Phase transitions in spin cross-over systems

Several complexes of iron and cobalt transform gradually or abruptly from a low-spin state to a high-spin state with increase in temperature and such transitions have been investigated widely in the past few years. Spin-state transitions in complexes are often accompanied by changes in enthalpy and crystal structure and many of them exhibit characteristics of first-order phase transitions. Transitions in certain complexes are affected by pressure, dopant metal ions and grinding and not infrequently show a plateau in the magnetic susceptibility-temperature plots. Seemingly unknown to coordination chemists, there is a fair body of information on the spin-state transitions in transition metal oxides and other extended solids. The transitions in rare earth cobaltates exhibit a plateau or a maximum in the inverse susceptibility-temperature plots, changes in enthalpy and crystal structure and other characteristics, not unlike those of the complexes. An attempt is made in this article, to bring together the essential features of spin-state transitions in metal complexes and oxides, particularly with respect to the nature of the phase transitions associated with spin cross-over. Models for spin-state transitions are examined and scope for further research indicated.