Abstract
Dynamic crossover above Tg has been recognized as a
characteristic feature of molecular dynamics of liquids approaching glass
transition. Experimentally, it is manifested as a change in Vogel−Fulcher−
Tammann dependence or a breakdown of the Stokes−Einstein and related
relations. In this paper, we report the exception from this rather general
pattern of behavior. By means of dielectric, ultrasonic, rheological, and
calorimetric methods, dynamics of two good ionic conductors
(BMIm)2[Co(NCS)4] and (EMIm)2 [Co(NCS)4] of less common
stoichiometry (2:1) was studied in a very broad temperature range.
However, none of the mentioned dynamic changes was observed in the
entire studied temperature range. On the contrary, the single VFT and the
same fractional Walden coefficient were found for conductivity and
viscosity changes over 12 decades. Moreover, ultrasonic studies revealed
that the data at temperatures which cover the normal liquid region cannot be fitted by a single exponential decay, and the Cole−
Cole function should be used instead.