Raman spectroscopic study of pressure effects on the spin-crossover coordination polymers Fe(pyrazine)[M(CN)4]•2H2O (M = Ni, Pd, Pt).: First observation of a piezo-hysteresis loop at room temperature

The spin state of a series of cyanide-bridged iron(II) spin-crossover coordination polymers of formula Fe(pyrazine)[M(CN)(4)].2H(2)O (M = Ni, Pd, or Pt) have been studied by recording their solid-state Raman spectra at room temperature as a function of pressure. For the first time, a reproducible piezo-hysteresis loop has been observed at room temperature for the Ni complex with characteristic spin transition pressures of P(1/2)up arrow = 1350 (+/- 50) bar and P(1/2)down arrow = 650 (+/- 50) bar. For the Pd and Pt complexes, the spin-state change occurs around 1800 (+/- 50) and 3500 (+/- 500) bar, respectively, and competes with a pressure-induced structural transformation. Pressure effects on the spin state have been interpreted within the framework of an adapted two-level Ising-like model.