Theoretical study of mixed LiCeX4 (X = F, Cl, Br, I) rare earth/alkali halide complexes

The structure and vibrational properties of the LiCeX4 (X = F, Cl, Br, I) complexes were studied using density functional theory in conjunction with polarised triple-zeta valence basis sets and relativistic effective core potentials for the heavy atoms. Three characteristic structures were found on the potential energy surface with one (C-3v), two- (C-2v) and three-fold coordination (C-3v) between the alkali metal and the bridging halide atoms. The bidentate structure was found to be the global minimum being more stable by 10-40 kJ/mol than the tridentate form representing a local minimum. The monodentate structure generally proved to be a second-order saddle-point except for the fluorides where it is a high-lying local minimum. The energy difference between the three structures as well as the dissociation energy and the Li...X binding energy decrease in the row F to I. The ionic bonding in the complexes was characterised by a topological analysis of the electron density distribution according to Bader's theorem. The calculated vibrational data indicate that vibrational spectroscopy may be an effective tool for experimental investigation and characterisation of the LiCeX4 molecules. (C) 2000 Elsevier Science Inc. All rights reserved.