A glass-forming molten salt exhibiting the universal dynamic properties in both ion conductivity relaxation and liquid–glass relaxation: Ca0.4K0.6(NO3)1.4

Abstract

The paradigmatic system of Ca0.4K0.6(NO3)1.4 (CKN) as a glass-forming molten salt, simultaneously showing multifaceted relaxation processes of both ion conductivity and liquid–glass transition, is pivotal to both fundamental physics and applied energy technologies, yet critical questions of relaxation dynamics persist after extensive studies over 50 years. The intricate relaxation dynamics of CKN, together with its structural complexity, provide unique testing grounds for the Coupling Model (CM), which has predicted universality of the dynamic processes and their properties in glass-forming materials irrespective of physical structure and chemical composition over the past 45 years, as reviewed recently in Ngai [Prog. Mater. Sci. 139, 101130 (2023)]. By unifying decades of experimental and theoretical insights on the relaxation behaviors of CKN, many universal properties in both conductivity and liquid–glass relaxation have been extracted for the first time and explained in this paper. We demonstrate that the dual relaxation processes of CKN conform well to CM-predicted universality, including several critical transitions of temperature-dependent properties at Tg and TB. Furthermore, the strong coupling of the conductivity relaxation of CKN to its structural relaxation processes is revealed by their related or shared properties. These positive results highlight the universal properties as predicted by the CM and enhance our understanding of the physical origins of structural and ion conductivity relaxation dynamics. Therefore, it not only advances the fundamental science of disordered glasses but also provides a template for studying relaxation behavior of complex systems.