Problems in the collection and analysis of accurate diffraction data have been explored in a careful study of urea. Initial neutron measurements on non-spherical crystals show that little confidence can be placed in the values of extinction parameters derived for a non-spherical crystal of a hydrogenous material used in a neutron-diffraction experiment. It is also shown that multiple Bragg scattering is an important source of error. Uncertainties in the absorption and extinction corrections can be overcome by using a spherical crystal with/~R-0"5. Corrections for the effects of multiple Bragg scattering were made by correlating complete sets of data measured at three different wavelengths. A number of models based on rigid-molecule vibrations and anharmonic interactions between atoms were used to treat the effects of thermal vibration. The best results were obtained from a refinement of the spherical-crystal data using general thermal parameters plus corrections for curvilinear motion, but a rigid-molecule treatment, with fewer vibrational parameters, was not much worse. Anharmonic effects did not seem to be appreciable. To look for possible 'bonding features' in urea, difference syntheses computed from X-ray data and calculated X-ray structure factors based on the neutron i esults were plotted. It was not possible to remove the deep negative differences at the atomic positions by adjusting a scale factor and a thermal diffuse scattering correction, and it is probable that a real discrepancy exists between the observed and calculated structure factors.

NH4Br, which has the caesium-chloride structure, and NH4I, which has the sodium chloride structure, have been studied by neutron diffraction, mainly to determine the orientation and vibrational behaviour of the NH4 groups. For NH4Br, a model with eight ½-hydrogen atoms in (111 ) directions, and isotropic, Gaussian libration of the hydrogen atoms, seemed to provide an adequate destription of the diffraction results with physically plausible parameters. For NH4I, the hydrogen distribution has broad peaks in the six (100) directions and unusually large vibration amplitudes for the ions; the most profitable method of refinement was to describe the hydrogen distribution by Kubic Harmonics.

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The sodium nitrate structure has been determined by neutron-diffraction methods between 20 and 290°C. Above the 275°C phase transition there is complete disorder between alternative orientations of the NO3 group.

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