Multistep transitions in spin crossover materials without long-range order from dimensional reduction

Most theories of multistep spin crossover (SCO) have focused on intermediate phases with long-range order. However, disordered intermediate phases have also been observed experimentally. Here we show that the interplay of crystallographically inequivalent SCO centres with elastic interactions between SCO centres can lead to an effective reduction in the dimensionality of the system. This is highly analogous to dimensional reduction in quantum magnetism. The resulting quasi-one-dimensional and quasi-zero-dimensional models naturally result in disorder at non-zero temperatures, explaining the absence of long-range order in our calculations. Furthermore, the low effective dimensionality can strongly suppress short-range correlations and hence diffuse scattering should not necessarily be expected to be observed experimentally in these disordered phases. Our model contains parameter regimes where disordered intermediate phases give rise to diffuse scattering and other regimes where disordered intermediate phases do not. Our results are in good agreement with experiments on \mat ~[Sciortino \textit{et al., Chem. Sci.}, 2017, 8, 701].