**Preface to "X-ray Diffraction of Functional Materials"**

With new functional materials being developed and their properties being strongly related to their microstructure, the characterization of the latter is of paramount interest. Thanks to its non-invasive character, adaptability to various environments, and high sensitivity to crystalline structure and deformation as well as to defects, X-ray diffraction is the ideal tool to investigate the microstructure of these new materials.

The last 15 years have seen the continuous development of X-ray sources, optics, and detectors, making X-ray microscopy of strain and defects a reality with realistic time scales. Thanks to the penetrating power of X-rays, in situ or even operando monitoring of the crystalline structure of materials and devices as a function of mechanical stimuli, temperature, gases, electric fields, etc., is being commonly performed. Such studies are invaluable to investigate the physical mechanisms at work in real or close-to-real conditions. This is the case of elastic and plastic properties, catalytic activity, ferroelectric domain structure, and many others.

Moreover, advances in detectors and computer hardware and software facilitate time-resolved X-ray diffraction studies of the transient behavior of the microstructure, phase transitions, and physical changes caused by external stimuli. The time range covers more than ten orders of magnitude—from sub-picoseconds to kiloseconds. The advent of free electron lasers opens the few femtoseconds range, enabling the study of electronic processes.

This issue is dedicated to the latest advances in X-ray diffraction using both synchrotron radiation as well as laboratory sources for evaluating the microstructure and structure-to-property relation in functional materials (functional oxides, organic and hybrid materials for energy, electronics, etc.). Particular focus will be placed on novel in situ or operando approaches.

> **Thomas Walter Cornelius, Souren Grigorian** *Editors*
