Understanding phase transition and vibrational mode coupling in ammonium nitrate using 2D correlation Raman...

Area of Science:

• Chemical Engineering
• Materials Science
• Spectroscopy

Background:

• Ammonium nitrate (AN) is crucial in fertilizers and explosives.
• AN's thermal instability causes significant safety concerns during handling and storage.
• A clear mechanistic understanding of AN's thermal behavior is lacking.

Purpose of the Study:

• To investigate the temperature-dependent behavior of ammonium nitrate.
• To elucidate the dynamic interactions between ionic and molecular species in AN at elevated temperatures.
• To provide a mechanistic insight into AN's thermal instability.

Main Methods:

• Utilized 2D correlation (2D Cos) Raman spectroscopy, a mathematical approach.
• Analyzed low-frequency libration and translational modes of nitrate ions.
• Employed moving window analysis to identify transition temperatures.

Main Results:

• Phase transition initiates with alterations in libration modes, followed by nitrate and ammonium modes, and then translational modes.
• 2D correlation successfully differentiated between mode splitting and shifting.
• Weakening of hetero-ionic coupling between NH4+ and NO3- ions was observed through changes in N-O deformation and stretching modes.

Conclusions:

• 2D correlation Raman spectroscopy provides a detailed dynamical view of AN's thermal behavior.
• The study clarifies the sequence of events during AN's phase transition.
• This research enhances understanding of AN's thermal instability, crucial for safety in its industrial applications.