Glassy dynamics of ${\mathrm{Rb}}_{0.40}$(${\mathrm{ND}}_4$${)}_{0.60}$${\mathrm{D}}_2$${\mathrm{PO}}_4$

The complex dielectric constant of ${\mathrm{Rb}}_{0.40}$(${\mathrm{ND}}_4$ ${)}_{0.60}$ ${\mathrm{D}}_2$ ${\mathrm{PO}}_4$ has been measured in the frequency interval from 1 mHz to 1 GHz at temperatures between 40 and 140 K. A novel frequency-temperature plot of the data is introduced to show that the behavior of the relaxation spectrum is reflected in the real part of the dielectric constant. It is found that the highest relaxation frequency ${\mathit{f}}_2$=1/2π${\mathrm{\tau }}_{\min }$ follows an Arrhenius behavior, whereas the lowest frequency ${\mathit{f}}_1$=1/2π${\mathrm{\tau }}_{\max }$ exhibits a critical decay according to the Vogel-Fulcher law ${\mathit{f}}_1$∝exp[-U/(T-${\mathit{T}}_0$)] with ${\mathit{T}}_0$=32.9 K.