Structure determination from powder data without prior indexing, using a similarity measure based on...

Area of Science:

• Crystallography
• Materials Science
• Computational Chemistry

Background:

• Refining organic crystal structures from powder diffraction data is crucial for materials characterization.
• Incorrect lattice parameters in experimental data pose a significant challenge for traditional structure refinement methods.
• Accurate crystal structure determination is essential for understanding material properties and designing new materials.

Purpose of the Study:

• To develop a robust method for refining organic crystal structures from powder diffraction data, specifically addressing issues with incorrect lattice parameters.
• To create a computational tool that automates and improves the accuracy of crystal structure determination from powder data.
• To demonstrate the versatility of the method across various challenging experimental conditions and sample types.

Main Methods:

• Development of a novel similarity measure based on cross- and auto-correlation functions between simulated and experimental powder patterns.
• Optimization of lattice parameters, molecular position, molecular orientation, and intramolecular degrees of freedom.
• Integration of the similarity-based refinement with subsequent Rietveld refinement.
• Implementation of the method in a program named FIDEL (FIt with DEviating Lattice parameters).

Main Results:

• Successful refinement of organic crystal structures even when initial lattice parameters are inaccurate.
• Demonstration of the method's applicability to unindexed powder data, low-quality diffraction patterns, and non-phase-pure samples.
• Validation of the approach using crystal structure predictions from force-field methods and leveraging data from isostructural compounds or varying experimental conditions (temperature, pressure).

Conclusions:

• The developed method provides a reliable and versatile approach for organic crystal structure refinement from powder diffraction data, overcoming limitations of incorrect lattice parameters.
• The FIDEL program offers an automated solution for accurate structure determination, applicable to a wide range of challenging crystallographic problems.
• This work significantly advances the capabilities of powder diffraction analysis for organic materials, enabling structure determination from diverse and imperfect datasets.