Correction factors in the photographic measurements of x-ray intensities in crystal analysis

Abstract

For the complete solution of the structures of complex crystals, and in particular of organic substances, it is essential to be able to measure at least the relative intensities of reflexion of homogeneous X-rays from the more important lattice planes with some degree of accuracy. In most crystal problems a number of parameters governing the positions of the atoms in the lattice remain to be found after the geometrical requirements of the symmetry have been satisfied. These can only be determined from intensity measurements, so that in general, the greater the number of reflexions measured, the more closely will the deduced structure approach to the truth. Until comparatively recently the only instrument of precision available for X-ray intensity work has been the Bragg ionisation spectrometer. It has, however, three disadvantages when used for this purpose, which may be briefly summarised:— (1) Only the strongest planes are measurable with any degree of accuracy, owing to the “swamping” effect of the unremovable background radiation in the case of the weaker reflexions. (2) It is clearly impossible to use it, without very special technique, in the case of crystals which are volatile or even liquid at normal temperatures. (3) The measurement of the true integrated intensity is a somewhat lengthy operation in practice, with the result that experimenters are tempted to determine the peak values of the intensities instead; these, in general, are not in the same ratio as the integrated reflexions, so that a false idea of the reflexions may be obtained.