Direct space approach in action: Challenging structure solution of microcrystalline materials using the EXPO software

Despite recent advances, the crystal structure solution process from powder X-ray diffraction (PXRD) data is still a challenging task. In this article, we present case studies of structure solutions using direct-space methods based on a simulated annealing approach. Representative examples of applications of the software EXPO are described in detail to provide methodological and computational tools useful to face complex cases of structure solutions from powders. We focus on the main practical aspects regarding how to build a structure model compatible with the expected molecular geometry, which tricks to use in case of low-quality diffraction data, how to decide that the obtained solution is the correct one and how to treat the most frequent sources of errors. Through six case studies, regarding the solution process of diltiazem hydrochloride, tetracycline hydrochloride, selexipag form I, catena-(Aluminophosphate hexadecakis(2-hydroxyethylamine), poly[[di-mu-aqua-bis-(glycinato-kappa 2N,O)calcium (II)] monohydrate] and lanthanum titanium aluminate, we give the reader, expert or new in the field, the necessary instructions for solving structure using EXPO with the help of auxiliary programs. The article highlights some of the most critical aspects of the PXRD solution process with direct-space methods for providing deep insights into both the powers and limits.