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

Sanchita Sil; Nikki Kuhar; Khokan Roy; Deepika Chaturvedi; Shigeaki Morita; Yukihiro Ozaki; Siva Umapathy

doi:10.1016/j.saa.2021.119581

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

Spectrochim Acta A Mol Biomol Spectrosc. 2021 Jun 5:254:119581. doi: 10.1016/j.saa.2021.119581. Epub 2021 Feb 14.

Authors

Sanchita Sil¹, Nikki Kuhar², Khokan Roy², Deepika Chaturvedi², Shigeaki Morita³, Yukihiro Ozaki⁴, Siva Umapathy⁵

Affiliations

¹ Defence Bioengineering & Electromedical Laboratory, DRDO, Bangalore 560093, India.
² Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560012, India.
³ Dept. of Engineering Science, Osaka Electro-Communication University, Osaka, Japan.
⁴ School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan; Toyota Physical and Chemical Research Institute, 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan.
⁵ Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560012, India; Dept. of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India. Electronic address: umapathy@iisc.ac.in.

PMID: 33706114
DOI: 10.1016/j.saa.2021.119581

Abstract

Ammonium nitrate (AN) is an important component of the chemical industry such as an active ingredient in fertilizers, as an oxidizer in explosive compositions and propellants, and as a blasting agent in civil explosives. Numerous accidents have been reported in the past which concerns its thermal instability and poses a big threat to its processing, transportation, and storage. Despite much literature being reported to understand its thermal instability, a mechanistic view remains unclear. In the present work, we have studied the behavior of AN to temperature change using a mathematical approach called 2D correlation (2D Cos) Raman spectroscopy to provide complete insight into the detailed dynamical nature of the interactions between the species (ionic or molecular) occurring with an increase in temperature. We have analyzed various libration and translational modes of nitrate in the low-frequency region using this mathematical tool. It is observed from 2D maps that the phase transition of AN starts with changes in libration modes followed by various nitrate modes and ammonium modes which further precedes low-frequency translational modes. Further, the 2D correlation could differentiate between modes splitting and shifting based on specific 2D Cos pattern. The changes occurring in the N-O deformation modes, symmetric stretching modes as well as anti-symmetric stretching modes which have been attributed to the weakening of the hetero-ionic coupling between the NH₄⁺ and the NO₃^- ions could be clearly distinguished in the 2D synchronous and asynchronous plots. Besides, moving window analysis was performed to visualize the transition temperature at which phase change of AN takes place.

Keywords: 2D-correlation spectroscopy; Ammonium nitrate; Asynchronous correlation; Hetero-ionic coupling; Low-frequency modes; Moving window; Phase transition; Raman spectroscopy; Synchronous correlation.