STATISTICAL APPROACH: TO COHERENT DIFFRACTION FROM A PERFECT CRYSTAL WITH A RANDOMLY DISTURBED LAYER. I. A. Vartanyants^l and J. -P. Guigay^{2, 1}Institute of Crystallography RAS, Moscow, Russia, ²Laboratoire Louis Neel, CNRS,Grenoble, France

X-ray intensity diffracted by statistically deformed crystals is usually considered as the sum of a coherent I_coh and an incoherent I_inc components t [1], which can be separated experimentally by using the technique of triple-crystal diffractometry. The present work deals with numerical calculations of the angular distribution of I_coh from a crystal with a statistical distribution of microdefects located in a surface layer, in the frame of the theory developed in [2]. The lattice spacing of the layer was taken slightly different from the lattice spacing of the perfect substrate. The statistical layer deformation is defined, according to [1] by a static Debye-Waller factor E and a correlation function g(t). We have used different models for g(t): an exponential model and a model corresponding to spherical amorphous clusters. In our calculations of the coherent intensity, I^s_coh diffracted by the substrate and I^l_coh diffracted by the layer correspond to different angular ranges because of the microscopic deformation of the layer. Our results show that I^l_coh depends strongly on the value of Kato's correlation length [[tau]]₀ and on the value of E only and practically does not depend on the model of correlation function. There was found and explained nonmonotonic behavior of I^s_coh as a function of parameter [[tau]]₀. The possibilities of independent measurements of E and [[tau]]₀. with the help of synchrotron radiation are discussed.

1. N. Kato, Acta Cryst. A36, 763-778 (1980).

2. V. A. Bushuev, Fiz. Tverd. Tela 31, 70-78, (1989); Sov. Phys. Solid State 31, 1877-1882 (1989).