Comprehensive Study of the Low-Temperature Transport and Thermodynamic Properties of the Cluster Compounds Ag x Mo 9 Se 11 (3.41 ≤ x ≤ 3.78)

We present a detailed study of the evolution of the electrical, galvanomagnetic, and thermodynamic properties of polycrystalline AgxMo9Se11 compounds for 3.4 <= x <= 3.8 at low temperatures (2-350 K). In agreement with density functional theory calculations, the collected data show an overall gradual variation in the transport properties from metallic to semiconducting behavior on going from x = 3.4 to 3.8. The results evidence subtle variations in the electronic properties with the Ag content, typified by both positive and negative phonon-drag effects together with thermopower and Hall coefficient of opposite signs. Analysis of the data suggests that these features may be due to peculiarities of the dispersion of the valence bands in the vicinity of the chemical potential. A drastic influence of the Ag content on the thermal transport was evidenced by a pronounced change in the temperature dependence of the specific heat below 10 K. Nonlinearities in the C-p(T-3) data are correlated to the concentration of Ag atoms, with an increase in x resulting in a more pronounced departure from a Debye law. The observed behavior mirrors that of ionic conductors, suggesting that AgxMo9Se11 for x >= 3.6 might belong to this class of compounds.