Proton dynamics of two-dimensional oxalate-bridged coordination polymers

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.