Microscopic origin of the nonexponential dynamics in a glassy crystal

Area of Science:

• Solid-state physics
• Materials science
• Molecular dynamics

Background:

• Understanding molecular motion in orientationally disordered crystals is crucial for predicting material properties.
• Dynamic heterogeneity and slow relaxation are key phenomena in glassy systems.
• Orthocarborane, a molecular crystal with quasi-icosahedral symmetry, presents a unique model for studying these dynamics.

Purpose of the Study:

• To investigate the origins of slow relaxation and dynamic heterogeneity in orthocarborane.
• To elucidate the complex reorientational motion of molecules within the crystal lattice.

Main Methods:

• Utilized multidimensional deuteron magnetic resonance spectroscopy.
• Analyzed the anisotropic reorientational motion of orthocarborane molecules.

Main Results:

• Revealed that large jump angles are dominant in the molecular reorientational motion.
• Identified a complex motion involving sequences of small-angle tilts around preferred axes.
• Observed symmetry-adapted threefold jumps contributing to the overall dynamics.
• Demonstrated that the intrinsic dynamics are nonexponential.

Conclusions:

• The complex, anisotropic reorientational motion, characterized by large jumps and tilt dynamics, fully accounts for the nonexponential relaxation and dynamic heterogeneity in orthocarborane.
• This study provides a detailed molecular-level understanding of dynamics in glassy crystals.