  Extensive studies have been performed on layered compounds, ranging from
layered cuprates to van der Waals materials with critical issues of
intergrowths and stacking faults. However, such structures have been studied
less because of experimental difficulty. We present characteristic defect
structures of intergrowths in the Ruddlesden-Popper
Ca$_{2.46}$Sr$_{0.54}$Ti$_2$O$_7$, which is known to exhibit hybrid improper
ferroelectricity. Transmission electron microscopy reveals that numerous
intergrowths composed of 7 and 15 layers are introduced in the ferroelectric
domains. Elemental maps demonstrate that Sr ions are selectively substituted
into the perovskite layers of intergrowths. Density functional theory
calculations support the site-selective substitution of Sr ions, favorably
located in the intergrowths. The stabilization of the Ruddlesden-Popper phase
and intergrowths via Sr substitution can be explained by the ionic-radius
difference between Ca and Sr ions. The study reveals detailed defect structures
originating from the layered perovskite structure of
Ca$_{2.46}$Sr$_{0.54}$Ti$_2$O$_7$, and shows the usefulness of elemental
mapping in probing the substitution effects in oxides.
