Enhancement of hydrogen absorption and hypervalent metal hydride formation in lanthanum using cryogenic ball milling

TL;DR

This study demonstrates that cryogenic ball milling of lanthanum precursor enhances hydrogen absorption and enables formation of hypervalent LaH4 at lower pressures, advancing superhydride synthesis.

Contribution

It introduces a cryogenic ball milling method to improve hydrogen reactivity and stabilize hypervalent lanthanum hydrides at reduced pressures.

Findings

Hydrogen absorption increases with milling time.

Hypervalent LaH4 phase formed at pressures up to 60 GPa.

Cryomilling suppresses rhombohedral distortion in LaH4.

Abstract

Rare earth superhydrides exhibit high temperature superconductivity but are difficult to characterize and use in applications due to their high formation and stability pressures, which are typically in excess of 100 GPa. We studied how modification of the rare earth precursor improves hydrogen reactivity and hydrogen uptake for forming such metal hydrides at lower pressures. An elemental lanthanum precursor was milled at liquid nitrogen temperatures for different time intervals. After exposure to gaseous hydrogen at 380 C and 100 bar, we found a systematic enhancement of hydrogen absorption with increasing ball milling time for forming the LaHx, x=2-3 phase. Exposing the precursor to pressures up to 60 GPa with an ammonia borane (BNH6) hydrogen source resulted in a hypervalent LaH4 phase. This LaH4 phase is associated with the suppression of a rhombohedral distortion of the Fm3-m cubic structure after cryomilling the precursor.