EXPERIMENTAL AND THEORETICAL ELECTRONIC-STRUCTURE INVESTIGATIONS ON ALPHA-NB3CL8 AND THE INTERCALATED PHASE BETA-NANB3CL8

The electronic structures of the cluster compound alpha-Nb3Cl8 and the intercalated phase beta'-NaNb3Cl8 have been studied by core level and valence band X-ray and ultraviolet photoelectron spectroscopy (XPS an d UPS), diffuse reflectance spectroscopy, and charge-self-consistent m olecular orbital (CSC-EH) and band structure (CSC-EH-TB) calculations. The crystal structures of the two compounds consist of layers of inte rconnected Nb3Cl13 units, XP and UP valence band spectra as well as th e band structure calculations show well separated sets of Cl 3p levels at lower energy (higher binding energy) and Nb 4d levels at higher en ergy (lower binding energy), indicative of mainly ionic Nb-Cl bonding. The UP spectra of alpha-Nb3Cl8 reveal a triple-peak structure for the Nb 4d levels, corresponding to the 1a(1), 1e, and 2a(1) metal-metal b onding orbitals of a seven-electron Nb-3 cluster as suggested by theor y. The valence band shapes are in good agreement with the theoretical density-of-states curve. The relative intensities in the XP valence ba nd spectra of beta'-NaNb3Cl8 evidence an additional electron in the Nb 4d orbitals. The better resolved UP spectra, however, show a broad pa ttern for the Nb 4d levels which is neither expected from a single clu ster model nor given by the theoretical density-of-states curve, Possi ble origins for this discrepancy between experiment and theory are dis cussed, Electrostatic interactions between Na+ and Cl- ions in beta'-N aNb3Cl8 lead to a narrowing of the Cl 3p core level and C1 3p valence band signals, the former being shifted to higher binding energy in com parison to alpha-Nb3Cl8. Two rather narrow absorptions in the optical spectra of alpha-Nb3Cl8 are assigned to the transitions from the 1a(1) and 1e levels into the singly occupied 2a(1) orbital. These absorptio ns are missing in the optical spectra of beta'-NaNb3Cl8, in agreement with the theoretical expectations for an eight electron Nb-3 cluster.