期刊
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
卷 16, 期 32, 页码 17295-17304出版社
ROYAL SOC CHEMISTRY
DOI: 10.1039/c4cp01432d
关键词
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资金
- Institute for Solid State Physics, the University of Tokyo
- National Science Foundation [DMR-0944772]
- Grants-in-Aid for Scientific Research [25790005, 26600026, 26248019, 26708007, 26810037] Funding Source: KAKEN
A two-dimensional porous coordination polymer (NH4)(2){HOOC(CH2)(4)COOH}[Zn-2(C2O4)(3)] (abbreviated as (NH4)(2)(adp)[Zn-2(ox)(3)] (adp = adipic acid, ox = oxalate)), which accommodates water molecules between the [Zn-2(ox)(3)] layers, is highly remarked as a new type of crystalline proton conductor. In order to investigate its phase behavior and the proton conducting mechanism, we have performed adiabatic calorimetry, neutron diffraction, and quasi-elastic neutron scattering experiments on a fully hydrated sample (NH4)(2)(adp)[Zn-2(ox)(3)]center dot 3H(2)O with the highest proton conductivity (8 x 10(-3) S cm(-1), 25 degrees C, 98% RH). Its isostructural derivative K-2(adp)[Zn-2(ox)(3)]center dot 3H(2)O was also measured to investigate the role of ammonium ions. (NH4)(2)(adp)[Zn-2(ox)(3)]center dot 3H(2)O and K-2(adp)[Zn-2(ox)(3)]center dot 3H(2)O exhibit higher order transitions at 86 K and 138 K, respectively. From the magnitude of the transition entropy, the former is of an order-disorder type while the latter is of a displacive type. (NH4)(2)(adp)[Zn-2(ox)(3)]center dot 3H(2)O has four Debye-type relaxations and K-2(adp)[Zn-2(ox)(3)]center dot 3H(2)O has two similar relaxations above each transition temperature. The two relaxations of (NH4)(2)(adp)[Zn-2(ox)(3)]center dot 3H(2)O with very small activation energies (Delta E-a < 5 kJ mol(-1)) are due to the rotational motions of ammonium ions and play important roles in the proton conduction mechanism. It was also found that the protons in (NH4)(2)(adp)[Zn-2(ox)(3)]center dot 3H(2)O are carried through a Grotthuss mechanism. We present a discussion on the proton conducting mechanism based on the present structural and dynamical information.
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