Structure of ice VII with Hirshfeld atom refinement

Area of Science:

• Crystallography
• Materials Science
• High-Pressure Physics

Background:

• Disordered hydrogen atoms in ice structures complicate crystal structure refinement.
• Neutron diffraction is typically preferred for precise ice structure determination due to hydrogen atom challenges.
• Previous work demonstrated Hirshfeld atom refinement (HAR) with X-ray data for ice VI.

Purpose of the Study:

• To investigate the crystal structure of high-pressure cubo-ice (ice VII) using HAR.
• To compare HAR results with neutron diffraction data for ice VII.
• To assess the feasibility of HAR for disordered ice structures.

Main Methods:

• Single crystals of cubo-ice (ice VII) were grown under pressure using diamond anvil cells.
• X-ray diffraction data were collected using synchrotron and laboratory diffractometers.
• Data were refined using Hirshfeld atom refinement (HAR).

Main Results:

• HAR of ice VII X-ray data yielded structural parameters in excellent agreement with neutron diffraction for bond lengths.
• Fair agreement was achieved for hydrogen atom anisotropic displacement parameters (ADPs).
• Unconstrained refinements with split-atom models were successfully performed.

Conclusions:

• Hirshfeld atom refinement (HAR) is a viable alternative to neutron diffraction for refining disordered ice structures like ice VII.
• X-ray diffraction combined with HAR can accurately determine key structural parameters, including hydrogen atom positions.
• This approach offers a powerful tool for studying complex ice polymorphs.