Heterogeneous Dynamics of Prototypical Ionic Glass CKN Monitored by Physical Aging

In this Letter, we investigate the time evolution of the conductivity relaxation process in a prototypical ionic glass former, $[\mathrm{Ca}({\mathrm{NO}}_3{)}_2{]}_{0.4}[{\mathrm{KNO}}_3{]}_{0.6}$ (CKN), undergoing physical aging. It is demonstrated that the heterogeneous nature of molecular dynamics is manifested by an increase in the slope of the high frequency wing of the conductivity relaxation peak as the sample is annealed below the glass transition temperature. This finding is also confirmed for other ionic glass formers. Additionally, we analyze the kinetics of the change in the ionic conductivity in glassy CKN to probe its structural relaxation.

Figure 1

(a) The dielectric loss modulus spectra due to conductivity relaxation of CKN at various temperatures in the liquid and glassy states. (b) The shift of the conductivity peak during physical aging at a temperature 313 K.

Figure 2

(a) Two $M^{\prime \prime }$ spectra measured at 303 and 308 K, both recorded after rapid cooling from $T=353\mathrm{K}$; and the final spectrum recorded during the physical aging at 308 K (after 20 h). Inset: evolution of ${\beta }_{\mathrm{KWW}}$ parameter during aging at 308 K. (b) Superposition of all $M^{\prime \prime }$ spectra recorded at the beginning of isothermal aging measurements depicted together with a fully aged spectrum at 308 K. The solid line is the Kohlrausch function with ${\beta }_{\mathrm{KWW}}=0.71$.

Figure 3

Comparison of the shape of the rapidly quenched and aged modulus spectra of three different ionic liquids. (a)–(b) insets: $\log {\tau }_{\sigma }(1000/\mathrm{T})$ dependences. In (c) inset: comparison of the changes of the ${\tau }_M$ during physical aging and rejuvenation experiments is presented.

Figure 4

(a) Change in ${\tau }_{\sigma }$ observed during physical aging at the indicated temperatures. Solid lines are fits by means of Eq. (1). (b) Temperature dependence of ${\beta }_{\mathrm{KWW}}$ parameter. Open circles are the values of ${\beta }_{\mathrm{age}}$ obtained from analysis of the aging data by Eq. (1). Closed circles are ${\beta }_{\mathrm{KWW}}$ parameters determined based on the enthalpy relaxation measurements presented in Ref. 28. Open squares present the values of ${\beta }_{\mathrm{KWW}}$ determined from fitting of KWW function to the dc-conductivity relaxation peaks measured in glassy and supercooled liquid states.

Figure 5

Conductivity relaxation times for CKN obtained after rapid cooling of the molten salt (filled circles) and slow cooling of the melt (open circles). Solid triangles are the structural relaxation times extracted from the change in ${\tau }_{\sigma }$ during physical aging. Additionally, ${\tau }_{\sigma }(T)$ and ${\tau }_{\alpha }(T)$ presented in Refs. 24, 29 are depicted. Solid and dashed lines are the Arrhenius fits to the ${\tau }_{\sigma }$ and ${\tau }_{\mathrm{age}}$, respectively. Dotted line is the Arrhenius fit with the activation energy following from the equation $E_{\alpha }=s^{-1*}{E}_{\sigma }$. Inset: ${\tau }_{\sigma }$ vs. ${\tau }_{\alpha }$ in a double logarithmic scale. Parameter $s=0.62$ is the exponent in the fDSE equation.