We propose an experimental study of the vibrational dynamics of Al6Ge5 (same structure as free defect Zn4Sb3) by neutron time-of-flight spectroscopy. Our interest is the establishment of the details in the low-energy inelastic responses supposed to be responsible for the reduced thermal conductivity of this compound. The effect of temperature on the mode frequencies is of importance in order to study the anharmonicity of the vibrations modes. This work will also permit to improve our understanding of the mechanisms behind the low thermal conductivity of Zn4Sb3 and notably the importance of anharmonicity vs defects. Our experimental efforts are guided and supported by ab-initio lattice dynamics calculations.