Low-Spin-High-Spin Transitions in Transition-Metal-Ion Compounds

Models for low-spin-high-spin transitions in transition-metal-ion compounds are presented. Within a molecular-field scheme, the transition between, and coexistence of, high-spin (Hund's-rule) and low-spin (crystal field) states are described. One- and two-sublattice-spin structures and the influence of magnetic interactions are considered. It is pointed out that the one-sublattice model, which was considered by Chestnut for the singlet-triplet system, applies to the transitions observed in certain organic complexes. We have found that the inclusion of magnetic interactions between ions can yield a magnetically ordered state above a certain transition temperature. This "heat magnetization" is not due to the coupling between levels but rather to a self-consistently determined crystal field splitting. The two-sublattice-spin-structure case exhibits a rich variety of behaviors. These include a low-spin to two-sublattice spin to high-spin transition with the order of the transitions varying with the parameters of the model. Applications to certain ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Co}}^{3+}$ compounds are noted.