There are hundreds of organometallic compounds that are classified as spin-crossover compounds. As the temperature is varied the average spin per molecule can undergo a transition between low-spin and high-spin states.
The figure below shows several classes of transitions that have been observed. The vertical axis represents the fraction of molecules in the high-spin state, and the horizontal axis represents temperature.
Taken from Gutlich, Garcia, and Spiering and reproduced by Nicolazzi and Bousseksou
b) There is sharp transition with the curve having a very large slope at T_{1/2}.
c) There is a discontinuous change in the spin fraction at the transition temperature, the value of which depends on whether the temperature is increasing or decreasing, i.e., there is hysteresis. The discontinuity and hysteresis are characteristic of a first-order phase transition.
d) There is a step in the curve when the high-spin fraction is close to 0.5. This is known as a two-step transition.
e) Although a crossover occurs, the system never contains only low- or high-spins.
But, there is more. Over the past decade, multiple-step transitions have been observed. An example of a four-step transition is below.
Hysteresis is present and is larger at lower temperatures.
In a few cases of multiple-step transitions on the down-temperature sweep, the first step is missing compared to the up-temperature step.
Given the diverse behaviour described above, including sharp transitions and first-order phase transitions, spin "crossover" is a misnomer.
More importantly, given the chemical and structural complexity materials involved, is there a simple model that can capture all this diverse behaviour?
Yes. An Ising model in a field. A preliminary discussion is here. I hope to discuss this in future posts. But first I need to introduce the simple two-state model and show what it can and cannot explain.
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