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.