Piezomagnetic Behavior of the Spin Crossover Prussian Blue Analogue CsFe[Cr(CN)₆]

The effect of pressure on the magnetic properties of the ferromagnetic mixed-valence Prussian blue analogue CsFe II [Cr III (CN) 6 ] has been studied by magnetic susceptometry. The abrupt spin crossover transition at T 1/2 ↓ = 207 K, induced by the change of Fe II from the high spin (HS) (t 2g 4 e g 2, S = 2) to the low spin (LS) (t 2g 6 e g 0, S = 0) configuration shifts to higher temperatures in a rigid fashion at a rate of 60 K kbar -1 with an unchanged width of the thermal hysteresis loop (Δ T = T 1/2 ↑ − T 1/2 ↓ ≈ 35 K). At the same time, the ferromagnetic transition temperature increases quasilinearly with increasing pressure at a rate of 0.1 K kbar -1 . Infrared spectroscopic measurements under pressure reveal that the Fe II −N−C−Cr III structural motif is preserved in the course of the internal spin rearrangements and no linkage isomerization of the cyanide bridge accompanies the spin crossover phenomenon. Conversion to the Fe II LS state is initially incomplete with a minority of inhomogeneously distributed Fe II HS domains surviving at ambient and low pressures. Their fraction rapidly decreases under compression until at ∼5 kbar all Fe II is in the LS state. The observed piezomagnetic response is reversible upon decompression.