Hydrogen Pairing and Anisotropic Potential for Hydrogen Isotopes in Yttrium

The potential for H isotopes in $\alpha$-phase (hexagonal) ${\mathrm{YH}\left(\mathrm{D}\right)\mathrm{}}_{0.18}$ has been probed by neutron spectroscopy. The results reveal a highly unusual potential, which is considerably softer and anharmonic along the $c$ axis than in the basal plane, even though the near-neighbor arrangement of metal atoms is only slightly distorted from regular tetrahedral symmetry. Low-temperature results show a splitting of the $c$-axis vibrational modes, consistent with the pairing of hydrogen on either side of yttrium atoms.