Q-dependent Collective Relaxation Dynamics of Glass-Forming Liquid Ca0.4K0.6(NO3)1.4 Investigated by Wide-Angle Neutron Spin-Echo

TL;DR

This study uses wide-angle neutron spin echo spectroscopy to analyze the Q-dependent relaxation dynamics of a glass-forming ionic liquid, revealing two-step relaxation, de Gennes narrowing, and temperature-dependent heterogeneity in the supercooled state.

Contribution

It provides new insights into the microscopic relaxation mechanisms and dynamic heterogeneity in ionic glass formers across different length scales and temperatures.

Findings

Observation of two-step relaxation process.

De Gennes narrowing in alpha relaxation variables.

Relaxation mechanisms modify above the bifurcation temperature.

Abstract

Employing wide-angle neutron spin echo spectroscopy, we measured the Q-dependent coherent intermediate scattering function of the prototypical ionic glass former Ca0.4K0.6(NO3)1.4, in the equilibrium and supercooled liquid states beyond the hydrodynamic regime. The data reveal a clear two-step relaxation: an exponential fast process, and a stretched exponential slow alpha process. de Gennes narrowing is observed in all characteristic variables of the alpha process: the relaxation time, amplitude, and stretching exponent. At all length scales probed, the relative amplitude of the alpha-relaxation decreases with increasing temperature and levels off in the normal liquid state. The temperature dependence of the stretching exponent and the relaxation time at different Q's indicate that modifications of the relaxation mechanisms at the local length scales, manifested as temperature independent dynamic heterogeneity and smaller deviations from Arrhenius behavior, have occurred even above the alpha-beta (Johari-Goldstein) bifurcation temperature.