Crystal Water Molecules as Magnetic Tuners in Molecular Metamagnets Exhibiting Antiferro–Ferro–Paramagnetic Transitions

We studied the effects of the number of crystal water molecules on the magnetic behavior of {[Ni(en)2]3[Fe(CN)6]2·xH2O}n (1–3) (where en = ethylenediamine and x = 3, 2, or 0) coordination polymers by 57Fe Mössbauer spectroscopy, single-crystal X-ray diffraction, and magnetization measurements. Magnetic phase diagrams constructed for all three compounds indicate that they behave as metamagnets exhibiting very rare field-induced antiferro–ferro–paramagnetic transitions. The number of crystal water molecules has a major effect on the Néel temperature, critical field, and magnetic hardness of the compounds in the ferromagnetic state. Moreover, the systems behave as molecular magnetic sponges, changing their magnetic properties due to the controllable and reversible dehydration/hydration process.