Unprecedented Reverse Volume Expansion in Spin-Transition Crystals.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Spin crossover (SCO) materials typically exhibit volume decrease upon transition from high-spin (HS) to low-spin (LS) states.
• This volume change is crucial for potential applications in sensors and actuators.
• Understanding SCO mechanisms is key to designing novel functional materials.

Purpose of the Study:

• To investigate the spin crossover phenomenon in the molecular complex [Fe(PM-pBrA)2(NCS)2].
• To characterize the structural and magnetic properties associated with spin transitions.
• To explore the implications of unusual volume changes for SCO material development.

Main Methods:

• Single-crystal and powder X-ray diffraction.
• Magnetic measurements.
• Analysis of thermal and photoexcited spin conversions.

Main Results:

• The complex [Fe(PM-pBrA)2(NCS)2] shows an unprecedented increase in unit-cell volume from HS to LS states.
• This abnormal volume change is associated with phenyl ring rotation and altered crystal packing.
• Light-Induced Excited Spin-State Trapping (LIESST) results in a high relaxation temperature (T(LIESST)) of 109 K.

Conclusions:

• The observed inverse volume change challenges conventional understanding of SCO mechanisms.
• The findings highlight the role of molecular flexibility and crystal packing in SCO behavior.
• This research expands the possibilities for designing advanced spin crossover materials with unique properties.