Introducing Ca or Sr ion into GdBa2Cu3Oy, Ca acting as a hole dopant at the R site and a hole filler at the Ba site. Tc is linearly dependent on the orthorhombicity of the unit cell for all three series and it is also correlated to the ∠O(1)–Ba–O(1) along the unit cell a-axis, which is the relative positions of the Ba and the epical O(1) sites.
