Thermodynamics of the spin transition in [Fe xZn 1− x(2-pic) 3]Cl 2 · 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.