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Recent experimental and theoretical work has demonstrated significant potential to tune the properties of silicon and germanium by adjusting the mesostructure, nanostructure, and/or crystalline structure of these group 14 elements. Despite the promise to achieve enhanced functionality with these already technologically important elements, a significant challenge lies in the identification of effective synthetic approaches that can access metastable silicon and germanium-based extended solids with a particular crystal structure or specific nano/meso-structured features. In this context, the class of intermetallic compounds known as Zintl phases has provided a platform for discovery of novel silicon and germanium-based materials. This review highlights some of the ways in which silicon and germanium-based Zintl phases have been utilized as precursors in innovative approaches to synthesize new crystalline modifications, nanoparticles, nanosheets, and mesostructured and nanoporous extended solids with properties that can be very different from the ground states of the elements.

In the "method of four coefficients," electrical resistivity (ρ), Seebeck coefficient (S), Hall coefficient (RH), and Nernst coefficient (Q) of a material are measured and typically fit or modeled with theoretical expressions based on Boltzmann transport theory to glean experimental insights into features of electronic structure and/or charge carrier scattering mechanisms in materials. Although well-defined and readily available reference materials exist for validating measurements of ρ and S, none currently exists for RH or Q. We show that measurements of all four transport coefficients—ρ, S, RH, and Q—can be validated using a single reference sample, namely, the low-temperature Seebeck coefficient Standard Reference Material® (SRM) 3451 (composition Bi2Te3+x) available from the National Institute for Standards and Technology (NIST) without the need for inter-laboratory sample exchange. RH and Q data for NIST SRM 3451 reported here for the temperature range 80–400 K complement the data already available for ρ and S and will therefore be of interest to researchers desiring to validate new or existing galvanomagnetic and thermomagnetic transport properties measurement systems. [ABSTRACT FROM AUTHOR]

We have designed a unique guided-inquiry-inspired course for entry-level graduate students using chemical research as a mechanism to teach research-oriented problem-solving skills. The course has been designed for flexibility around a shared research experience. The curriculum can be modified each year by incorporating a new research project into the framework of the course. Advanced graduate students and postdoctoral scholars serve as course instructors, providing significant teaching and mentoring opportunities for them. The benefits of the inquiry-driven approach have been reinforced through careful selection of instructors and students. We have been able to create a positive learning environment and a highly beneficial award system for students and instructors by offering an opportunity to publish class results in a scholarly journal. The course serves as a template for the implementation of similar graduate coursework at comparable research institutions.

We have carried out an evaluation of the lattice thermal conductivity for 19 different type-I intermetallic clathrate compositions at temperatures near or above their respective Debye temperatures. Although the low temperature data (T < 300 K) show that the thermal transport can vary from glasslike to crystalline depending on the composition, the high temperature lattice thermal conductivity values (T > 300 K) appear to be relatively well described by a simple model that assumes only anharmonic three-phonon scattering processes. The analysis suggests the large number of atoms in the unit cell accompanied by a significant degree of anharmonicity and relatively low Debye temperatures produce low lattice thermal conductivities in these materials above 300 K, regardless of composition or the nature and influence of guest rattling behavior on thermal transport below room temperature. The dominant mechanism underlying the universally low lattice thermal conductivity of type-I intermetallic clathrates (ranging from ~0.6 to ~2 W/m K) in the temperature regime of importance for thermoelectric power generation is therefore clarified. [ABSTRACT FROM AUTHOR]

Unusual specular X-ray diffraction patterns have been observed from certain thin film intergrowths of metal monochalcogenide (MX) and transition metal dichalcogenide (TX2) structures. These patterns exhibit selective "splitting" or broadening of selected (00l) diffraction peaks, while other (00l) reflections remain relatively unaffected [Atkins et al., Chem. Mater. 24, 4594 (2012)]. Using a simplified optical model in the kinematic approximation, we illustrate that these peculiar and somewhat counterintuitive diffraction features can be understood in terms of additional layers of one of the intergrowth components, MX or TX2, interleaved between otherwise "ideal" regions of MX-TX2 intergrowth. The interpretation is in agreement with scanning transmission electron microscope imaging, which reveals the presence of such stacking "defects" in films prepared from non-ideal precursors. In principle, the effect can be employed as a simple, non-destructive laboratory probe to detect and characterize ultrathin layers of one material, e.g., 2-dimensional crystals, embedded between two slabs of a second material, effectively using the two slabs as a highly sensitive interferometer of their separation distance. [ABSTRACT FROM AUTHOR]