Synthesis and Characterization of Type II Silicon and Germanium Clathrates

Clathrate materials comprise compounds in which guest atoms or molecules can be encapsulated inside atomic cages formed by host framework polyhedra. The unique relationship that exists between the guest species and its host results in a wide range of physical phenomena, and offers the ability to study the physics of structure-property relationships in crystalline solids. Clathrates are actively being investigated in fields such as thermoelectrics, superconductivity, optoelectronics, and photovoltaics among others. The structural subset known as type II clathrates have been studied far less than other clathrates, and this forms the impetus for the present work. In particular, the known “composition space” of type II clathrates is small, thus the need for a better understanding of possible compositions is evident. A basic research investigation into the synthesis and characterization of silicon and germanium type II clathrates was performed using a range of synthetic, crystallographic, chemical, calorimetric, and transport measurement techniques. A series of framework substituted type II germanium clathrates has been synthesized for the first time, and transport measurements indicate that these compounds show metallic behavior. In the course of the investigation into type II germanium clathrates, a new zeolite-like framework compound with its corresponding novel crystal structure has been discovered and characterized. This compound can be described by the composition Na1-xGe3 (0 < x < 1), and corresponds to a new binary phase viii in the Na-Ge system. One of the most interesting aspects of type II clathrates is the ability to create compounds in which the framework cages are partially occupied, as this offers the unique opportunity to study the material properties as a function of guest content. A series of type II sodium-silicon clathrates NaxSi136 (0 < x < 24) has been synthesized in higher purity than previously reported for as-synthesized products. The transport properties of the NaxSi136 clathrates exhibit a clear dependence on the guest content x. In particular, we present for the first time thermal conductivity measurements on NaxSi136 clathrates, and observe evidence that the guest atoms in type II clathrates affect the thermal transport in these materials. Some of the crystalline NaxSi136 compounds studied exhibit very low thermal conductivities, comparable in magnitude to amorphous materials. In addition, for the first time clear evidence from transport measurements was found that resonance phonon scattering may be present in type II clathrates, as is also the case in the type I subset.