Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.