Structure and superconducting properties of Ga-substituted YBa₂Cu₃O_7−δ and YBaSrCu₃O_7−δ systems

Abstract

In YBa₂Cu_3−yGa_yO_7−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 T_c; the same is true when Y is partly substituted by Ca, as in Y_1−xCa_xBa₂Cu_3−yGa_yO_7−δ,(0 .0<x≤0.2). When one Ba is replaced by Sr as in YBaSrCu_3−yGa_yO_7−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 YBa₂Cu_3−yGa_yCu₃O_7−δ,. Both the hole concentration and T_c 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 Y_1−xCa_xBaSrCu_3−yGa_yO_7−δ,, the material is superconducting even when y=0.3. All these Ga-substituted cuprates are in the underdoped region and accordingly T_c increases with increase in hole concentration.