Thermodynamics of the spin transition in [FexZn1-x(2-pic)3]Cl2.EtOH

Abstract With a new type of calorimeter we reinvestigated the specific heat C p of the mixed crystals [Fe x Zni 1− x (2-pic) 3 ]Cl 2 · EtOH in the temperature range from 15 to 300 K. On the basis of a phenomenological thermodynamic model the anomaly in the C p ( T ) curve of the mixed crystal ( x = 0.73) can be quantitatively described as a result of the spin transition ( 1 A 1 (low spin, LS) → 5 T 2 (high spin, HS)). The parameters of the Gibbs free energy were independently derived from the temperature dependence of the HS fraction γ in [Fe x Zn 1− x (2-pic) 3 ]Cl 2 · EtOH. The pure iron complex ( x = 1) exhibits a two-step spin transition. In the temperature range between the two steps the entropy obtained from the C p values is lower than expected from a random mixture of HS and LS complexes in the crystal. This reduction of entropy is interpreted in terms of an ordering of HS and LS molecules.