Haven ratio for correlated ion hopping in oxide glasses from NMR spin-lattice relaxation.

Abstract

The ionic transport in glasses and supercooled liquids exhibits complex dynamical behavior characterized by non-exponential correlation functions, often described by stretched exponential decay. This study investigates the connection between the Haven ratio HR, which measures the deviation of diffusivity of modifier alkali cations from random walk due to their backward-correlated hopping, and the stretching exponent β of the orientational correlation function associated with the nuclear magnetic resonance spin-lattice relaxation (NMR SLR) of these alkali nuclides. By analyzing the temperature-dependent NMR SLR rate data of alkali ions in a wide range of supercooled oxide network liquids, this study reveals a hitherto unknown approximate equality between HR and β. This relationship is shown to be consistent with a model of backward-correlated hopping of mobile modifier ions in a temporally frozen oxide network. Estimation of NMR SLR β for individual alkali ions in mixed-alkali systems offers a pathway to estimate species-specific HR values that are otherwise experimentally inaccessible. These findings, when taken together, suggest that β can serve as a proxy for HR and offer new insight into the microscopic nature of glassy ion transport.