首页|Isotopic dependence of the frequency of optical vibrations in molybdenum monohydride
Isotopic dependence of the frequency of optical vibrations in molybdenum monohydride
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NSTL
Elsevier
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 hy-drogen vibrations, which was considered to be responsible for the large deviation of the ratio of the fun-damental optical frequencies omega(H)/omega(D) = 1.51 from the harmonic value root 2 approximate to 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 omega(H)/omega(D) = 1.44 is close to the harmonic value root 2 approximate to 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 agree-ment 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 similar to 0.07 of deuterium atoms partially occupying the tetrahedral interstices. (C) 2021 Elsevier B.V. All rights reserved.
NSTL
Elsevier
