STRUCTURE AND SUPERCONDUCTING PROPERTIES OF GA-SUBSTITUTED YBA2CU3O7-DELTA AND YBASRCU3O7-DELTA SYSTEMS

In YBa2Cu3-yGayO7-delta, 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 Y1-xCaxBa2Cu3-yGayO7-delta (0.0 < x less-than-or-equal-to 0.2). When one Ba is replaced by Sr as in YBaSrCu3-yGayO7-delta, 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-delta. 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 Y1-xCaxBaSrCu3-yGayO7-delta, 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.