NATURE OF THE PHASE-TRANSITION IN SPIN CROSSOVER COMPOUNDS

Starting from the phenomenological free energy describing the spin equilibrium of continuous or gradual high spin (HS) ⇌ low spin (LS) transitions a reduced equation of state has been derived which is of the type known in mean field theories. The continuous HS ⇌ LS transitions of [Fe(2-pic-ND2)3]Cl2·EtOD (2-pic = 2-picolylamine) at ambient pressure and p = 1200 bar and of [Fe(2-pic)3]Cl2-MeOH can be classified as isobars above the critical point of the system. Around and below the critical point a complex behaviour is expected for thermodynamic reasons combined with the consequences of an elastic interaction mechanism between the HS and LS complex molecules in the crystal. The unusual spin transitions of [FexZn1 - x(2-pic)3]Cl2·EtOH (two-step transition x ~1) and of [Fe(2-pic)3]Br2-EtOH (narrow hysteresis only in the upper part of the transition curve) have been shown to be below and close to above the critical point, respectively. Results from Mössbauer and susceptibility studies of the two-step transition at different concentrations, x = 1.0, 0.98, 0.94, 0.89, 0.86, 0.78 and applied pressures of p = 600 and 1350 bar are strong evidence for the intrinsic properties of the interaction mechanism being responsible for the observed complex behaviour rather than special lattice properties serving as triggers. Static density waves of the HS fraction in the transition region as observed for hydrogen in metal systems, are suggested to be responsible for the observed complex behaviour. © 1990.