Accurate X-ray structure factors of CaO, SrO and BaO were measured at 293 K from single crystals and analysed together with the data of Sanger [Acta Cryst. (1969), A25, 694-702] on MgO. The oxygen ion is seen to suffer an average compression and a fourth-order cubic harmonic type deformation, as compared to the atomic superposition model, yielding a lower average charge density in the surrounding region and a charge transfer from [ 111 ] to the [ 100] direction. These features strengthen with increasing atomic number of the cation. Mg and Ca show a weak broadening. About nine electrons are localized around the oxygen position. The tenth electron of 02-is more widely distributed. The electron counts of the cation correspond most closely to Mg +, Ca 2+, Sr 2+ and Ba 2+. The results confirm the dominance of ionic character and increase of ionicity with increasing atomic number of the cation.

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Accurate X-ray diffraction data on LiH and LiD measured at three different temperatures are analysed in terms of multipolar radial densities searching for phenomenological indications on the nature of bonding. The average spherical charge density around the atomic positions shows typical features of an ionic crystal. The central peaks are slightly contracted compared with superimposed free ions. The Li + peak contains, however, a small but significant excess of electrons, and the H-/D-peak is low and diffuse. Li + is spherical and its Debye-Waller parameters agree with the neutron diffraction values obtained by Vidal& Vidal-Valat [Acta Cryst. (1986), B42, 131-137]. This indicates that, within the experimental accuracy, Li + is rigid. The non-spherical multipoles are significantly stronger in the hydrogen than in the deuterium derivative. They accumulate charge along the (100) directions giving a phenomenological indication of 'long-distance covalency' of H-H and D-D bonding with Li ÷ ions embedded in the middle but not contributing to the covalency. The significant deviation of the charge distribution of the anion in LiH from that in LiD indicates breakdown of the Born-Oppenheimer approximation due to coupling of the vibrations and the electronic states, which is much stronger in LiH. This is the first case -and probably the only possible -where such a breakdown can be seen by X-ray diffraction.

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X-ray diffraction intensities were measured from a BeO single crystal with wurtzite structure specially treated to reduce extinction effects. Preliminary leastsquares refinement yielded experimental structure factors on an absolute scale, corrected for extinction and anomalous dispersion, and a reference model Be2+O 2-with position parameter z = 0.3778 (4) and anisotropic Debye-Waller factors corresponding to atomic mean-square displacements with spherical average (U 2) "--0"0045 (3) and 0.00353 (1) A 2 and prolateness (u 2)-(u 2) = 0.0008 (4) and 0.00005 (9) A2 for Be 2+ and O z-, respectively. The residual differences Fo-Fc (of these experimental structure factors and those of the reference model) were analysed in terms of site-symmetrized multipole expansions. The phases of Fo were refined iteratively to take into account the effect of the significant multipole components found. The contribution of the Be 2+ ion was subtracted to make possible a study of the oxygen lattice. Significant multipole components of third, fourth and sixth order indicate that the oxygen in BeO forms bonded hexagonal 0 2-planes. They build up diffuse O-O bridges strongly bent towards the stabilizing Be z+ ions above the plane. The Be 2+ ions exhibit a stretching quadrupole-like deformation suggesting some covalency of the bonding they build between the O 2-planes. There is some evidence of anharmonicity of atomic vibrations, particularly in the direction of the hexagonal axis, AbstractA simple relationship has been developed for determining values for the moduli of normal order deviates for sample size n from the values of the normal order deviates for sample size 2n + 1. This relationship can be expressed as a quadratic 'correction' to the normal order values. This approximation is in error by <0.001 for values of n > 8.