  Temperature measurements of dielectric permittivity are performed for
nonstoichiometric ferroelectric lead germanate Pb$_{4.95}$Ge$_3$O$_{11}$ and
multiferroic solid solution [N(C$_2$H$_5$)$_4$]$_2$CoClBr$_3$. Unlike the heat
capacity data, the analysis of the dielectric permittivity of ferroics is
usually performed at the assumption that the dielectric "background" is
negligible compared with its critical part. In this work we quantitatively
interpret the dielectric properties of the single crystals mentioned above and
the appropriate literature data for multiferroic Sr$_2$IrO$_4$ crystals, using
generalized Curie-Weiss formulas that combine corrections due to a nonlinear
temperature-dependent dielectric background, a modified critical index of
electric susceptibility, and a diffuse character of phase transition. We argue
that taking account of the temperature dependent dielectric background can
improve notably the quantitative analysis of PTs for a number of classes of the
ferroic materials.
