Predicting the Pressure-Induced Isosymmetric Phase Transition of Sulfamic Acid by Applying Periodic Density Functional Theory Calculations

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Abstract
Sulfamic acid (SA) is extensively utilised in industry as a component in the production of flameproof materials, a catalyst for swift and highly efficient synthesis, in dye and pigment manufacturing processes, or as herbicide. Under ambient conditions, this compound exists as a solid in zwitterionc form, undergoing pressure-induced isosymmetric polymorphic phase transition (IPT), starting at approximately 10.0 GPa. In this work, multiple computational approaches were used to predict and describe this transition. While geometry optimisation at an increased pressure using periodic DFT-level calculations have not resulted in the anticipated IPT, the comparison of the experimental and theoretical Raman spectra confirmed this transformation. Thermodynamic calculations enabled the comparison of the stability of the modelled phases and explained the experimental observations. Ab initio molecular dynamics simulations revealed the mechanisms behind the observed transition. This work presents a complex methodology that can be successfully used to predict the IPT of molecular crystals.

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