Studies of electric field induced structural and electron-density modifications by X-ray diffraction

During the last two decades, a number of X-ray diffraction studies on the response of a crystal to an applied electric field have been carried out. In a few cases, the electron-density polarizations could be determined. The analysis of the induced variations of the structural properties on an atomic scale are of prime importance in order to acquire a better understanding of physical properties like the piezoelectric and dielectric properties of crystals. This article reviews the experimental technique used and the modelling methods of the Bragg scattering variations induced by the field. Some noteworthy results are presented that illustrate the possibility of detecting Asubtle structural changes, for example as small as 0.1(degree) in bond angles arising from applying a strong field, 10-40  kV  cm^-1, as well as the pitfalls of such an approach for clarifying the relevance of the structural properties in physical mechanisms.