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 Na
1-xGe3 (0 < x < 1), and corresponds to a new binary phase
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 Na
xSi136 (0 < x < 24) has been synthesized in
higher purity than previously reported for as-synthesized products. The transport
properties of the Na
xSi136 clathrates exhibit a clear dependence on the guest content x. In
particular, we present for the first time thermal conductivity measurements on Na
xSi136
clathrates, and observe evidence that the guest atoms in type II clathrates affect the
thermal transport in these materials. Some of the crystalline Na
xSi136 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.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-5061 |
| Date | 07 March 2006 |
| Creators | Beekman, Matthew K. |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
| Rights | default |
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