Concerning order parameters in the statistical dynamical theory of diffraction

The development of the statistical dynamical theory of diffraction by real crystals involves two order parameters: a long-range and a short-range one. These parameters play a fundamental role in practical applications of the theory to real diffraction data. A reasonable probabilistic model is proposed to describe the phase correlation at two given scattering positions. This model allows for a tractable solution of the propagation equations for the beams.