The effect of the lattice expansion on high spin ⇌ low spin transitions

Abstract The dilution effect on the high spin (HS) ⇌low spin (LS) transition in [Fe x Zn 1− x (2-pic) 3 ]Cl 2 · EtOH has been studied on the basis of the theory of elastic interactions of the HS and LS molecules of iron (II) and zinc complexes in the crystal. The decrease in the elastic self energy of an incompressible sphere in a hole in an expanding crystal leads to a deviation of the HS fraction γ from the value determined by the Boltzmann population of the HS and LS energy levels. The constant Γ of the interaction term −Γγ 2 of the free energy of the system determines the bulk modules K = 1.4 X 10 10 N m−2. The slope of the critical temperature T c = T (γ = 1 2 ) versus the concentration x , which strongly depends on differences in the size of the HS, the LS, and the Zn complex molecules, is in agreement with the differences deduced from the X-ray data of the iron compound ( x = 1) and the zinc compound ( x = 0).