From Negative to Zero Thermal Expansion IR Transparent Materials Using the Defect Chemistry of ReO3-Type Fluorides

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Non-technical AbstractThermal expansion, which is change in dimensions with temperature, plays a very important role in determining if a material will be suitable for a particular application. Low or zero thermal expansion is necessary when dimensional stability is important, such as in the fabrication of precision optical devices, and it is also highly beneficial in ceramics that are exposed to rapid changes in temperature, such as those used in cookware and in some engine and aerospace applications. Materials that shrink on heating (display negative thermal expansion) can be used to compensate for positive thermal expansion in other components. This research focuses on enhancing our fundamental understanding of the relationship between atomic structure and a material's properties, finding new compositions with interesting properties, developing chemical strategies for controlling thermal expansion, and using them to create infrared transparent zero thermal expansion materials. Such materials are potentially of value in thermal imaging, communications and laser applications. Graduate and undergraduate students will be trained in the wide variety of synthetic and characterization techniques needed to prepare and understand the properties of these materials. They will be introduced to important concepts and methods in materials chemistry/science, and engaged in activities that develop general professional skills.Technical AbstractWith support of the Solid State and Materials Chemistry program, the project will focus on the control of thermal expansion in metal fluorides with an ReO3-type framework, as this structure type is relatively simple, which helps facilitate both experimental and computational approaches to the elucidation of underlying principles, a wide range of different chemical compositions can be prepared, and metal fluorides typically have good transparency in the infrared. In early 2015, the team reported that calcium zirconium fluoride displays extremely strong negative thermal expansion over a wide temperature range. The family of materials which calcium zirconium fluoride belongs to has considerable compositional and structural flexibility. This provides for the control of thermal expansion by 1) cation substitutions, 2) cation disorder and 3) movement away from the ideal stoichiometry to form fluoride excess solids, which either have interstitial fluoride or cation vacancies. The control of thermal expansion by the deliberate generation of such structural defects is novel and potentially very powerful. Variable temperature / pressure neutron and x-ray scattering methods will be used to establish the links between atomic level structure and properties.