Structural refinements of Yb₂BaCuO₅ and Lu₂BaCuO₅ by powder neutron diffraction

The structure of the "211" or "green phases," Yb₂BaCuO₅ and Lu₂BaCuO₅, have been refined from time-of-flight powder neutron diffraction data. Both crystallize in the orthorhombic system, space group Pnma, with lattice parameters a = 12.0588(1), Å, b = 5.6123(1), Å, c = 7.0535(1), Å, and a = 12.0314(1), Å, b = 5.5989(1), Å, c = 7.0380(1), Å, respectively. The phases are isostructural with Y₂BaCuO₅, consisting of a three-dimensional framework of MO₇ units (M = Yb, Lu) enclosing cavities occupied by the Ba and Cu atoms. Ba is 11-coordinate and Cu 5-coordinate, in a distored squarepyramidal environment.