Phonon spectrum ofYH3: Evidence for a broken symmetry structure

Total-energy calculations for ${\mathrm{YH}}_3$ indicate that small symmetry-breaking hydrogen displacements can lower the total energy of the structure deduced from neutron powder diffraction (NPD) experiments on ${\mathrm{YD}}_3.$ However, such displacements were not observed in subsequent low-temperature NPD experiments. Here we present the results of parameter-free lattice dynamics calculations for ${\mathrm{YH}}_3.$ Comparison of the theoretical vibrational densities of states calculated for the ${\mathrm{HoD}}_3$ and broken-symmetry structures with the recently reported experimental spectrum determined by inelastic neutron scattering yields evidence that symmetry lowering has in fact been seen.