Nonlinear background corrections to dielectric permittivity of ferroics and multiferroics

Temperature measurements of dielectric permittivity are performed for nonstoichiometric ferroelectric lead germanate Pb_(4.95)Ge_3O_(11) and multiferroic solid solution [N(C_2H_5)_4]_2CoClBr_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 Sr2IrO4 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