Désordre linéaire dans les cristaux (cas du silicium, du quartz, et des pérovskites ferroélectriques)

Many crystals produce a diffuse scattering of X-rays which is localized in a series of relplanes. It is shown that the corresponding linear disorder in the crystal may have various origins. In silicon, the scattering is due to thermal vibrations and is well explained by the elastic properties of the crystal. In neutron irradiated quartz the radiation damage is responsible for the major part of the scattering. The case of BaTiO3 and KNbO3 is discussed in detail. A linear disorder is proposed which accounts better for the different distributions of the scattering in the 4 allotropic phases than the alternative explanation of the soft mode. In spite of some neutron inelastic scattering results it is not yet possible to distinguish between static and dynamic disorder.