Structure and superconducting properties of Ga-substituted YBa₂Cu₃O_7–δand YBaSrCu₃O_7–δsystems

In YBa2Cu3–yGayO7–δ, Ga can be substituted at the Cu(1) site up to y= 0.1 without change in structure, but this is accompanied by a slight decrease in the hole concentration and Tc; the same is true when Y is partly substituted by Ca, as in Y1–xCaxBa2Cu3–yGayO7–δ(0.0<x⩽0.2). When one Ba is replaced by Sr as in YBaSrCu3–yGayO7–δ, however, Ga can be substituted at the Cu(1) site to a much greater extent (up to y= 0.6). In this system, Ga substitution changes the structure from orthorhombic to tetragonal, unlike in YBa2Cu3–yGayCu3O7–δ. Both the hole concentration and Tc decrease with an increase in y and the material becomes non-superconducting for y> 0.2. The y= 0.3 and 0.4 compositions show metal–semiconductor transitions at 60 and 120 K, respectively, while the y= 0.6 composition is a semiconductor. When Y is partly substituted by Ca as in Y1–xCaxBaSrCu3–yGayO7–δ, the material is superconducting even when y= 0.3. All these Ga-substituted cuprates are in the underdoped region and accordingly Tc increases with increase in hole concentration.