Phonon spectrum and soft-mode behavior ofMgCNi3

Temperature-dependent inelastic neutron-scattering measurements of the generalized phonon density-of-states for superconducting ${\mathrm{MgCNi}}_{3},$ ${T}_{c}=8\mathrm{K},$ give evidence for a soft-mode behavior of low-frequency Ni phonon modes. Results are compared with ab initio density-functional calculations. They suggest an incipient lattice instability of the stoichiometric compound with respect to Ni vibrations orthogonal to the Ni-C bond direction which is not directly linked to the superconducting state.