Proton dynamics in lithium-ammonia solutions and expanded metals

Area of Science:

• Condensed Matter Physics
• Materials Science
• Physical Chemistry

Background:

• Understanding proton dynamics is crucial for various chemical and physical processes.
• Lithium-ammonia solutions exhibit unique liquid and solid phases with potential applications.
• Previous studies using Nuclear Magnetic Resonance (NMR) provided insights into these systems.

Purpose of the Study:

• To investigate proton dynamics in lithium-ammonia solutions across different concentrations and phases.
• To determine diffusion coefficients and rotational correlation times using quasielastic neutron scattering.
• To correlate observed dynamics with the electronic and ionic solvation in the system.

Main Methods:

• Quasielastic neutron scattering (QENS) was employed to probe atomic motions.
• Experiments were conducted on lithium-ammonia systems at varying metal concentrations (0-20 mole percent metal).
• Measurements covered both homogeneous liquid and solid (expanded metal) phases at different temperatures.

Main Results:

• Proton self-diffusion coefficient in liquid ammonia initially increases with metal concentration, peaking at 12 MPM, then slightly decreases at saturation (20 MPM).
• In the liquid state above melting point (100 K), proton dynamics fit a jump-diffusion model (l=2.1 Å, τ=3.1 ps).
• Solid phases exhibit jump diffusion (Phase I) and localized rotational motion (Phases IIa and IIb) with distinct correlation times.

Conclusions:

• Proton mobility in lithium-ammonia solutions is influenced by metal concentration and phase transitions.
• The observed diffusion behavior is consistent with competing electron and ion solvation effects.
• QENS provides detailed insights into the temperature and phase-dependent proton dynamics in these unique systems.