Identifying vibrations that destabilize crystals and characterize the glassy state

Area of Science:

• Materials Science
• Solid State Physics
• Chemistry

Background:

• Zeolites are crystalline microporous materials with diverse applications.
• Understanding their vibrational dynamics is crucial for predicting their behavior under pressure and in amorphous states.
• The Boson peak is a characteristic feature of glasses, but its origin in zeolitic glasses is not fully understood.

Purpose of the Study:

• To identify the primary sources of low-frequency vibrations in zeolite crystals.
• To investigate the role of these vibrations in the compressive amorphization of zeolites.
• To elucidate the connection between vibrational modes, the Boson peak, and structural transformations in zeolitic glasses.

Main Methods:

• High-resolution inelastic neutron scattering (INS) was employed to probe vibrational modes.
• Analysis focused on dispersed and nondispersed vibrational modes in crystalline zeolites and their amorphized counterparts.
• Comparison with silica glass was performed to validate findings.

Main Results:

• Both dispersed and nondispersed vibrational modes were identified in zeolites.
• These modes were prominent during early amorphization but diminished in dense glasses.
• Dispersed modes correlate with vibrations of secondary building units, contributing to the Boson peak in zeolitic glasses.
• Nondispersed, librational modes, retained in glasses and observed in silica, destabilize the zeolite structure, driving low- to high-density phase transitions and influencing the Boson peak.

Conclusions:

• Low-frequency vibrations in zeolites, particularly librational modes, are critical for structural destabilization and amorphization.
• The Boson peak in zeolitic glasses arises from vibrations within various-sized connected rings.
• These findings provide a molecular-level understanding of the amorphization process and glass properties in zeolites.