Effect of Ca doping for Y on structural / microstructural and   superconducting properties of YBa2Cu 3O7-d
  We report systematic studies of structural, microstructural and transport properties of Y1-xCaxBa2Cu3O7-d bulk samples with 0.20 x 0.0. The partial substitution of Ca2+ at Y3+ site in YBa2Cu3O7-d leads towards slightly overdoped regime, which along with disorder in CuO2 planes is responsible for decrease in superconducting transition temperature (Tc) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation. 
Rajiv Giri, H.K. Singh, O.N. Srivastava (BHU, Varanasi, India), V.P.S.   Awana, Anurag Gupta, B.V. Kumaraswamy and H. Kishan...