Development of the Variable-Cell Powder Difference (VC-PWDF) Method and Applications in the Comparison and Determination of Crystal Structures

Abstract

The identification and classification of crystal structures is fundamental in materials science, as the crystal structure is an inherent factor of what gives solid materials their properties (conductivity, magnetism, hardness, solubility, etc.). Being able to identify the same crystallographic form from unique origins (e.g. different temperatures, pressures, or in silico-generated) is a complex challenge. In particular, the use of simulated powder diffractograms for this purpose has not seen general success due to the intimate relationship between the lattice dimensions and peak positions, which are strongly affected by experimental conditions. Herein is presented the development and application of the VC-PWDF (Variable-Cell PoWder DiFference) method to resolve this scientific problem. This new approach of comparing crystal structures using their powder diffractograms involves an automated series of steps that identifies the lattice distortion necessary to align the two crystal structures being compared, provided one exists (i.e. provided they are the same form). The quantitative value (VC-PWDF score) yielded by the protocol provides a measure of similarity more accurate than other available methods of structure comparison based on powder diffractograms. For a set of nearly 45,000 structure pairs in the Cambridge Structure Database (CSD), the VC-PWDF method is shown to be as successful as the COMPACK method, which compares atomic positions, in distinguishing the same form under disparate conditions from a different polymorph structure. When comparing known polymorphs to in silico-generated structures from Crystal Structure Prediction (CSP) studies, the VC-PWDF method is shown to be a valuable complementary method to COMPACK, which is prone to false negatives that VC-PWDF readily identifies as true positives, determining two missed matches from the 6th CSP blind test. Finally, the ability of the VC-PWDF approach to match an experimental powder diffractogram collected on a regular laboratory diffractometer to a crystal structure (from the CSD or CSP) via its simulated powder diffractogram is demonstrated (Variable-Cell eXperimental PoWder DiFference, VC-xPWDF), outlining a path for structure determination from powder data. The VC-(x)PWDF methods are anticipated to become commonplace tools for crystal structure comparison and determination in academia and industry alike.