Proton dynamics of two-dimensional oxalate-bridged coordination polymers

Satoshi Miyatsu, Maiko Kofu, Atsushi Nagoe, Takeshi Yamada, Masaaki Sadakiyo, Teppei Yamada, Hiroshi Kitagawa, Madhusudan Tyagi, Victoria García Sakai, Osamu Yamamuro

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Abstract

A two-dimensional porous coordination polymer (NH4)2(HOOC(CH2)4COOH)[Zn2(C2O4)3] (abbreviated as (NH4)2(adp)[Zn2(ox)3] (adp = adipic acid, ox = oxalate)), which accommodates water molecules between the [Zn2(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)[Zn2(ox)3]·3H2O with the highest proton conductivity (8 × 10-3 S cm-1, 25 °C, 98% RH). Its isostructural derivative K2(adp)[Zn2(ox)3]·3H2O was also measured to investigate the role of ammonium ions. (NH4)2(adp)[Zn2(ox)3]·3H2O and K2(adp)[Zn2(ox)3]·3H2O 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)[Zn2(ox)3]·3H2O has four Debye-type relaxations and K2(adp)[Zn2(ox)3]·3H2O has two similar relaxations above each transition temperature. The two relaxations of (NH4)2(adp)[Zn2(ox)3]·3H2O with very small activation energies (ΔEa < 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)[Zn2(ox)3]·3H2O are carried through a Grotthuss mechanism. We present a discussion on the proton conducting mechanism based on the present structural and dynamical information.

Original languageEnglish
Pages (from-to)17295-17304
Number of pages10
JournalPhysical Chemistry Chemical Physics
Volume16
Issue number32
DOIs
Publication statusPublished - 23 Jul 2014

Cite this

Miyatsu, S., Kofu, M., Nagoe, A., Yamada, T., Sadakiyo, M., Yamada, T., Kitagawa, H., Tyagi, M., García Sakai, V., & Yamamuro, O. (2014). Proton dynamics of two-dimensional oxalate-bridged coordination polymers. Physical Chemistry Chemical Physics, 16(32), 17295-17304. https://doi.org/10.1039/c4cp01432d