Electrochemical capacitance and cubic-rhombohedral phase transition of sodium intercalated ferrimagnetic manganese hexacyanoferrate based open framework material

We report a synthesis of a highly crystalline, sodium intercalated manganese hexacyanoferrate open framework structure based on Prussian blue analogues with chemical stiochiometry of Na2xMn[Fe(CN)6]2(x+1)/3·zH2O (x = 0, 0.5, and 1) using controlled co-precipitation method. The effect of Na intercalation on structural properties, electrochemical performance along with magnetic ordering are prersnted. The formation of uniformly cube shaped sodium‑manganese hexacyanoferrate is evident from field-emission scanning electron microscopy. Infra-red spectroscopy shows presence of characteristic absorption peaks of MnII-C ≡ N-FeIII bonds, confirm the formation of manganese hexacyanoferrate. The Rietveld refinement of x-ray data reveals that the compounds are crystallized in Fm3m cubic symmetry up to x = 0.5, whereas, excess Na intercalation (x = 1) leads to a R-3 mH, rhombohedral symmetry. The dc magnetization measurements show ferrimagnetic nature of these compounds. The magnetic transition temperature increases with Na intercalation and was found to be ∼11, 12.5, and 13.4 K for x = 0, 0.5, and 1. The Mössbauer spectroscopy study confirms low and intermediate spin states of both Fe+2 and Fe+3 ions in the studied compounds, consistence with their magnetic properties. The electrochemical performance of the compounds with Na intercalation in terms of charging-discharging are studied using cyclic voltammetry. The compounds show capacitance values of 182.6 F/g at 10 A/g, 117.5 F/g at 6A/g, and 110.1 F/g at 2.8 A/g for x = 0, 0.5, and 1, respectively, suggesting their suitability as electrochemical capacitors.