Anharmonic lattice dynamics of superionic lithium nitride

Abstract

Superionic crystals reach an ionic conductivity comparable to liquid electrolytes following a superionic transition at high temperature. The physical mechanisms that lead to this behaviour remain poorly understood. It has been proposed that superionic transitions are accompanied by the breakdown of specific phonon modes linked to characteristic diffusion processes. Any changes in vibrational properties across the superionic transition may therefore provide insights into the underlying physics of this phenomenon. Here, we apply a combination of lattice dynamics and ab initio molecular dynamics to probe the vibrational properties of the archetypal superionic conductor Li3N. We assess harmonic, quasi-harmonic, and anharmonic descriptions of the phonons. The harmonic and quasi-harmonic models show no change in features across the superionic transition. The fully anharmonic model, however, exhibits a phonon breakdown for all modes above the superionic transition temperature. The implications for developing lattice-dynamics based descriptors for superionic conductors are discussed.

Understanding fast-ion conduction in solid electrolytes

“…In other materials, fast-ion conduction has been associated with specific host-framework dynamics, such as dynamical ‘breathing’ of diffusion-limiting bottlenecks [ 22 ], or polyanion rotations that may directly couple to the motion of the charge-carrying ions [ 24 , 36 – 38 ]. Investigations of the vibrational properties of fast-ion conductors have identified correlations between simple descriptors computed from the vibrational densities of states and ionic conductivities within families of solid electrolytes [ 9 ], while other studies have shown dramatic changes in the phonons of solid electrolytes as they undergo phase transitions from a low-temperature poorly conducting phase to a high-temperature ‘superionic’ phase [ 39 – 41 ]. Another interesting observation is that in a number of materials the mobile ions exhibit dynamical behaviours that are qualitatively similar to those in supercooled glass-forming liquids [ 16 , 18 , 21 , 31 , 42 , 43 ], with, for example, both classes of systems displaying ‘dynamic heterogeneity’, where some fraction of atoms participate in rapid highly correlated diffusion while other atoms are significantly less mobile [ 44 , 45 ].…”