The Ba sub (2-x) La sub x YCu0 sub (7+-delta) perovskite compounds, crystal chemistry.

We report the crystal chemical properties of solid solutions obtained by substitution of lanthanum for barium in the high T sub c superconductor Ba sub 2 YCu sub 3 0 sub 7. An orthorhombic to tetragonal symmetry change occurs at x=0.4 in Ba sub (2- x) La sub x YCu sub 3 0 sub (7+-delta), with the tripled perovskite supercell maintained. Charge compensation is achieved through reduction of the formal copper valence (delta=0) in the orthorhombic phase and by a mixture of excess oxygen accommodation (delta > 0) and formal valence reduction in the tetragonal phase. The resistivity increases by several orders of magnitude, even though the average formal copper valence changes very little.