  Diffraction experiments with holographic gratings recorded in SiO$_2$
nanoparticle-polymer composites have been carried out with slow neutrons. The
influence of parameters such as nanoparticle concentration, grating thickness
and grating spacing on the neutron-optical properties of such materials has
been tested. Decay of the grating structure along the sample depth due to
disturbance of the recording process becomes an issue at grating thicknesses of
about 100 microns and larger. This limits the achievable diffraction efficiency
for neutrons. As a solution to this problem, the Pendell\"{o}sung interference
effect in holographic gratings has been exploited to reach a diffraction
efficiency of 83% for very cold neutrons.
