Neutron spectroscopic evidence of a low-temperature phase transition in C15-type ZrCr2Hx (x=0.2 and 0.45)

The vibrational spectra of hydrogen in the cubic (C15-type) ZrCr2H0.2 and ZrCr2H0.45, and in the hexagonal (C14-type) ZrCr2H0.5 have been studied by inelastic neutron scattering at 5 and 85 K. For the C15-type compounds, the shape of the hydrogen vibrational spectrum is found to change strongly between 85 and 5 K. This suggests the existence of a low-temperature phase transition, most probably related to hydrogen ordering accompanied by displacements of H atoms. For the hexagonal modification of the same system, C14-type ZrCr2H0.5, the hydrogen vibrational spectrum is found to be nearly unchanged between 85 and 5 K.