Lattice dynamics of lithium hydride and lithium deuteride: effect of long-range three-body forces

Abstract

Some new experimental data (optical measurement and elastic constant determination) have shown discrepancies with model calculations, in LiH and LiD crystals. Using the classical shell model, the author has shown that second-neighbour H-H interactions are very important in this crystal. In order to correct the frequencies, particularly on the TO branches in the (1, 1, 1) direction, shell deformation effects have been introduced into the model and it has been shown that there exist no short-range forces able to improve the calculation. It is then necessary to introduce long-range three-body forces to explain all the dynamic properties of these two crystals.