  The charged domain walls in ferroelectric materials exhibit intriguing
physical properties. We examine herein the charged-domain-wall structures in
Ca$_{3-x}$Sr$_x$Ti$_2$O$_7$ using transmission electron microscopy. When viewed
along the [001] axis, the wavy charged domain walls are observed over a wide
range ($>$5 $\mu$m). In contrast, short charged-domain-wall fragments (from 10
to 200 nm long) occur because they are intercepted and truncated by the
conventional 180$^{\deg}$ domain walls. These results reveal the unusual
charged domain structures in Ca$_{3-x}$Sr$_x$Ti$_2$O$_7$ and will be useful for
understanding their formation process.
