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

Abstract

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.

Lighter Alkali Hydride and Deruteride. Electronic Properties of Pure Solids

“…A reliable neutron diffraction study of these compounds was made by Vidal-Vilat and Vidal [44], where the Debye-Waller factors obtained could be interpreted in terms of conventional lattice dynamical models and the isotopic effect could be attributed to the difference in mass. Very recently Vidal-Vilat et al [31] obtained accurate X-ray diffraction data on LiH and LID measured at three temperatures. Detailed data on experimental and theoretical structure factors at different temperatures have been presented.…”

Are lithium hydride clusters purely ionic? Study using model potentials and density-functional theory

“…On the contrary cubic structures are by large the most stable in the case of Li 14 H 13 ϩ . As for interatomic distances, the experimental Li-H lattice parameter is 4.08 Å, 27 which makes the Li-H distance equal to 2.04 Å. The interatomic distances calculated for Li 4 H 4 , Li 6 H 6 , and Li 14 H 13 ϩ for instance are between 1.82 and 1.86 Å, therefore, are still greatly smaller in our clusters than the bulk equilibrium distance, but larger than the 1.63 Å distance calculated in the Li-H molecule.…”

Direct Observation of Born−Oppenheimer Approximation Breakdown in Carbon Nanotubes

“…Although the BO approximation is valid in most materials and molecular systems, there are a few situations in which it does not hold, including some low atomic weight compounds [2][3][4] , intercalated graphite 5 , and graphene 6 . Clean, defect-free single-walled carbon nanotubes (SWNTs) are systems which can be used to verify fundamental phenomena such as Wigner crystallization 7 and spin-orbit coupling 8 , and are ideal candidates for testing fundamental physical predictions.…”