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The diffraction pattern for a double hexagonal close-packed crystal containing deformation stacking faults is derived under the assumption that the crystal is infinite in size with faults covering entire basal planes and distributed at random. It is found that such faulting broadens only...
A computerized procedure based on the Evjen method for the direct summation of Madelung energies of centrosymmetric structures is described. The values obtained are strictly according to the definition of the Madelung function and the accuracy depends only on the number of terms taken into...
A chart is presented on which the appropriate zero-level Weissenberg photograph of a pseudo-merohedral twin may be superimposed. This enables the separation of twin-pairs of reflexions at any point on the film to be related to the twin obliquity angle. A separate chart which may be used with the...
The contribution of the thermal diffuse scattering to the measured X-ray intensities of cubic powder patterns is calculated without making the approximation that T . Correction curves are given which are valid at all temperatures.
An improved method of including accidentally absent reflexions in the least-squares refinement of crystal structures, so as to accelerate the initial convergence of the procedure, is discussed. It is shown that when the calculated structure factors of these reflexions are less than the observed...
Expressions which enable data to be `corrected' for twinning are examined. If the volume ratio of the twin individuals is known, structure factors may be calculated and used in the normal way. Particular care must, however, be taken with weighting. For least-squares refinement or for comparing...
Neutron and X-ray intensities of NH4F were measured at -196DegreesC and -155DegreesC respectively. The wurtzite type structure and space group P63mc were confirmed. The displacement of the two h.c.p. sublattices, formed by each of the F-- and NH+4- ions, is such that all bond-distances are...
The calculation of molecular form factors from ab initio molecular electronic wavefunctions is discussed, and a scheme for application to X-ray diffraction structure analysis is given. The method is used to calculate the form factor of the NH+4 molecular ion from three accurate molecular...
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