Neutron vibrational spectroscopy of the Pr₂Fe₁₇-based hydrides

Neutron vibrational spectroscopy measurements of Pr₂Fe₁₇H_x and Pr₂Fe₁₇D_x (x ≤ 5) reveal dynamic features consistent with the interstitial hydrogen locations previously determined by neutron diffraction. In particular, for Pr₂Fe₁₇H₃, two peaks centered at ≈85.4 and 106.0 meV correspond to the normal-mode vibrational energies of hydrogen in octahedral (o) sites comprised of a near-square planar arrangement of four Fe atoms and two apical Pr atoms. Based on bond distances and preliminary first-principles calculations, the lower-energy feature is assigned to the H_o vibration along the c-oriented Fe{single bond}H_o{single bond}Fe axis. The higher-energy feature is assigned to the other two normal-mode H_o vibrations along the orthogonal Fe{single bond}H_o{single bond}Fe and Pr{single bond}H_o{single bond}Pr axes in the basal plane. For Pr₂Fe₁₇H₄ and Pr₂Fe₁₇H₅, the lower-energy H_o mode softens considerably by ≈6 and 10 meV, respectively. This is in part due to the c-axis expansion to accommodate the additional hydrogen occupying the neighboring distorted tetrahedral (t) sites comprised of two Fe atoms and two Pr atoms. For Pr₂Fe₁₇H₅, in addition to slightly softened H_o basal-plane modes centered at ≈104.7 meV, there is extra scattering intensity evident near ≈112 and 123.7 meV due to two of the H_t normal modes. The analogous Pr₂Fe₁₇D₅ spectrum suggests that the third H_t normal mode is located near 103.3 meV, obscured by the 104.7 meV H_o peak. A comparison of Pr₂Fe₁₇H_x and Pr₂Fe₁₇D_x vibrational energies indicates that the o-site bonding potential is largely harmonic, whereas the t-site bonding potential is more anharmonic.