Enhanced absorption of hydrogen in ball milled lanthanum for hydride formation*

Superhydride compounds tend to exhibit high temperature superconductivity. A challenge with forming these compounds is that it is hard to incorporate the large over-stoichiometric amounts of hydrogen needed to promote superconductivity into a reacted product. High pressures and temperatures are required to form the presently known superhydride compounds with high transition temperatures. We attempt to improve the reactivity to hydrogen in elemental lanthanum using a cryogenic ball milling technique. A lanthanum precursor was milled for different lengths of time and exposed to solid ammonia borane. The resulting mix was exposed to high pressure in a diamond anvil cell at room temperature and X-ray diffraction was performed at HPCAT to explore any phase transformations and to measure the unit cell volume in order to infer the hydrogen content. With increased milling time, we were able to improve the hydrogen loading so as to form LaHx of various stoichiometries.