  Crystallography and dielectric properties in Dion-Jacobson layered
perovskites, CsNdNb$_2$O$_7$ and RbNdNb$_2$O$_7$, have been examined in dense
polycrystalline samples, and polarization hysteresis loops that substantiate
ferroelectricity have been observed at room temperature. The theoretical
mechanism for the spontaneous polarization, "hybrid improper ferroelectric
mechanism,'' induced by a combination of two types of non-polar octahedral
rotations, is confirmed in these two phases. Our samples show remanent
polarizations of 2-3 $\mu$C/cm$^2$, which are much larger than those obtained
in polycrystalline samples with the hybrid improper ferroelectricity reported
so far. A dielectric constant in CsNdNb$_2$O$_7$ exhibits an anomaly at 625 K,
corresponding to the ferroelectric transition, as previously revealed by x-ray
and neutron diffractometry. No dielectric anomaly is observed for
RbNdNb$_2$O$_7$ throughout the temperature range studied here ($\leq$ 773 K),
which is consistent with the previous diffractometry showing the persistence of
polar $I2cm$ symmetry up to 790 K.
