Low-energy vibrational excitations of Mo₆S₃I₆nanowires revealed by low-temperature specific heat

We report on measurements of the low-temperature heat capacity of Mo6S3I6 nanowire-network samples in the form of a pressed pellet under different conditions of annealing. In the temperature (T) range of liquid He4 and below, specific heat is dominated by three contributions: (i) phonons, for T>2 K, (ii) a nuclear hyperfine contribution, below 0.3 K, and (iii) a series of Schottky anomalies for 2 K>T>0.1 K. In pristine samples, the phonon contribution varies as T2.6, which indicates a strongly anisotropic atomic structure. In the sample annealed at 800 °C, this term varies as T2.3, in agreement with a more one-dimensional (1D) structure, consistent with the transport properties. In comparison to other quasi-1D compounds, the departure from the 3D limit (corresponding to the usual T3 Debye law) is here very large, indicating a considerably loose interchain connection. We discuss the influence of stoichiometric disorder, especially the role of the position of the iodine atoms in the structure in both kinds of samples, the impact of topological imperfections and the possible origin of the observed Schottky anomalies.