Speaker
Description
Cooperativity plays a crucial role in shaping the spin crossover (SCO) transition behavior, influencing its abruptness, hysteresis, and thermal response—features that are critical for applications such as solid-state barocaloric cooling. Previous studies have demonstrated a strong link between intermolecular interactions (e.g., hydrogen bonding, π–π stacking) and cooperativity. However, the connection between lattice dynamics and cooperative behavior remains largely unexplored.
In this work, we investigate the structural and dynamic origins of cooperativity in the SCO complex Fe(PM-BiA)₂(NCS)₂ (PM = N-2'-pyridylmethylene, BiA = 4-aminobiphenyl), which exists in two polymorphs: an orthorhombic phase (Pccn) showing an abrupt spin transition (high cooperativity) around 177 K, and a monoclinic phase (P2₁/c) exhibiting a gradual transition (low cooperativity) near 208 K.
To gain insight into the role of lattice dynamics, we employ a combination of inelastic neutron scattering , nuclear inelastic scattering , and quasi-elastic neutron scattering to probe complete lattice vibrations, Fe-specific phonon density of states, and local dynamic motions across a wide energy scale. Notably, we identify a Fe–N stretching phonon mode that couples strongly with the degree of cooperativity, suggesting a direct link between specific vibrational dynamics and the sharpness of the transition. By correlating these findings with spin-state changes, we quantify the electronic, vibrational, and configurational entropy contributions and their relationship to cooperativity. Our results offer new insight into how lattice dynamics mediate cooperativity in SCO transitions and provide a foundation for designing advanced caloric materials.
