Sprecher
Beschreibung
The barocaloric effect (BCE) is characterized as a thermal response (variation of temperature or entropy) in solid-state materials induced by external hydrostatic pressure and cooling technologies based on the BCE have emerged as a promising alternative to conventional vapor-compression cooling. Recently, spin crossover (SCO) transitions, where the low spin (LS) and high spin (HS) states can be switched by hydrostatic pressure, were proposed as a potential mechanism to generate outstanding BCE. The considerable overall entropy change across the SCO transition is primarily attributed to the significant change in lattice vibration, which is directly linked to the dynamic features.
In this study, we correlate the structural changes of a classic SCO complex Fe(PM-BiA)2(NCS)2 (with PM = N-2’- pyridylmethylene and BiA = 4-aminobiphenyl) in the vicinity of a spin transition as functions of temperature and pressure with the dynamic properties and aim at a better understanding of the role of cooperativity on the transition. The two polymorphs of Fe(PM-BiA)2(NCS)2 are ideal in this respect, as the orthorhombic polymorph (Pccn) features a high cooperativity indicated by an abrupt transition, while the gradual transition in the monoclinic polymorph (P21/c) hints towards a low cooperativity. From the structural studies, we determined which intermolecular interactions (hydrogen bridges, π-π interactions, van der Waals interactions) play a key role at the spin transition in both polymorphs.
We investigate the changes in dynamic features at the temperature- and pressure-induced spin transition using different spectroscopic methods on a wide range of energy and time scales in combination with ongoing ab-initio modelling. Raman and IR spectroscopy give access to the energy difference of vibrational energy levels in the molecules. The Fe-related phonon density of states in the monoclinic polymorph is obtained through nuclear inelastic scattering (NIS) to extract the Fe-related vibrational entropy change in the energy range well below 100 meV. The result from quasi-elastic neutron scattering (QENS) confirms the existence of dynamic disorder in the order of picoseconds corresponding to sub-meV energy. Our combined approach allows us to unravel the different entropy contributions (electronic, vibrational, configurational) to the overall entropy change at the spin transition.
