Low-spin state structure of [Fe(chloroethyltetrazole)6](BF4)2 obtained from synchrotron powder diffraction data

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Spin-crossover (SCO) compounds exhibit reversible switching between high-spin and low-spin states.
• Understanding the structural changes accompanying SCO is crucial for material design.
• The iron(II) complex [Fe(teec)6](BF4)2 (teec = chloroethyltetrazole) is known to display SCO behavior.

Purpose of the Study:

• To investigate the structural dynamics of the [Fe(teec)6](BF4)2 complex during its two-step spin-crossover transition.
• To elucidate the relationship between structural changes and spin-state transitions.
• To analyze the anisotropic unit-cell contraction and its dependence on cooling rate.

Main Methods:

• Time-resolved synchrotron powder diffraction experiments were performed across the 300-90 K temperature range.
• Crystal structure models were generated using the parallel tempering technique.
• Rietveld refinement was employed to determine the precise low-spin state structure at 90 K.

Main Results:

• A two-step complete spin-crossover transition was observed between 300 K and 90 K.
• Detailed crystal structure models were obtained for different spin states.
• A remarkable anisotropic unit-cell parameter contraction was identified, sensitive to the cooling rate.

Conclusions:

• The structural characteristics of the low-spin state were elucidated and compared to the high-spin state.
• The findings highlight the significant influence of cooling rate on structural changes during SCO.
• This study provides critical insights into the interplay between structural dynamics and spin-crossover phenomena.