SECONDARY EXTINCTION OF X-RAY AND NEUTRON DIFFRACTION IN MOSAIC CRYSTALS *. Hua-Chen Hu, China Institute of Atomic Energy, P. O. Box 275(18), Beijing 102413, People's Republic of China

Universal expressions by using three dimensionless parameters for the secondary extinction factors in X-ray and neutron crystallography are developed based on the exact solutions of the power transfer equations. These methods can be applied to reflections of all possible values of extinction factor, reflection symmetry and the absorption to scattering cross-section ratio of the crystal. The representation by three parameters gives a clear and definite physical meaning to the concept of extinction. The theory has been extended to treat the extinction of a spherical crystal. As a demonstration of the feasibility of using these expressions, the diffraction data on LiF and MgO plane mosaic crystals measured by Lawrence [(l972) Acta Cryst. A28, 400-404, and (1973) Acta Cryst. A29, 208-210.] are reanalyzed by this method. All the reflections including the strongest ones (Y_O down to 0.026) are reanalyzed simultaneously with single-valued particle size and mosaic spread as fitting parameters and allowing for primary extinction if necessary. The results (R factor =0.014 and 0.053 for LiF and MgO, respectively) are unprecedentedly good. Furthermore, in disagreement with Lawrence, the extinction of LiF is found to be of secondary type, and in the case of MgO both primary and secondary extinction should be considered. The analysis shows that Darwin's transfer equations are valid in a range much broader than previous anticipated.

* Project supported by National Nature Science Foundation of China and Science I Foundation of Nuclear Industry of China.