Dielectric relaxation of mixed crystals of Rb_1-x(NH₄)_xH₂PO₄ at microwave frequencies

Measurements of the clamped dielectric constant on mixed single crystals of Rb1-x(NH4)xH2PO4, x=0.35, are reported up to 11 GHz. Between 24 and 60 K, the dielectric dispersion can be fitted to a Cole-Cole relaxation, the parameters of which indicate a temperature-dependent distribution of relaxation times consistent with Vogel-Fulcher freezing. Both the audio and the microwave measurements can be scaled up to ≊ 100 MHz, with a freezing temperature of To≅8.4 K. In the GHz range, a relaxation process in addition to the low-frequency freezing mode contributes to the dielectric response. © 1988 Springer-Verlag.