Cooperative spin-crossover-like transitions in the inhomogeneous chain of exchange clusters.

A statistical theory of temperature-induced spin-crossover-like transitions has been developed for spin chains with a few exchange clusters in a unit cell. Using static magnetic susceptibility measurements, a general expression for the effective magnetic moment μ(eff) of exchange clusters is obtained for an arbitrary structure of cluster terms. For heterospin Cu(hfac)2L(R) complexes ("breathing crystals") with a "head-to-head" chain motif, a two-point approximation for a partition function of the Jahn-Teller Cu(+2) paramagnetic center (spin 1/2) is suggested. Theoretical results are compared with available experimental data on thermal spin-crossover-like transitions in the "breathing crystal" compounds. It is shown that the model developed is able to successfully describe both of the cases of smooth and abrupt (cooperative) spin transitions for chains of three-spin exchange clusters. Spin-crossover-like transitions of a new type with step-wise changes in a positive exchange integral are predicted.