THERMODYNAMICS OF THE SPIN TRANSITION IN [FEXZN1-X(2-PIC)3]CL2.ETOH

With a new type of calorimeter we reinvestigated the specific heat C(p) of the mixed crystals [FexZn1-x(2-pic)3]Cl2.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 (1A1(low spin, LS) --> 5T2(high spin, HS)). The parameters of the Gibbs free energy were independently derived from the temperature dependence of the HS fraction gamma in [FexZn1-x(2-pic)3]Cl2.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.