  We study the temperature dependence of dielectric permittivity along the
polar axis for ferroelectric ammonium fluoroberyllate (AFB) crystal in the
vicinity of its phase transition points. The experimental data within
incommensurately modulated phase of AFB is compared with the predictions of
phenomenological models known from the literature: the Curie-Weiss (CW) law,
the generalized Curie-Weiss (GCW) law, and the models by Levanyuk and Sannikov
(LS) and by Prelov\v{s}ek, Levstik and Filipi\v{c} (PLF) suggested for improper
ferroelectrics. It is shown that the LS approach describes the temperature
behavior of the dielectric permittivity for the AFB crystal better than the CW,
GWC and PLF models. The main physical reasons of this situation are elucidated.
