Abstract
The thermal spin transition in spin-crossover compounds is accompanied by a change in most of their physical properties, like optical, magnetic and structural properties. Iron (II) spin-crossover crystals provide insight into some most interesting phenomena of solid-state physics [P. Gütlich, A. Hauser, and H. Spiering, Angew. Chemie 106 (1994), pp. 2109–2141; Topics in Current Chemistry: “Spin Crossover in Transition Metal Compounds I, II and III”, P. Gütlich and H.A. Goodwin eds., Vols. 233, 234 and 235, 2004.]. To fine-tune the zero-point energy difference between the high-spin (HS) and the low-spin (LS) states, external pressure can be used as a tool [H.G. Drickamer and C.W. Frank, Electronic Transitions and the High Pressure Chemistry and Physics of Solids, Chapman and Hall, London, 1973; P. Adler, A. Hauser, A. Vef, H. Spiering, and P. Gütlich, Hyperfine Interactions 47 (1989), pp. 343–356. P. Adler, H. Spiering, P. Gütlich, J. Phys. Chem. Solids 50 (1989), pp. 587–597.]. The large difference in metal–ligand distances between HS and LS complexes, typically of 0.2 Å [B. Gallois, J.A. Real, C. Hauw, and J. Zarembowitch, Inorg. Chem. 29 (1990), pp. 1152–1158.] and the concomitant large volume difference, typically of 30 Å3 [L. Wiehl, H. Spiering, P. Gütlich, and K. Knorr, J. Appl. Cryst. 23 (1990), pp. 151–160.] is a general feature of iron (II) spin-crossover systems. In fact, the macroscopic change in volume of the crystal that accompanies the spin transition is evidenced by X-ray crystallography [L. Wiehl, H. Spiering, P. Gütlich, and K. Knorr, J. Appl. Cryst. 23 (1990), pp. 151–160.]. In this paper, we discuss the thermodynamic behaviour, which is the energy restructuration between the units in an iron (II) molecular crystal that undergoes a spin-crossover transition due to an external perturbation such as temperature or pressure. The results are given in comparison with Slichters’ and Drickamers’ expression for a regular solution extended to three components [H.G. Drickamer, and C.W. Frank, Electronic Transitions and the High Pressure Chemistry and Physics of Solids, Chapman and Hall, London, 1973; P. Adler, A. Hauser, A. Vef, H. Spiering, and P. Gütlich, Hyperfine Interactions 47 (1989), pp. 343–356. P. Adler, H. Spiering, P. Gütlich, J. Phys. Chem. Solids 50 (1989), pp. 587–597; J. Jeftić, H. Romstedt, and A. Hauser, J. Phys. Chem. Solids 57 (1996), pp. 1743–1750.]. The expression of the Gibbs’ free energy of the system is discussed in relation to its structure.
Acknowledgements
Swiss National Science Foundation, CNRS and French Ministry of Education and Science are gratefully acknowledged for their financial support.