Ba2-xLaxYCu3O7±δ perovskite compounds: Crystal chemistry

We report the crystal chemical properties of solid solutions obtained by substitution of lanthanum for barium in the high-Tc superconductor Ba2YCu3O7. An orthorhombic-to-tetragonal symmetry change occurs at x=0.4 in Ba2-xLaxYCu3O7±δ with the tripled perovskite supercell maintained. Charge compensation is achieved through reduction of the formal copper valence (δ=0) in the orthorhombic phase and by a mixture of excess oxygen accommodation (δ>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.