Structural changes and superconducting properties of Gd1−2xPrxCaxBa2Cu3O7−δHTSCs

We report on structural changes and the properties of Gd1−2xPrxCaxBa2Cu3O7−δ (0≤x≤0.3) high temperature superconductors which were prepared and characterized. The transition temperature (Tc) was found to decrease roughly linearly with respect to the Pr and Ca concentration x. 65Cu2+(3d9) and 63Cu2+(3d9) ions from impurity phase BaCuO2 give rise to X-band electron spin resonant (ESR) two-quartet absorption peaks centred at ∼310 mT (g∼2.020) which are superposed by the broad ESR peak of Gd3+(4f7) at g∼2.000 in the undoped Gd123. Both sets of ESR peaks shift towards low fields as the temperature is lowered, which is due to the change in crystal field experienced by the ions. It should be noted here that even at 77 K there still is no magnetic ordering from Gd3+ ions. However, at x = 0.025 doping, a new six-peak hyperfine multiplet ESR signal arising from 141Pr4+ (4f1,S = 1/2,I = 5/2) appears at ∼323 mT (g∼2.029). Typical simulated ESR parameters of Gd3+ in Gd1−2xPrxCaxBa2Cu3O7−δ at x = 0.025, are gx = 2.4879, gy = 2.3921 and gz = 2.2590, Axx = 12.50, Ayy = 23.50, Azz = 32.00 G and the zero-field splitting parameter (D) of 66.0 G, respectively. This signal arises from some Pr ions entering the Gd site with a valence of four, thus lending support to the hole-filling and pair-breaking models.