Abstract
Refractive-index dispersion data below the interband absorption edge in more than 100 widely different solids and liquids are analyzed using a single-effective-oscillator fit of the form , where is the photon energy, is the single oscillator energy, and is the dispersion energy. The parameter , which is a measure of the strength of interband optical transitions, is found to obey the simple empirical relationship , where is the coordination number of the cation nearest neighbor to the anion, is the formal chemical valency of the anion, is the effective number of valence electrons per anion (usually ), and is essentially two-valued, taking on the "ionic" value eV for halides and most oxides, and the "covalent" value eV for the tetrahedrally bonded zinc-blende- and diamond-type structures, as well as for scheelite-structure oxides and some iodates and carbonates. Wurtzite-structure crystals form a transitional group between ionic and covalent crystal classes. Experimentally, it is also found that does not depend significantly on either the bandgap or the volume density of valence electrons. The experimental results are related to the fundamental spectrum via appropriately defined moment integrals. It is found, using relationships between moment integrals, that for a particularly simple choice of a model spectrum, viz., constant optical-frequency conductivity with high- and low-frequency cutoffs, the bandgap parameter in the high-frequency sum rule introduced by Hopfield provides the connection between the single-oscillator parameters () and the Phillips static-dielectric-constant parameters (), i.e., . Finally, it is suggested that the observed dependence of on coordination number and valency implies that an understanding of refractive-index behavior may lie in a localized molecular theory of optical transitions.
- Received 30 July 1970
DOI:https://doi.org/10.1103/PhysRevB.3.1338
©1971 American Physical Society