A two-step spin transition with a disordered intermediate state in a new two-dimensional coordination polymer.

Area of Science:

• Materials Science
• Solid State Chemistry
• Magnetism

Background:

• Spin crossover (SCO) compounds are materials that can switch between low-spin and high-spin states.
• Two-dimensional (2D) polymeric SCO materials offer unique properties for potential applications.
• Understanding the transition mechanisms in SCO compounds is crucial for their technological development.

Purpose of the Study:

• To synthesize and characterize a novel 2D polymeric SCO compound, Fe(py)2[Ag(CN)2]2.
• To investigate the spin transition behavior, including its steps, hysteresis, and thermodynamic properties.
• To elucidate the structural and electronic factors contributing to the observed two-step spin transition.

Main Methods:

• Synthesis of the 2D polymeric compound Fe(py)2[Ag(CN)2]2.
• Magnetic susceptibility measurements to detect spin transitions and hysteresis.
• Heat capacity measurements to determine thermodynamic parameters (enthalpy, entropy).
• X-ray diffraction (XRD) and Mössbauer spectroscopy to analyze structural and electronic properties across temperature ranges.

Main Results:

• The synthesized Fe(py)2[Ag(CN)2]2 compound exhibits a two-step spin transition.
• Step 1 (high-temperature) occurs at 146.3 K without hysteresis; Step 2 (low-temperature) occurs at 84 K (cooling) and 98.2 K (heating) with a 10 K hysteresis.
• Evidence of significant residual high-spin (HS) species (23%) and HS state trapping at higher cooling rates was observed.
• Thermodynamic data (DeltaH, DeltaS) were determined for both transition steps.
• XRD and Mössbauer spectroscopy confirmed a single crystallographic site for iron(II) and ruled out structural phase transitions as the cause of the two-step behavior.

Conclusions:

• The two-step spin transition in Fe(py)2[Ag(CN)2]2 is an intrinsic property of the compound, not driven by crystallographic phase transitions.
• A disordered state, characterized by a mixture of HS and LS species without long-range order, is proposed to exist between the two transition steps.
• The findings provide insights into the complex spin transition mechanisms in 2D SCO materials.