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

Sil, S and Kuhar, N and Roy, K and Chaturvedi, D and Morita, S and Ozaki, Y and Umapathy, S (2021) Understanding phase transition and vibrational mode coupling in ammonium nitrate using 2D correlation Raman spectroscopy. In: Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy, 254 .

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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. © 2021

Item Type:	Journal Article
Publication:	Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
Publisher:	Elsevier B.V.
Additional Information:	The copyright for this article belongs to Elsevier B.V.
Keywords:	Chemical industry; Explosives; Phase transitions; Raman spectroscopy; Thermodynamic stability, 2D correlation; 2D-correlation spectroscopy; Ammonium nitrate; Asynchronoi correlation; Hetero-ionic coupling; Low-frequency modes; Moving window; Synchronoi correlation; Thermal instabilities; Translational modes, Nitrates
Department/Centre:	Division of Chemical Sciences > Inorganic & Physical Chemistry Division of Physical & Mathematical Sciences > Instrumentation Appiled Physics
Date Deposited:	22 Apr 2021 10:20
Last Modified:	22 Apr 2021 10:20
URI:	http://eprints.iisc.ac.in/id/eprint/68500

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