Phonon density of states of the quasi-one-dimensional molybdenum selenides M2Mo6Se6 (M=Tl, In, Rb, and vacancy): Evidence of Einstein-like M-atom modes in the superconducting compounds

We have measured the generalized phonon density of states (DOS) G(ω) of the quasi-one-dimensional compounds M2Mo6Se6 (M=Tl, In, Rb, and vacancy) by inelastic neutron scattering on polycrystalline samples. From the comparison with the specific-heat data, we derived the unweighted density of states g(ω). The DOS spectra can be consistently interpreted as the superposition of a low-energy peak corresponding to the Einstein-like vibrations of the M atoms and of the characteristic spectrum of the Mo6Se6 skeleton. The particular features of the In2Mo6Se6 and Tl2Mo6Se6 spectra are discussed in connection with the occurrence of superconductivity in these compounds (Tc≅3 and 6.5 K, respectively)