Hydrogen and deuterium site separation in fcc-based mixed-isotope rare-earth hydrides

The structural arrangements and optic-vibrational densities of states of H and D in various fcc-based, mixed-isotope, rare-earth hydrides as measured by neutron powder diffraction and neutron vibrational spectroscopy clearly indicate preferential isotopic occupation of H in the octahedral interstices and D in the tetrahedral interstices. Although lessened by entropic contributions at high temperature, the degree of isotopic separation increases with decreasing temperature, presumably driven by the differences in the zero-point energies of H and D in the different interstitial sites. Decreasing the temperature below ≈150 K leads to a “freezing in” of the H and D lattice configuration prevalent near this temperature, kinetically precluding the complete isotopic site separation expected at temperatures approaching 0 K.