The search for new efficient materials and refrigeration mechanisms is a key challenge toreplace the conventional vapor compression technology. An attractive alternative technologyuses the caloric refrigeration cycle, which is based on the adiabatic temperature and isothermalentropy change upon tuning an external parameter such as pressure, electric field or magneticfield. Recently, spin crossover (SCO) compounds have been recognized as promising candidates,which exhibit large barocaloric effects: Large isothermal entropy changes have been reportedfor some of these SCO compounds at fairly low hydrostatic pressures (< 1.2 GPa) [1]. In SCOcomplexes the central metal ion switches between a low spin (LS) state at low temperature /high pressure and a high spin (HS) state at high temperature / low pressure. The LS to HStransition involves an increase of the spin entropy, but a larger part of the entropy changeoriginates from changes in the intramolecular vibrations [2].In this work, we report on magnetization measurements and single crystal synchrotronradiation diffraction on SCO complexes consisting of Fe+2 as a central ion bound to six nitrogenatoms. Our focus is Fe(PM-Bia)2(NCS)2, PM-Bia = (N-(2′-pyridylmethylene)-4-amino-biphenyl),which crystallizes in two polymorphs depending on thesynthesis route. Polymorph P1 crystallizes orthorhombic(Pccn) and undergoes an abrupt spin transition around 170 K.Polymorph P2 crystallizes monoclinic (P21/c) and undergoes agradual spin transition around 200 K [3].From the structural data, we extracted the temperaturedependence of the Fe-N distances (Figure 1), which can thenbe used to determine the high spin fraction. From the fitting ofthe temperature dependence of the high spin fraction, weobtained the change in entropy (ΔS), the change in enthalpy(ΔH), and the cooperativity (Г). The values obtained for ΔS andΔH on the basis of the structural data are substantiallydifferent from the values of the entropy as deduced from heatcapacity measurements [4]. The width of the transition region,differs strongly between the two polymorphs. This indicatesthe importance of intermolecular interactions for the spintransitions in both polymorphs
