Microscopic modeling of a spin crossover transition

TL;DR

This paper develops a microscopic model for spin crossover transitions in Fe(II) coordination polymers, highlighting the importance of magnetic exchange and elastic couplings in the phase transition.

Contribution

It introduces a new microscopic explanation for the cooperative phase transition, emphasizing the role of magnetic exchange alongside elastic interactions.

Findings

Magnetic exchange is as crucial as elastic couplings in the phase transition.

Reliable model structures for polymeric spin crossover compounds were developed.

Ab initio density functional theory analysis supports the importance of magnetic interactions.

Abstract

In spin crossover materials, an abrupt phase transition between a low spin state and a high spin state can be driven by temperature, pressure or by light irradiation. Of a special relevance are Fe(II) based coordination polymers where, in contrast to molecular systems, the phase transition between a spin S=0 and a spin S=2 state shows a pronounced hysteresis which is desirable for technical applications. A satisfactory microscopic explanation of this large cooperative phenomenon has been sought for a long time. The lack of X-ray data has been one of the reasons for the absence of microscopic studies. In this work, we present an efficient route to prepare reliable model structures and within an ab initio density functional theory analysis and effective model considerations we show that in polymeric spin crossover compounds magnetic exchange between high spin Fe(II) centers is as important as elastic couplings for understanding the phase transition. We discuss the relevance of these interactions for the cooperative behavior in these materials.