Least-Squares Analysis of the Dielectric Permittivity for Improper Ferroelectric Ammonium Fluoroberyllate

Using a nonlinear least-squares technique and statistical tests, we compare different theoretical models for description of temperature dependence of the dielectric permittivity in the incommensurate phase of improper ferroelectric crystal, ammonium fluoroberyllate. According to reduced chi-square and Wald-Wolfowitz tests, a model suggested by Levanyuk-Sannikov turns out to outperform a model by Prelovsek, Levstik and Filipic. Since the anomalous dielectric peak for the ammonium fluoroberyllate crystals is relatively small, a constant background contribution to the dielectric permittivity is taken into account in the nonlinear fitting.