The effect of pressure on the thermal hysteresis of the first‐order spin transition in bis(1,10‐phenanthroline‐2‐carbaldehyde phenylhydrazone) iron (II) complexes

The effect of pressure on the thermal hysteresis of the high‐spin (5T2) ∏ low‐spin (1A1) transition in [Fe(phy)2](BF4)2 and [Fe(phy)2](ClO4)2 has been studied (phy=1, 10‐phenanthroline‐2‐carbaldehyde phenylhydrazone). An increase of the hysteresis width ΔTc as well as of the residual fractions nres5T2 and (1−n5T2)res with pressure is observed. The quantity (ΔTc)1/2 shows a linear dependence on pressure p as expected from the Landau theory. The increase of the residual fractions is explained by the assumption of decreasing size of the cooperative region with pressure.