Elusive structure of HCl monohydrate.

Area of Science:

• Solid-state chemistry
• Crystallography
• Computational materials science

Background:

• Crystalline hydrogen chloride monohydrate (HCl·H2O) exhibits structural disorder, a topic of ongoing scientific debate.
• Previous X-ray diffraction studies suggest disorder, but its precise nature remains unclear.
• Understanding the atomic arrangement is crucial for predicting material properties.

Purpose of the Study:

• To elucidate the debated structural disorder in crystalline HCl monohydrate.
• To reconcile conflicting interpretations of experimental diffraction data.
• To provide a detailed atomic-level model of the HCl monohydrate crystal structure.

Main Methods:

• Computational investigations using empirical potentials and density functional theory (DFT) with Gaussian basis sets (QUICKSTEP).
• On-the-fly simulations to explore alternative crystal structures.
• Experimental analysis of Fourier-transform infrared (FTIR) spectra from crystal nanoparticles.

Main Results:

• Simulations of FTIR spectra and X-ray diffraction patterns were performed.
• The study proposes a crystal structure model comprising ferroelectric domains.
• These domains are interconnected by 'boundary tissue' with an antiferroelectric structure.

Conclusions:

• The proposed ferroelectric domain model with antiferroelectric boundaries successfully explains the observed disorder in crystalline HCl monohydrate.
• Computational and experimental data are consistent with this refined structural model.
• This work clarifies the nature of structural disorder in HCl monohydrate, offering insights into its solid-state behavior.