Proton Dynamics of Two-Dimensional Oxalate-Bridged Coordination Polymers

Published

August 28, 2014

Author(s)

Satoshi Miyatsu, Maiko Kofu, Atsushi Nagoe, Takeshi Yamada, Masaaki Sadakiyo, Teppei Yamada, Hiroshi Kitagawa, Madhu Sudan Tyagi, Victoria Garcia Sakai, Osamu Yamamuro

Abstract

A two-dimensional porous coordination polymer (NH₄)₂HOOC(CH₂)₄COOH}[ZN₂(Co₂O₄)₃] (abbreviated to ZnADP), which accommodates water molecules between the [Zn₂(C₂O₄)₃] layers, is highly remarked as a new-type crystalline proton conductor. In order to investigate its phase behavior and proton conducting mechanism, we have performed adiabatic calorimetry, neutron diffraction, and quasi-elastic neutron scattering experiments on a fully hydrated sample ZnADP&#82263H₂O with the highest proton conductivity (8 x 10^{-3 Scm}u-1^, 25 °C, 98% RH). Its isostructural derivative ZnKADP&#82262H₂O, in which ammonium ions are substituted by potassium ions, we also measured to investigate the role of ammonium ions. ZnADP&#82263H₂O and SnKADP&#82263H_{2Oexhibit higher-order transition at 86 K and 138 K, respectively. From the magnitude of transition entropy, the former is of an order-disorder type while the later of a displacive type. ZnADP°3H}d2O has four Debye-type relaxations and ZnKADP&#82263H₂O two similar relaxations above each transition temperature. The two relaxations of ZnADP&#82263H₂ with very small activation energies (Δ}Ε < 5 kJmol⁰¹) are due to rotational motions of ammonium ions and play important roles in the proton conduction mechanism. It was also found that the protons in ZnADP°3H₂O are carried through a Grotthus mechanism. We present a discussion on the proton conducting mechanism based on the present structural and dynamical information.

Citation

Physical Chemistry Chemical Physics

Volume

16

Issue

32

Pub Type

Journals

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Keywords

neutron scattering, polymers, dynamics, hopping

Citation

Miyatsu, S. , Kofu, M. , Nagoe, A. , Yamada, T. , Sadakiyo, M. , Yamada, T. , Kitagawa, H. , Tyagi, M. , Garcia Sakai, V. and Yamamuro, O. (2014), Proton Dynamics of Two-Dimensional Oxalate-Bridged Coordination Polymers, Physical Chemistry Chemical Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=915859 (Accessed December 26, 2024)