Structure-property relationships and the mechanisms of multistep transitions in spin crossover materials and frameworks

TL;DR

This paper investigates the mechanisms behind multi-step spin crossover transitions in frameworks, revealing how elastic interactions influence the number of steps and ordering patterns, with implications for designing materials with specific properties.

Contribution

It introduces a simple elastic interaction model that identifies thirty-six distinct spin-state phases and explains the structure-property relationships governing multi-step transitions.

Findings

Thirty-six spin-state ordered phases identified.

Transitions range from one to eight steps, including sharp and crossover types.

Long-range elastic interactions increase the number of transition steps and diversity.

Abstract

Spin crossover frameworks and molecular crystals display fascinating collective behaviours. This includes multi-step transitions with hysteresis and a wide variety of long-range ordered patterns of high-spin and low-spin metal centres. From both practical and fundamental perspectives it is important to understand the mechanisms behind these collective behaviours. We study a simple model of elastic interactions and identify thirty six different spin-state ordered phases. We observe spin-state transitions with between one and eight steps. These include both sharp transitions and crossovers, and both complete and incomplete spin crossover. We demonstrate structure-property relationships that explain these differences. These arise because through-bond interactions are antiferroelastic (favour metal centres with different spin-states); whereas, through-space interactions are typically ferroelastic (favour the same spin-state). In general, rigid materials with longer range elastic interactions lead to transitions with more steps and more diverse spin-state ordering, which explains why both are prominent in frameworks.