Graduation Year

2006

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Physics

Major Professor

George S. Nolas, Ph.D.

Co-Major Professor

Sarath Witanachchi, Ph.D.

Committee Member

Lilia Woods, Ph.D.

Keywords

clathrate, thermal conductivity, transport properties, materials science, silicon

Abstract

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.

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