Accurate crystal structure of ice VI from X-ray diffraction with Hirshfeld atom refinement
Michal L Chodkiewicz1, Roman Gajda1, Barbara Lavina2
1Biological and Chemical Research Centre, Department of Chemistry, University of Warsaw, Żwirki i Wigury, Warszawa 02-089, Poland.
Iucrj
|September 8, 2022
Summary
Accurate crystal structures of ice VI were determined using X-ray diffraction and Hirshfeld atom refinement (HAR). This method successfully resolved hydrogen atom disorder, offering a cost-effective alternative to neutron diffraction for studying ice polymorphs.
Area of Science:
- Crystallography
- Materials Science
- Physical Chemistry
Background:
- Water's solid forms (ices) are crucial but present challenges in determining hydrogen atom positions and thermal motions.
- Existing methods like neutron diffraction, DFT calculations, and X-ray diffraction struggle with hydrogen disorder and anisotropic displacement parameters (ADPs) in many ice structures.
- The full spectrum of ice phases remains incomplete, with theoretical calculations suggesting undiscovered forms.
Purpose of the Study:
- To accurately determine the crystal structures of H2O, D2O, and mixed H2O/D2O ice VI.
- To assess the efficacy of Hirshfeld atom refinement (HAR) combined with X-ray diffraction for resolving hydrogen atom disorder and determining ADPs.
- To establish HAR-X-ray diffraction as a viable and accessible alternative to neutron diffraction for ice structure analysis.
Main Methods:
- Single-crystal X-ray diffraction data collection under high pressure using synchrotron and laboratory sources.
- Application of Hirshfeld atom refinement (HAR) to analyze the X-ray diffraction data.
- Comparison of HAR-derived O-H/D bond lengths and ADPs with existing neutron diffraction data.
Main Results:
- Accurate crystal structures of H2O, D2O, and mixed ice VI were obtained using HAR and X-ray diffraction.
- The method successfully determined O-H/D bond lengths and ADPs for disordered hydrogen atoms.
- Results showed excellent agreement with corresponding single-crystal neutron diffraction data.
Conclusions:
- Hirshfeld atom refinement (HAR) combined with X-ray diffraction is a powerful technique for studying polymorphic ice forms, comparable to neutron diffraction.
- This approach offers a more accessible and cost-effective alternative to neutron diffraction, facilitating the verification of known ice polymorphs and the discovery of new ones.
- HAR-X-ray diffraction can be applied to other hydrogen-rich compounds beyond water ice.
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