Evidence on the breakdown of the Born-Oppenheimer approximation in the charge density of crystalline 7LiH/D

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 and Vidal-Valat. 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. (orig.)