Site dependent hardening of the lanthanum metal lattice by hydrogen absorption

The compressibility of lanthanum (La) metal and its hydrides were measured at room temperature by high pressure synchrotron X-ray diffraction. La metal pressurized in a hydrogen medium forms a hydride with an fcc metal lattice, which likely contains hydrogen at a concentration close to 3.0 and persists over the measured pressure span up to 21 GPa. Equations of state have been determined by helium compression experiments for LaH2 with tetrahedral interstitial sites fully occupied with hydrogen atoms and for LaH2.46 with octahedral interstitial sites partially occupied with hydrogen atoms and tetrahedral sites fully occupied. Both hydrides possess fcc metal lattices. The bulk modulus values B0B0 are 66.7 ± 1.2 GPa for LaH2 and 68.4±1.0 GPa for LaH2.46. These values are three times larger than that of La metal and are very close to each other despite the difference in hydrogen occupation states. The hardening of the metal lattice by hydrogenation is attributed predominantly to hydrogen–metal interactions at the tetrahedral sites and is most pronounced for La, which has the largest ionic radius among rare-earth metals.

Research highlights
► The metal lattice of LaHx becomes 3 times harder than that of the parent La metal.
► Bulk modulus B0B0 becomes almost unchanged as the O-site hydrogen increases.
► Lattice hardening of the La–H system is attributed to the T-site hydrogen atoms.
► The O-site hydrogen atoms have a negligible influence on compressibility.