Resistivity, Seebeck coefficient, and Hall measurements were performed on densified nanocrystalline composite materials of undoped and Ag-doped PbTe nanocrystals to investigate the physical mechanisms responsible for Seebeck coefficient enhancement in nanocrystalline systems. The unique temperature dependence of the resistivity and mobility for these PbTe nanocomposites suggests that grain-boundary potential barrier scattering is the dominant scattering mechanism. We propose that carrier trapping in the grain boundaries forms energy barriers that impede the conduction of carriers between grains, essentially filtering charge carriers with energy less than the barrier height. These nanocomposites therefore demonstrate an enhanced Seebeck coefficient as compared to single crystal or polycrystalline PbTe at similar carrier concentrations.

Crystalline open-framework and nanoporous materials have long attracted the attention of chemists, physicists, and materials scientists. The intriguing structures such materials exhibit are often intimately related to the unique physical properties they possess. This article reviews recent developments in the preparation and characterization of one such class of inorganic materials, those comprised of group 14 elements crystallizing with the clathrate-II crystal structure. These materials correspond to expanded forms and, in some cases, metastable allotropes of Si, Ge, and Sn. Just as interesting as the structure and properties that group 14 clathrate-II materials display is the diverse range of synthetic techniques used to prepare them and these techniques are discussed. We review the work to date characterizing the physical properties of group 14 clathrate-II materials with emphasis placed on structure–property relationships, in particular with regard to the electronic and transport properties.

Thermal conductivity, Seebeck coefficient, resistivity, and Hall measurements on polycrystalline ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-y}{\mathrm{Sb}}_y$ with $0\le y\le 0.37$ are reported. In these materials, Sb substitutes for Si in the antifluorite structure. As the Sb content increases, vacancies are formed in the lattice on Mg sites, which also contribute to variations in the transport properties. With increasing Sb content, both the absolute Seebeck coefficient and electrical resistivity first decrease, but then increase due to vacancy formation, while the thermal conductivity decreases monotonically with increasing Sb content. We investigate the lattice thermal conductivity by modeling the data using the Debye approximation in order to discern the contributions from different phonon-scattering mechanisms. We discuss the results in the context of potential thermoelectric applications.

Systematic dc magnetization studies using the Banerjee criterion, Kouvel-Fisher, and magnetocaloric effect methods provide physical insights into the origin of the magnetic anomaly and the tunneling effect of europium on the ferromagnetic ordering in Eu8Ga16Ge30 type-I clathrates. We show that Eu8Ga16Ge30 undergoes a second-order magnetic transition (SOMT) at TC~$\sim${}~35 K, resulting from the magnetic interaction between the Eu2+ ions at the Eu2 sites, followed by a secondary magnetic transition at TL~$\sim${}~10 K (indicated as a magnetic anomaly in previous studies), as a result of the magnetic interaction between the Eu2+ ions at the Eu1 and Eu2 sites. The critical exponent $\beta${} = 0.388 is close to that predicted from the three-dimensional Heisenberg model ($\beta${} = 0.365), while the critical exponent $\gamma${} = 0.956 is close to that predicted from the mean-field model ($\gamma${} = 1). The substitution of Sr2+ for Eu2+ retains the SOMT but largely reduces the transition temperatures (TC~$\sim${}~15 K and TL~$\sim${}~5 K), with the critical exponents $\beta${} = 0.521 and $\gamma${} = 0.917 close to those predicted from the mean-field model ($\beta${} = 0.5 and $\gamma${} = 1). These results point to the important fact that the tunneling of Eu2+ between the four equivalent sites in the tetrakaidecahedral cage tends to prevent the occurrence of a long-range ferromagnetic ordering in the type-I clathrate materials.