Structural and dynamical properties of YH3

On the basis of parameter-free electronic structure calculations for YH3, it was predicted that the total energy of the high symmetry HoD3 structure deduced from neutron powder diffraction (NPD) experiments on YD3 could be lowered by small displacements of the hydrogen atoms. Subsequent, more detailed NPD experiments failed to observe any such symmetry-breaking displacements, but neither could they be ruled out. Moreover, a new broken symmetry structure which is slightly different to that predicted by total energy calculations was proposed. Analysis of the phonon modes measured very recently using Raman spectroscopy yields the first clearcut experimental evidence for symmetry breaking. Here we present the results of parameter-free lattice dynamics calculations for each of three structures currently being considered for YH3. The results are obtained within the harmonic model starting from a force field which is calculated from first principles in a supercell geometry. Comparison of the calculated phonon densities of states with the experimental spectrum determined by inelastic neutron scattering gives clear evidence for a broken symmetry structure. The Debije-Waller factors for some of the hydrogen atoms are exceptionally large and we speculate on the importance of the large zero-point motions of these atoms for the structure of YH3.