Control of Thermal Expansion, Behavior on Compression, and Guest Loading in Framework Materials

By Baxter, Samuel J.

Georgia Institute of Technology

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Advisors: Angus Wilkinson, Younan Xia, John Zhang, Christopher Jones, Henry La Pierre

Each chapter in this thesis focuses on open framework systems found to display anomalous properties relating to their unique structures. The systems investigated were chosen for their open framework form, flexibility, and modifiability. These three key factors can allow for the structural control necessary for tunable thermal expansion and phase stability to be achieved in materials that inherently display large negative thermal expansion (NTE). Concepts necessary for the understanding of thermal expansion research will be covered in sufficient detail as well as examples from the different materials classes being investigated in this thesis. Definitions, mechanisms, applications, and real examples of thermal expansion, behavior upon compression, and composite research are reviewed in chapter 1. Chapters 2, 3 and 4 examine the control of thermal expansion in ReO3-type metal fluoride systems. ReO3-type materials exhibit the simplest crystalline structure found to exhibit strong NTE. Because of its inherent simplicity, modification and analysis of these structures is relatively straightforward. This being the case, the coefficients of thermal expansion (CTEs) of ReO3-type materials have been found to range from strongly positive to strongly negative. This fact reveals deeper complications involving the ionicity, bond strength, and general flexibility of these structures that lead to diverse thermodynamic behavior. In these chapters certain metal fluorides were chosen that have previously been found to exhibit both structural tunability and large NTE. The structural tunability allows for certain flexible structural components to be replaced by more rigid counterparts (systematically) to control the properties related directly to flexibility, such as thermal behavior. Specifically, Mg1-xZr1+xF6+2x and Sc1-xZrxF3+x solid solutions were found to display tunable thermal expansion and phase stability upon the introduction of edge-sharing polyhedra within their structures. The introduction of this specific defect is mediated by the metal ratio within these systems. The use of defects to control thermodynamic behavior is a common theme in this thesis, but the work in chapter 4 shows that the techniques involved in behavioral control must be modified specifically for each system they are being applied to. In chapter 4, Ca[Zr(IV)1-xNb(V)x]F6+x solid solutions are investigated using the same general technique applied successfully to the materials of chapters 1 and 2. Interestingly, it is shown that this specific variation of the technique does not appear to introduce the same defect that was found in the previous two systems. This new defect introduced leads to changes in the structural behavior that are non-ideal for application but demonstrates the importance of developing a diverse toolkit when approaching thermodynamic behavioral modification. Chapters 5 and 6 investigate an even broader range of thermal behavior control techniques, which span from structural scaffold modification to guest inclusion. A comparison of how effective these techniques are for a diverse set of metal organic frameworks (MOFs) is drawn and previously unexplored routes of structural control are introduced for these systems. While some techniques facilitated direct tunability of thermal expansion from positive to negative within a measured temperature range, others simply showed non-systematic changes in the thermal expansion or just the controlled inhibition of pronounced NTE. Insights into the origin of these differences in thermal behavior are obtained through an in-depth analysis of synchrotron-radiation total scattering and diffraction experiments, as well as complementary molecular simulations performed by collaborators. The implications of these works on the prospects for MOFs as an emergent material class for NTE-related applications are also discussed. While several metal-organic frameworks are known to display negative thermal expansion, there have been no reports where the thermal expansion of a MOF has been tuned continuously from negative to positive through the formation of single-phase solid solutions. In the system Zn-DMOF-TMx, Zn2[(bdc)2-2x(TM-bdc)2x][dabco], the introduction of increasing amounts of TM-bdc, with four methyl groups decorating the benzene dicarboxylate linker, leads to a smooth transition from negative to positive thermal expansion in the a-b plane of this tetragonal material. The temperature at which zero thermal expansion occurs evolves from ~186 K for the Zn-DMOF parent structure (x = 0) to ~325 K for Zn-DMOF-TM (x = 1.0). The formation of mixed linker solid solutions is likely a general strategy for the control of thermal expansion in MOFs and its prospects are also discussed. Finally, Chapter 7 expands upon the recent discovery of a new class of hybrid ReO3-type fluoride perovskites that contain a neutral molecule (He) in the perovskite A-site. These experiments involve taking a prototypical NTE metal fluoride, ScF3, and exposing it to high pressure helium at high temperature. The pore aperture and activation barriers for diffusion are compared to that of the first known hybrid fluoride perovskite with the formula [He]CaZrF6. Evidence for the inclusion of He in ScF3 is reviewed and future directions for the project are discussed.

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Researchers should cite this work as follows:

  • Baxter, Samuel J. (2021), "Control of Thermal Expansion, Behavior on Compression, and Guest Loading in Framework Materials,"

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