Structural insight into the cooperativity of spin crossover compounds.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Spin-crossover (SCO) compounds exhibit potential for industrial applications, but a deeper understanding of their transition mechanisms is crucial for property tuning.
• Microscopic insights into SCO transitions are essential for designing materials with desired macroscopic physical properties.

Purpose of the Study:

• To correlate macroscopic physical properties with microscopic structural changes in orthorhombic and monoclinic polymorphs of Fe(PM-Bia)2(NCS)2.
• To investigate the influence of intermolecular interactions (hydrogen bonding, π-π, van der Waals) on SCO properties.
• To explore particle size dependence, thermal exchange, and kinetic behavior, and propose a model for non-equilibrium spin-phase fraction.

Main Methods:

• Single-crystal X-ray diffraction
• Magnetization measurements
• Differential Scanning Calorimetry (DSC)
• Slichter-Drickamer model for thermodynamic calculations
• Analysis of magnetization dependence on temperature scan rate

Main Results:

• Macroscopic properties were successfully correlated with microscopic structural changes in the two polymorphs.
• Cooperativity, entropy, enthalpy, and hydrogen bonding effects were quantified.
• A theoretical model for non-equilibrium spin-phase fraction was proposed.
• Synchrotron radiation dose was found to affect the SCO process, shifting the transition to lower temperatures, indicating tunability via radiation damage.
• A scan-rate-dependent two-step behavior was observed in the orthorhombic polymorph, absent in the monoclinic one.

Conclusions:

• Microscopic structural changes significantly influence the macroscopic properties of SCO compounds.
• Intermolecular interactions play a critical role in the cooperativity of SCO transitions.
• Radiation damage offers a novel method for tuning SCO transition temperatures.
• Polymorphism in SCO compounds leads to distinct behaviors, such as the scan-rate-dependent transition in the orthorhombic form.