or BaCuO₂ coexist. The ternary eutectic lies on the low-temperature side of the triangle. Melting experiments around this primary phase field allow documentation of the composition range in Y123 formed in equilibrium with a liquid and in pure O₂. The highest temperature around the primary phase field is 1027°C; Y123 there is the most V-rich (Cu: V slightly below 3). Liquids in equilibrium with Y123 and V211 descend from the maximum to an invariant point at 955°C at which CuO also forms. Ba: Y of Y123 decreases to 1.88; Cu: Y remains near 3. Liquids in equilibrium with Y123 and CuO join that invariant point to the eutectic (L = Y123 + CuO + BaCuO₂). Y123 on the coprecipitation curve increases in Cu : V to a maximum at the 910°C eutectic (Cu : Y = 3.28). Cu: Y of Y123 along the other circuit of the triangle has limited variation but decreases to 2.96 near the invariant point at which BaCuO₂ joins the crystallizing assemblage. These are greater variations in cation composition than previously documented and raise issues of how superconducting properties of Y123 vary with cation composition.