Low-frequency dielectric dispersion of (NH4)2BeF4 and Rb2ZnCl4 crystals

Low-frequency dielectric dispersion was observed on AFB and Rb2ZnCl4 crystals above room temperature. Similar results obtained by Stumpe et al. on perovskites at higher temperatures were explained by a boundary-layer model which may hold even in our case. If, however, the logarithms of β' and β'' of the complex dielectric impermeability 1/ε* are plotted vs. the logarithm of the frequency, curves are obtained very similar to those for the complex shear modulus of glasses above the glass temperature. The plot of the logarithm of the frequency of β'' maximum vs. temperature may be understood in terms of the Arrhenius formula in the former case or it can be interpreted as a frequency-temperature diagram of a transition between solid and yield properties of glass-like matter which can be fitted by the WLF equation. This latter interpretation is suggested by the glass-like anomalies of the heat capacity and thermal expansion of A2BX4 crystals.