AB-INITIO STUDY OF OPTICAL-RESPONSE PROPERTIES OF NONSTOICHIOMETRIC LITHIUM-HYDRIDE AND SODIUM-FLUORIDE CLUSTERS

Structural and optical response properties of LinHn-m and NanFn-m (n = 2-6, m = 1,2) clusters containing one- and two-excess electrons are s tudied using ab-initio methods accounting for electron correlation. Th e common feature of the optical response obtained for the most stable structures of NanFn-1 (n = 2-6) clusters is the appearance of a domina nt intense transition in the infrared regime independently whether the single excess electron is localized at the cuboid corner vacancy (sur face F-center) or at the external atom attached to the filled cuboid. In contrast, LinHn-1 (n = 2-6) clusters exhibit substantially differen t spectroscopic patterns with respect to halides also for the cases wi th the common structural properties. Optical response features of LinH n-2 (n = 3-6) clusters with two-excess electrons are characterized by dominant transitions in the visible regime reflecting segregation in ' 'metallic'' and ionic parts. In contrast, NanFn-2 (n = 3-6) can be div ided according to their optical and structural properties into cuboid ''lattice'' defect species (Na4F2, Na6F4) and segregated metallic-ioni c systems. For the former, the intense transitions occur in the infrar ed-visible, and for the latter only in the visible regime. It will be shown that the calculated absorption patterns are excellent fingerprin ts of structural and bonding properties. (C) 1997 Elsevier Science B.V .