  Sliding ferroelectricity is a recently observed polarity existing in
two-dimensional materials. However, due to their weak polarization and poor
electrical insulation in these materials, all available experimental evidence
till now are indirect, with most based on transport properties in the nanoscale
or piezoresponse force microscopy. We report the direct observation of sliding
ferroelectricity, using a high-quality amphidynamic single crystal,
(15-Crown-5)Cd$_3$Cl$_6$, which possesses a large band-gap and so allows direct
measurement of P-E hysteresis. This coordination polymer is a van der Waals
material, which is composed of inorganic stators and organic rotators as
measured using XRD and NMR characterisation. From DFT calculations, we find
that after the freezing of rotators an electric dipole is generated in each
layer driven by the geometric mechanism, meanwhile a comparable ferroelectric
polarization originates from the interlayer sliding. The net polarization of
these two components can be directly measured and manipulated. Our finding
provides insight into low-dimensional ferroelectrics, especially the
controlling of synchronous dynamics of rotating molecules and sliding layers in
solids.
