Mg₃[M(CN)₆]₂·xH₂O with M = Fe, Co: Synthesis, crystal structure, and hydrogen sorption

The titled materials were obtained neutralizing in situ prepared aqueous solutions of H₃[M(CN)₆] (M = Fe, Co) by adding Mg(OH) ₂ and evaporating the solvent under vacuum. Their crystal structure was solved and refined from powder X-ray diffraction data. The two solids crystallize with a cubic unit cell, in the Pm$\bar{3}$m space group, which corresponds to an ordered distribution of [M(CN)₆] vacancies. The materials structure is preserved when the crystal water is removed by moderated heating. The formed open porous framework was evaluated from nitrogen and hydrogen adsorption isotherms. The amount of hydrogen adsorbed is similar to that found for transition metal Prussian blue analogues. The structural study was complemented with infrared, thermo-gravimetric, and Mössbauer spectroscopy data.