  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.
