Comprehensive Study of the Low-Temperature Transport and Thermodynamic Properties of the Cluster Compounds Ag_xMo₉Se₁₁ (3.41 ≤ x ≤ 3.78)

Abstract

We present a detailed study of the evolution of the electrical, galvanomagnetic, and thermodynamic properties of polycrystalline Ag x Mo 9 Se 11 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 Ag x Mo 9 Se 11 for x ≥ 3.6 might belong to this class of compounds.

Transport Properties of Polycrystalline p-type SnSe

“…Such feature has been observed in several families of thermoelectric compounds including cage-like materials such as clathrates or skutterudites and in materials with complex unit cells. [24][25][26][27][28][29][30][31][32] SnSe follows this trend as shown in Figure 4 where a pronounced peak centered near 14 K is observed. To investigate in more detail this peak, the 3 / T C p data were fitted to the sum of a Debye contribution with two additional low-energy Einstein modes to describe the main features of the low-energy phonon spectrum.…”

X-ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag2Tl2Mo9Se11

“…3,22 This conclusion holds true for mixed Mo 6 and Mo 9 cluster compounds. 13,48,49 Although molybdenum clusters are connected to each other via shared chalcogen atoms, interactions between neighboring clusters are not strong enough to perturb the MO pattern of the isolated Mo 9 cluster.…”

“…21,23 In Ag x Mo 9 Se 11 , the possibility to tune the electrical properties from metallic to semiconducting by varying x enabled optimizing the carrier concentration and hence the power factor α 2 /ρ. 21,22 The combination of these properties resulted in interesting ZT values at high temperatures with a maximum of 0.65 at 800 K for x = 3.9. 21 In a continuing effort to explore these families of compounds, we tried to partially substitute Tl for Ag in Ag x Mo 9 Se 11 .…”

Evaluation of95Mo Nuclear Shielding and Chemical Shift of [Mo6X14]2–Clusters in the Liquid Phase

“…The corresponding 95 Mo nuclear shieldings were evaluated using the scalar ZORA Hamiltonian in combination with the TZ2P basis set and are compared with the GIPAW results in Figure 9. 10 ] 2− clusters, one obtains a mean and a standard deviation of −624 and 71 ppm, respectively. The former value allows us to shift the curves of Figure 5 by +51 ppm which improves by the same value the agreement between the chemical shifts calculated in acetone and acetonitrile as compared with the experimental values.…”