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

Abstract

The spin state of a series of cyanide-bridged iron(II) spin-crossover coordination polymers of formula Fe(pyrazine)[M(CN)4]·2H2O (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 P1/2↑ = 1350 (±50) bar and P1/2↓ = 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.