Diffusion of water in molecular magnet Cu_0.75Mn_0.75[Fe(CN)₆]·7H₂O

Abstract

Here we report the dynamical behaviour of water in Prussian blue analogue (PBA) Cu_0.75Mn_0.75[Fe(CN)₆]·7H₂O molecular magnet in the temperature range 260–360 K as studied using the quasielastic neutron scattering technique. While significant quasielastic broadening is observed in the hydrated sample, no broadening was observed in the dehydrated one. Data analysis showed that the observed quasielastic broadening in Cu_0.75Mn_0.75[Fe(CN)₆]·7H₂O corresponds to the dynamics of the non-coordinated water molecules at the 32f site and the coordinated water molecules at the 24e site, existing in the cavities created by the absence of Fe(CN)₆ units. The non-coordinated water molecules at 8c interstitial sites do not contribute to the broadening, suggesting that they are immobile at least within the time window of the spectrometer used. Behaviour of the elastic incoherent structure factor is consistent with the model where the water molecules undergo translational diffusion localized within the cavity of 5.1 Å. While all the non-coordinated water molecules at the 32f site are dynamic over the entire range of temperatures, the coordinated ones at the 24e site become progressively dynamic with temperature. The water molecules were found to undergo hindered (∼1.16 × 10^-5 cm² s^-1 at 300 K) diffusion compared to bulk water and the diffusivity followed Arrhenius behaviour within the measured temperature range with an activation energy of 1.26 kcal mol^-1.