Anharmonicity in Zirconium Hydrides: a first-principles study combined with inelastic neutron scattering

Zirconium Hydrides and Deuterides are candidates for neutron moderators and fuel-rod cladding materials. We investigate anharmonic phenomena in these materials using inelastic neutron scattering (INS) and lattice dynamics calculations within the framework of density functional theory (DFT). We observe multiple sharp peaks below harmonic (free) multi-phonon bands in the experimental spectra, which do not show up in the simulated INS spectra based on the harmonic approximation. We have thus carried out a detailed study of the anharmonicity by exploring the 2D potential energy surface with DFT calculations and solving the corresponding 2D single-particle Schrodinger equation of the H/D atom in zirconium hydrides/deuterides to get the eigenfrequencies. The results describe well the experimental INS spectra, showing harmonic behavior of the fundamental modes and strong anharmonicity at higher energies. The DFT calculations were carried out with the real-space multigrid (RMG) and VASP codes, and the experimental INS spectra were measured with the VISION spectrometer at the Spallation Neutron Source, Oak Ridge National Laboratory.