Two-step spin-crossover phenomenon under high pressure in the coordination polymer Fe(3-methylpyridine)2[Ni(CN)4]

The temperature dependence of the magnetic susceptibility and Mössbauer spectra of the compounds FeL2[Ni(CN)4] (L = 2, 3 or 4-methylpyridine) have been investigated. In each case the iron(II) ions are high spin at room temperature, but in the 3-methylpyridine compound about half of the iron ions becomes diamagnetic around 90 K. The application of external pressure allowed to uncover the underlying two-step nature of this spin transition. For further increase of the pressure the plateau between the two steps disappeared. Both observations were successfully interpreted in the framework of an Ising-like model with two iron sites displaying different volume changes.