The structure of cadmium selenide has been determined from X-ray intensity data obtained with extendedface crystals. The wurtzite parameter u was found to be 0.37679_ 0.00012. Comparison of the intensities of equivalent reflexions provided a test for the internal consistency of the measurements. Equivalent reflexions in two specimens differed on average by 1.4 and 0"6% from the mean measured intensity, attesting to the high internal consistency of measurements from extended-face crystals. Comparison of 36 structure factors derived from data obtained from both specimens showed their average deviation from the mean to be 0"9%. An attempted least-squares refinement of the dispersion corrections of both atoms from observed Bijvoet ratios failed, presumably on account of correlation. When the dispersion corrections of one atom were held constant, it did prove possible to refine the dispersion corrections of the other atom.

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Analysis of recently published X-ray and neutron diffraction data for LiOH reveals that there are pronounced non-spherical deformations of the electrondensity about the oxygen pseudo-atom. Comparison with a theoretical calculation of the electron density for the isolated hydroxide ion shows that these deformations are due to 7r-like bonding of the oxygen to the neighbouring hydrogen pseudo-atom. The observed features have shapes consistent with the non-spherical electron distribution of an isolated hydroxide ion undergoing rigid-body libration and translation. It is shown that an important effect of the rigid-body libration is a reversal in sign of the effective quadrupole component of the oxygen pseudo-atom electron density.

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Highly accurate absolute measurements of the X-ray structure factors of silicon [Aldred & Hart, Proc. R. Soc. London, Ser. A, (1973), 332, 223-238] have been used to analyse a number of models for the electron distribution. Initially, the valence-electron distribution (with the neon core assumed to be unmodified from that of the isolated silicon atoms) was built up with a radial basis of the form r ~ exp (-Q') and non-sphericity was allowed for by inclusion of octupole and hexadecapole terms. Improved representation was achieved with related models in which deformations from the total isolated-atom electron density were refined instead. The exact shape of the deformation electron density in the region of the bond was sensitively dependent on the monopole deformation term. The anomalous-dispersion contributions (Af') to the scattering factors were refined and found to be in agreement with recent interferometric measurements, but not with recent calculations. The octupole density term is slightly sharper at 293-2 than at 92.2 K, and the structure factor for the 222 reflection is predicted to be larger at the higher temperature. These effects may be due to a failure of the convolution approximation or to uncertainties in the anharmonic corrections to the structure-factor data.

Determination of the crystal polarity of non‐centrosymmetric crystals by comparison of the intensities of Friedel‐related pairs of reflections from planes inclined to an extended crystal face is described. Measurements on cubic ZnS are used to illustrate the method. The general case of absorption for a reflection from planes inclined to an extended crystal face is discussed.