A modified Bragg and Williams approximation of the two-step spin crossover Original Research Article

The expressions for free energy of a binary mixture with chemical equilibrium in a simple cubic lattice have been obtained as functions of energy spacings and degree of ordering supposing that the partition function of a molecule in a lattice centre is dependent on the nature of two nearest neighbours along each normal coordinate. The derived equations contain linear, square and cubic (besides logarithmic) terms with respect to mole fraction of reactants as well as quadratic terms with respect to the degree of ordering. Under simplifying conditions these equations are transformed into the equations of regular solutions theory. The Bragg–Williams approximation of binary ordered system employing regular solution theory expressions for free energy qualitatively predicts a two-step spin crossover. The use of modified expressions for free energy in a Bragg and Williams approximation results in a quantitative description of literature data on the two-step spin-crossover in [Fe(2-pic)3]Cl2EtOH.