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The crystal and molecular structure of tetraiodoethylene (C214) at 4 K has been determined by neutron powder diffraction with Rietveld's profile-refinement method. Three independent data sets at different wavelengths were collected and used in the calculations. The measurements were sufficiently accurate to locate the C atoms in the molecules, and to show that the C=C bonds in both molecules are fully ordered and aligned approximately parallel to a*. The dimensions of the unit cell at 4 K and several interatomic distances are reported. The estimated standard deviations derived by this method are discussed and a normalization and weighting scheme for combining results from multiple power-data sets is suggested.1765

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A neutron guide tube and diffractometer for measurements of small- angle Bragg diffraction are described. Application to studies of biological structures is demonstrated by preliminary diffraction data from the membranes of the myelin sheath of dog sciatic nerve.

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