Browsing by Author "Lee, K"

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    Comprehensive study of carbon dioxide adsorption in the metal–organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)
    (Royal Society of Chemistry, 2014-08-28) Queen, WL; Hudson, MR; Bloch, ED; Mason, JA; Gonzalez, MI; Lee, JS; Gygi, D; Howe, JD; Lee, K; Darwish, TA; James, M; Peterson, VK; Teat, SJ; Smit, B; Neaton, JB; Long, JR; Brown, CM
    Analysis of the CO2 adsorption properties of a well-known series of metal–organic frameworks M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host–guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal–O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)–CO2 adducts. © 2014, The Royal Society of Chemistry.
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    Pressure-modulated magnetism and negative thermal expansion in the Ho2Fe17 intermetallic compound
    (American Chemical Society, 2023-05-25) Cao, YL; Zhou, H; Khmelevskyi, S; Lin, K; Avdeev, M; Wang, CW; Wang, B; Hu, F; Kato,; Hattori, T; Abe, J; Ohara, K; Kawaguchi, S; Li, Q; Fukuda, M; Nishikubo, T; Lee, K; Koike, T; Liu, Q; Miao, J; Deng, JX; Shen, B; Azuma, M; Xing, X
    Hydrostatic and chemical pressure are efficient stimuli to alter the crystal structure and are commonly used for tuning electronic and magnetic properties in materials science. However, chemical pressure is difficult to quantify and a clear correspondence between these two types of pressure is still lacking. Here, we study intermetallic candidates for a permanent magnet with a negative thermal expansion (NTE). Based on in situ synchrotron X-ray diffraction, negative chemical pressure is revealed in Ho2Fe17 on Al doping and quantitatively evaluated by using temperature and pressure dependence of unit cell volume. A combination of magnetization and neutron diffraction measurements also allowed one to compare the effect of chemical pressure on magnetic ordering with that of hydrostatic pressure. Intriguingly, pressure can be used to control suppression and enhancement of NTE. Electronic structure calculations indicate that pressure affected the top of the majority band with respect to the Fermi level (EF), which has implications for the magnetic stability, which in turn plays a critical role in modulating magnetism and NTE. This work presents a good example of understanding the effect of pressure and utilizing it to control properties of functional materials. © 2024 American Chemical Society