Thermal- and pressure-induced spin transition coupled to crystallographic phase transition in some Fe(II) molecular crystals

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. Gutlich, 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. Gutlich 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. Gutlich, Hyperfine Interactions 47 (1989), pp. 343-356. P. Adler, H. Spiering, P. Gutlich, 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 [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 303 [L. Wiehl, H. Spiering, P. Gutlich, 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. Gutlich, 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. Gutlich, Hyperfine Interactions 47 (1989), pp. 343-356. P. Adler, H. Spiering, P. Gutlich, J. Phys. Chem. Solids 50 (1989), pp. 587-597; J. Jeftic, 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.