BH4− self-diffusion in liquid LiBH4.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Neutron Scattering

Background:

• Understanding hydrogen dynamics in ionic solids like lithium borohydride (LiBH4) is crucial for energy storage applications.
• Previous studies on LiBH4 have focused on its structural properties, with limited information on the detailed mechanisms of hydrogen motion.

Purpose of the Study:

• To elucidate the hydrogen dynamics in both solid and liquid phases of LiBH4.
• To characterize the diffusion mechanisms and their temperature dependence.

Main Methods:

• Incoherent quasielastic neutron scattering (IQNS) was employed to probe atomic motions.
• Measurements were conducted across various temperatures, including solid and molten phases of LiBH4.

Main Results:

• Rotational jump diffusion of the borohydride (BH4-) anion was identified in solid LiBH4 on the picosecond timescale.
• A significant decrease in the characteristic time constant for rotational diffusion was observed upon transitioning from the low- to high-temperature solid phase (383 K).
• Translational diffusion of BH4- units was detected in molten LiBH4 (above 553 K), with diffusion coefficients around 10(-5) cm2/s at 700 K, similar to molten alkali halides.

Conclusions:

• The study reveals distinct hydrogen diffusion mechanisms in solid and liquid LiBH4.
• The observed Arrhenius behavior for diffusion in the molten phase (Ea = 88 meV, D0 = 3.1 × 10(-4) cm2/s) provides quantitative insights into ion transport.