Most lanthanides have a hcp crystal structure.
The simplest way of loading the film with hydrogen is by putting the sample in hydrogen gas. First a fcc dihydride is formed, which upon further loading transforms into a trihydride. For the low atomic mass lanthanides this again has an fcc structure while for the high molecular weight lanthanides this trihydride has a hcp structure. In all cases only the reaction from dihydride to trihydride is reversible. Dehydrogenation can be carried out by putting the sample in vacuum or air.