Joint structure refinement of x-ray and neutron diffraction data on disordered materials: application to liquid water

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• X-ray diffraction provides structural data for liquids and solids, complementing neutron diffraction.
• X-ray scattering from electrons is complicated by Q-dependent form factors, especially in polarized molecules like water.
• Empirical Potential Structure Refinement (EPSR) is used to reconcile X-ray and neutron diffraction data.

Purpose of the Study:

• To develop a single atomistic structural model consistent with both X-ray and neutron diffraction data for liquid water.
• To investigate the constraints X-ray diffraction data place on structural models of water.
• To refine understanding of water's molecular structure and hydrogen bonding.

Main Methods:

• Utilizing Empirical Potential Structure Refinement (EPSR) to model liquid water.
• Analyzing X-ray diffraction data with varying Q-ranges (up to ~17.0 Å⁻¹).
• Analyzing neutron diffraction data with extended Q-ranges (up to ~30 Å⁻¹).

Main Results:

• X-ray data strongly constrain water's structural models but limited Q-ranges and data variations prevent precise determination of the OO radial distribution function's first peak.
• Neutron data, despite high Q-ranges, introduce minor uncertainties in hydrogen bond peak positions.
• Combined data suggest classical water potentials may have overly repulsive short-distance cores.

Conclusions:

• Simultaneous analysis of X-ray and neutron diffraction data refines structural models of liquid water.
• A softer core potential is proposed for water models to better represent short-range interactions.
• Accurate structural modeling requires careful consideration of diffraction data limitations and potential deviations in electronic form factors.