  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.
