Optical Hydrogen Vibrations Research Articles

Isotopic dependence of the frequency of optical vibrations in molybdenum monohydride

It is currently known that three hydrides – PdHx, MoHx, and TiHx – exhibit an inverse isotope effect in superconductivity. Namely, the phase with a heavier hydrogen isotope, deuterium, has a higher critical temperature. Hydrides and deuterides of palladium have intensively been studied both experimentally and theoretically, but the origin of the isotope effect has not been established with certainty. The commonly accepted explanation is that the effect is likely to be due to the strong anharmonicity of the optical hydrogen vibrations, which was considered to be responsible for the large deviation of the ratio of the fundamental optical frequencies ωH/ωD = 1.51 from the harmonic value 2 ≈ 1.41. In the present paper, powder samples of MoH1.1(1) and MoD1.07(3) were synthesized under a hydrogen / deuterium pressure of several gigapascals and studied by inelastic neutron scattering (INS) at ambient pressure and T = 10 K. The INS study demonstrated that optical vibrations of H atoms in MoH1.1 and D atoms in MoD1.07 are harmonic and the ratio of fundamental optical frequencies ωH/ωD = 1.44 is close to the harmonic value 2 ≈ 1.41. This shows that anharmonicity is not a necessary condition for the presence of the inverse isotope effect. The MoD1.07 sample was additionally studied by neutron diffraction (ND) at ambient pressure and T = 100 K. In agreement with previous ND results for MoH1.2, the ND study of MoD1.07 showed that deuterium atoms occupied almost all octahedral interstitial sites in its hexagonal close-packed metal lattice and formed a NiAs-type crystal structure with the composition close to MoD. The overstoichiometric composition MoD1.07 of the deuteride is likely to result from a small fraction D/Mo ~ 0.07 of deuterium atoms partially occupying the tetrahedral interstices.

Anharmonicity of optical hydrogen vibrations in RhH

A polycrystalline sample of nearly stoichiometric fcc rhodium hydride was synthesised under high hydrogen pressure and then studied at ambient pressure and 5 K by inelastic neutron scattering in the range of energy transfers 35–380 meV. The fundamental band of optical hydrogen vibrations was shown to consist of a strong peak centred at 74 meV and broadened towards higher energies. The second and third band demonstrated essentially anharmonic behaviour, which is not typical of monohydrides of 3 d- and 4 d-metals with close-packed metal lattices.

Crystal structure and lattice dynamics of chromium hydrides

Samples of fcc (γ) CrH and hcp (ɛ) CrH have been prepared by cathodic electrodeposition of chromium and studied by powder neutron diffraction at 8 K and by inelastic neutron scattering (INS) at 5 K in the range of energy transfers 6–400 meV. A profile analysis of the neutron diffraction patterns showed that hydrogen occupies octahedral interstitial sites in the fcc metal lattice of γ-CrH with a = 3.854(3) Å and in the hcp metal lattice of ɛ-CrH with a = 2.719(3) Å and c = 4.433(4) Å. No magnetic ordering was detected in either hydride at 8 K. The INS investigation demonstrated that the fundamental band of optical hydrogen vibrations in both hydrides consists of a strong peak with a broad shoulder towards higher energies. The peak position is 118 meV for γ-CrH and 122 meV for ɛ-CrH.

Neutron scattering studies of [formula omitted]-CoH

A powder sample of fcc ( γ ) cobalt hydride was synthesised under a pressure of 9 GPa and a temperature of 350 ° C and then studied at ambient pressure by neutron diffraction at 95 K and by inelastic neutron scattering (INS) at 5 K in the range of energy transfers 30–380 meV. A profile analysis of the neutron diffraction pattern showed that hydrogen occupies octahedral interstitial sites in the fcc metal lattice of γ -CoH with a = 3.7124 ( 5 ) Å. The INS investigation demonstrated that the fundamental band of optical hydrogen vibrations consists of a strong peak at 102 meV with a broad shoulder towards higher energies. This intensity distribution is typical of hcp and fcc monohydrides of 3 d- and 4 d-metals, and the peak position as a function of the hydrogen–metal distance agrees with the dependence established for those hydrides earlier.

Lattice dynamics of high-pressure hydrides of the group VI–VIII transition metals

Monohydrides of the group VI–VIII transition metals Cr, Mn, Fe, Co, Ni, Mo, Rh and Pd are synthesised under high hydrogen pressures and elevated temperatures and studied by inelastic neutron scattering (INS) at ambient pressure and low temperatures. In the measured INS spectra, the energy of the main optical H peak exhibits a strong monotonic increase as a function of the nearest hydrogen–metal distance in the hydrides of both 3d- and 4d-metals, respectively. The spectra for FCC NiH and PdH appear strongly anisotropic at energy transfers above the fundamental band of optical hydrogen vibrations, while those for FCC PdD do not show a significant directional dependence at energies of the second optical D band.

Neutron spectroscopy ofReH0.09

A powder and a single-crystalline sample of a solid hydrogen solution in hcp rhenium with an atomic ratio of $\mathrm{H}/\mathrm{R}\mathrm{e}=0.09$ were synthesized under a high pressure of gaseous hydrogen and studied by inelastic neutron scattering in the range of energy transfers from 25 to 380 meV. The fundamental band of optical hydrogen vibrations was found to be split into two sharp peaks centered at 100 and 130 meV. The peaks have an intensity ratio of about 1:2 and the corresponding vibrations are polarized along the c axis and in the basal plane of the crystal lattice, respectively.

Neutron spectroscopy of [formula omitted] manganese hydride

The vibrational spectrum of fcc γ-MnH 0.41 synthesized under high pressure of gaseous hydrogen was studied by inelastic neutron scattering at 2 K in the range of energy transfers from 25 to 400 meV. The fundamental band of optical hydrogen vibrations consists of a peak at 111 meV with a broad shoulder towards higher energies, which extends up to about 140 meV. At higher energy transfers, the spectrum originates from multiphonon neutron scattering and exhibits approximately harmonic behaviour. The results are compared with the available data for other metal hydrides.

Neutron-scattering studies on the vibrational excitations and the structure of ordered niobium hydrides: The ɛ phase

Inelastic and elastic neutron-scattering techniques have been used to study the structure and optical hydrogen vibrations of the ordered niobium hydrides ${\mathrm{NbH}}_{x}$ in the concentration range $0.1<x<0.73$ and at temperatures between 4 and 250 K. A splitting of the fundamental vibrational modes with an intensity ratio of 2:1 could be observed in the case of the \ensuremath{\varepsilon} phase. The neutron vibrational spectra show clearly that this splitting is due to the existence of two inequivalent hydrogen sites rather than H-H coupling effects. This is confirmed by neutron-diffraction data, which show the presence of two hydrogen sites with different distortions of the surrounding niobium tetrahedra. Our results also provide new information on the ${\mathrm{NbH}}_{x}$ phase diagram and the anharmonicity of the hydrogen potential in these hydrides.

Inelastic Neutron Scattering Spectra from Lanthanum Dihydride and Lanthanum Trihydride

The optical hydrogen vibrations in lanthanum dihydride and lanthanum trihydride have been investigated by the energy-gain scattering of cold neutrons. The approximate frequency distributions, derived from the observed time-of-flight spectra, show peaks which correlate well with the known crystal structures. In fcc lanthanum trihydride the optical bands are peaked at 940 and 515 cm−1 corresponding to hydrogen vibrating at tetrahedral and octahedral sites, respectively, and in lanthanum dihydride there is a single peak at 825 cm−1 due to tetrahedral hydrogen. Other weak peaks have been observed and are thought to be due to vibrations of hydrogen atoms dissolved in the metal phase; this phase was identified by x rays in both samples. A large width of the optical levels, a common feature in all metallic hydrides, was also observed for LaH2 and LaH3.

Vibration Spectra of Vanadium Hydride in Three Crystal Phases by Inelastic Neutron Scattering

The vibration spectra of vanadium hydride in three crystal phases have been investigated by the energy-gain scattering of cold neutrons. The measured spectra are generally split into two broad bands above and below about 300 cm− 1, which are primarily associated with optical hydrogen vibrations and metal-atom vibrations, respectively. Pseudofrequency distributions for the hydrogen vibrations were derived from the measured neutron spectra. These indicate broad optical vibration bands peaked at 970 ± 50 and ∼1400 cm− 1 in the α (bcc) phase, 440 ± 20, 970 and ∼1400 cm− 1 in the β (bct) phase, and ∼1300 cm− 1 in the γ (fcc) phase. The width of the 440-cm− 1 band increases with hydrogen concentration, possibly due to hydrogen–hydrogen interactions. The “metal-atom” vibration spectra also show changes in peak positions and widths as the distribution of phases is changed. The changes in the neutron spectra with temperature and as a function of composition between VH0.20 and VH1.71 correlate quite well with the previously measured phase diagram. A comparison of the present results with neutron spectra for hydrides with known hydrogen positions suggests that in the α phase hydrogens only occupy tetrahedral-type sites, while in the β phase both octahedral and tetrahedral sites are occupied. The results for VH1.49 and VH1.71 indicate that in the γ phase the hydrogens occupy regular tetrahedral sites corresponding to the fluroine positions in a CaF2-type structure.

Vibration Spectra of Yttrium and Uranium Hydrides by the Inelastic Scattering of Cold Neutrons

The vibration spectra of the hydrides and deuterides of yttrium and uranium have been investigated by the energy-gain scattering of cold neutrons. The measured spectra are all split into two bands, one at higher energies, due to the optical hydrogen vibrations, and another at lower energies due to metal—atom vibrations. Values have been obtained for the peaks and widths of the single optical hydrogen bands in YH2, YD2, UH3, and UD3 by the calculation of approximate frequency distributions from the observed neutron time-of-flight spectra. The derived peak frequencies in cm−1 are: YH2, 1025±60; YD2, 725±40; UH3, 970±60; UD3, 710±40. The relative width at half-maximum for the hydrogen band in UH3 is about double that for YH2. The distribution of hydrogen vibrations in hexagonal YH3 and YD3 is considerably broader, and exhibits several maxima, due to hydrogens at different sites in the lattice. The hydrogen modes in all the compounds can be correlated reasonably well with the known crystal structures. The results are in satisfactory agreement with hydrogen frequencies calculated previously from heat-capacity data.