Ba2-xLaxYCu3O7+/- 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/O/sub 7/. An orthorhombic-to-tetragonal symmetry change occurs at x = 0.4 in Ba/sub 2-//sub x/La/sub x/YCu/sub 3/O/sub 7//sub +- //sub 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.