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

1992; Elsevier BV; Volume: 53; Issue: 2 Linguagem: Inglês

10.1016/0022-3697(92)90055-i

1879-2553

R. Jakobi, H. Spiering, Philipp Gütlich,

Metal-Catalyzed Oxygenation Mechanisms

With a new type of calorimeter we reinvestigated the specific heat Cp of the mixed crystals [FexZni1−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 Cp(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 γ 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 Cp 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.