Crystal chemical design rules for tailored chemical expansivity

Anderson, Lawrence

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https://hdl.handle.net/2142/122123 Copy

Description

Title

Crystal chemical design rules for tailored chemical expansivity

Author(s)

Anderson, Lawrence

Issue Date

2023-11-29

Director of Research (if dissertation) or Advisor (if thesis)

Perry, Nicola H

Doctoral Committee Chair(s)

Perry, Nicola H

Committee Member(s)

• Shoemaker, Daniel
• Trinkle, Dallas
• Ertekin, Elif

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• perovskite
• chemical expansion
• strain
• stoichiometry
• tolerance factor
• covalency
• hybridization
• octahedral tilt
• hydration
• redox
• oxide
• fuel cell
• electrochemical

Abstract

Chemo-mechanical coupling relates a material’s mechanical response to changes in its chemical species concentrations, and vice versa. Although reversible ion exchange enables functional properties that are useful in energy storage, electronics, chemical sensing, and chemical separation, the mechanical response can have deleterious effects. Large chemical strains can offer better signal-to-noise ratios for chemical sensing applications, but small strains can benefit long-term stability by limiting strain-induced material failure and performance degradation in electrochemical devices. The diverse needs in various devices for either chemical strain minimization or maximization, or interfacial strain matching requires the ability to tune chemo-mechanical coupling for optimal material design. The compositional and functional flexibility of perovskite oxides allows for a wide range of properties, but this customizability comes at the cost of complex structure-property relationships. This body of work is focused on improving the understanding of these relationships, and on developing design rules to tailor chemo-mechanical coupling in perovskite oxides. In CHAPTER 1, I begin by introducing stochiometric chemical expansion, the key methods used for the quantification of chemical expansion coefficients (CCE), and the design rules that have been previously proposed to tailor CCEs. The high temperature applications for functional oxides involve thermal expansion that is coupled with chemical expansion during thermal cycling, so appropriate methodology when quantifying CCEs is necessary to decouple these contributions for an accurate assessment of material properties. The limited body of work related to quantifying and explaining CCEs is insufficient to establish reliable crystal-chemical design principles, so my work focuses on the refinement of existing design principles and the development of new design principles for 1) redox-induced chemical expansion, and 2) hydration-induced chemical expansion. In CHAPTER 2, I contribute to a collaborative study started by a prior student, which hypothesizes that lowering symmetry in perovskite-structured proton conductors will lower their CCEs. This is achieved by systematically varying the Goldschmidt tolerance factor through B-site substitution in the prototypical BaCe0.9-xZrxY0.1O3-δ (0 ≤ x ≤ 0.9) solid-solution. X-ray diffraction (XRD) confirms symmetry decreases with decreasing Zr content. CCEs are measured by isothermal XRD, dilatometry, and thermogravimetric analysis (TGA) in varied pH2O over 430-630 °C. With decreasing Zr content, CCEs monotonically decreased. Density functional theory simulations on endmember BaCe1-yYyO3-δ and BaZr1-yYyO3-δ compositions show the same trend. I perform in-situ, high temperature XRD on the lowest symmetry BaCe0.9Y0.1O3-δ composition and observe anisotropic chemical expansion that indicates both microstructural and crystal chemical contributions to low CCEs. Lower CCEs in this solid solution correlate to decreasing symmetry, increasing unit cell volume, increasing oxygen vacancy radius, decreasing bulk modulus, and inter- vs. intra-octahedral hydrogen bonding. In CHAPTER 3, I explore the extent to which lowering tolerance factor will lower CCE in perovskite-structured proton conductors by further extending the tolerance factor range, relative to the study in CHAPTER 2, with the SrCe0.9-xZrxY0.1O3-δ (x = 0, 0.45, 0.9) solid solution. In addition to preparing this solid solution, I fabricate another BaZr0.9Y0.1O3-δ sample and measure it to verify the consistency of CCEs with the new dilatometry and TGA instruments that are used in this study. I also utilized in-situ high-temperature neutron diffraction (ND) to determine atomic lattice positions before and after hydration. A comparison of the results between CHAPTER 2 and CHAPTER 3 indicate an optimum tolerance factor (0.93 – 0.95) to obtain the lowest hydration CCEs, and the lowest CCE composition for this solid solution (SrZr0.9Y0.1O3-δ) correlates to a more significant decrease in B-O-B bond angles (in the ABO3 perovskite) after hydration. Structural distortions that are present before hydration likely contribute to defect accommodation via subtle octahedral tilts, rather than a simple change in bond length, which serves to lower CCE. In CHAPTER 4, I demonstrate near-zero redox coefficients of chemical expansion (CCEs) for mixed- and triple-conducting Pr-oxide perovskites. I synthesize PrGa0.9Mg0.1O3-δ (PGM) and BaPr0.9Y0.1O3-δ (BPY), having Pr on the A- and B-site respectively, and characterize each with in-situ high-temperature, variable atmosphere X-ray diffraction, dilatometry, and thermogravimetric analysis, to obtain isothermal stoichiometry changes, chemical strains, and CCEs. Despite empirical model predictions of smaller CCEs for Pr on the A-site, both compositions yield unprecedented low average CCEs (0.004-0.011), 2-5x lower than the lowest reported perovskite redox CCEs. Simple empirical models assume pseudo-cubic structures and full charge localization on multivalent cations, like Pr. To evaluate actual charge distribution, I perform in-situ impedance spectroscopy and collaborate with density functional theory specialists at UIUC for further interpretation of the results. Results indicate the anomalously low CCEs in these compositions likely derive from a combination of decreased crystal symmetry (vs. cubic), partial charge delocalization through hybridization of Pr-4f and O-2p orbitals, and redox/multivalence on O rather than just Pr (with or without hybridization). On this basis, I suggest band structure design principles for near-zero redox-strain perovskites, highlighting the benefit of locating holes partially or fully on oxygen. In CHAPTER 5, I chose two compositions, Y0.8¬Ca0.2FeO3 (YCF) and La0.8¬Ca0.2FeO3 (LCF), to vary B-O bond hybridization and determine the effect of charge distribution on redox CCE. I initially hypothesize that A-site substitution of Y for La will decrease B-O-B bond angle, leading to less Fe-O orbital overlap, decreased hybridization, and higher CCEs. Decreasing oxygen hole concentration, achieved through lower O-2p hybridization, is expected to decrease the valence change of oxygen during redox, and increase the valence change of iron, leading to a higher CCE. While synchrotron XRD confirms lower B-O-B bond angles in YCF, an analysis of the O K-edge X-ray absorption spectroscopy (XAS) results indicate that YCF has a higher degree of hybridization than LCF. Dilatometry and TGA are used to quantify redox CCEs across various isotherms, and our results show lower CCEs for YCF than LCF. Potential crystal-chemical contributions to CCE are discussed, and I conclude that increasing B-O hybridization and increasing lattice distortion (lower B-O-B bond angle) are the primary factors that lower CCE between the two compositions studied. In CHAPTER 6, I conclude this dissertation with a summary of the key findings from these works, and I propose a few future research directions for the development of crystal-chemical design rules for tailored CCEs.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Lawrence Anderson

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