Graduation Year

2009

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Physics

Major Professor

George S. Nolas, Ph.D.

Keywords

Clathrates, Intermetallics, Silicon, Germanium, Thermoelectrics, Photovoltaics

Abstract

Crystalline open-framework intermetallics 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. The present work is focused on the preparation and characterization of open-framework intermetallic materials based on group 14 elements, in particular those crystallizing in clathrate and related structures such as the clathrate-II phases. Materials possessing the clathrate-II crystal structure have received increased attention in recent years, as a result of both the unique properties they exhibit as well as potential for use in technologically important applications such as thermoelectrics, photovoltaics, and optoelectronics. However, in comparison with other clathrate structure types, characterization of clathrate-II materials has in general been far less extensive.

Moreover, many conceivable compositions have yet to be realized. The purpose of this work is to expand the current knowledge of the structural, chemical, and physical properties of these materials, while simultaneously exploring new compositions and synthetic routes to clathrate-II phases. One of the unique and promising aspects of clathrate-II materials is the ability to vary the guest concentration, which is shown to have significant implications for the structural and physical properties of NaxSi136 (0< x< 24) materials. It is demonstrated that new compositions can be explored by novel approaches to chemical design. Framework substitution in clathrate-II compounds is explored in an effort to assess possibilities for influencing the physical properties of these materials.

A novel zeolite-like framework phase, Na1-xGe3+z, has been discovered, and is shown to be a new low-thermal conductivity crystalline solid, suggesting a new approach to the design of crystalline intermetallic materials with low thermal conductivity. New directions in synthesis of intermetallics are identified, with emphasis on unconventional preparative methods and the opportunities they offer. Processing of reactive precursors by spark plasma sintering is demonstrated as a new preparative tool for crystal growth, identifying the first method for preparation of clathrate-II Na24Si136single-crystals since the discovery of these compounds more than four decades ago.

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