705 {1969}. Van Vechten, Phys. Rev. 182, 891 (1969). Wemple and M. DiDomenico, Jr. , to be pub-sentially the same normalized value of optiealfrequency conductivity. In our model only bandwidth differences distinguish ionic from covalent crystals. Neither the energy gap nor the internuclear spacing enter into the computation of o. The bandwidth parameters b, and b; measure the width of those parts of the conduction and valence bands which play important roles in determining crystal structure.

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... the more covalent crystals Phillips" has correlated crystal structure with a dielectrically defined bond ionicity. Here we have found from a spectroscopic analysis of covalent and ionic crystals that the N, and Z, of Pauling's classical resonating-bond theory" play an important role. In both cases the usual picture of crystal structure as determined only by the energies of occupied valence states is discarded in favor of relationships between structure and the optical spectrum. We wish to thank J. C. Phillips and J. A. Van Vechten for stimulating discussions and helpful comments on the manuscript. lished; for data on some oxides see M. Dioomenico, Jr. , and S. H. Wemple, J. Appl. Phys. 40, 720 (1969). 5We have excluded from Table I all small-band-gap semiconductors since it is difficult to determine with sufficient accuracy the free-carrier and/or photoionization contribution to the observed refractive-index dispersion for such materials. The results tabulated for Ge may in fact be influenced by such extrinsic effects. 6In magnetic transition-metal compounds we would expect weaker oscillator strength for transitions from the occupied d or f orbitals. For example, in Euo Sd = 9 eV rather than approximately 25 eV as observed in 6-coordinated nonmagnetic oxides. In complex crystals containing anion radicals we find, for example, that Sd = 16 eV for several phosphates and hd = 20 eV for a group of iodates and carbonates. Based on available refractive-index data the P values for LiF and AgCl fall slightly outside the limits given for p;. The P value for Ge lies slightly below the lower bound given for P~. This may be due to a small freecarrier contribution to the refractive index data. The reflectance of solid Ar has been measured at 20'K for an angle of incidence of 15' in the photon energy range from 10 to 30 eV using the synchrotron radiation of DESY. The reflectance data reveal a spin-orbit-split exciton series with sharp maxima converging to about 14 eV together with broader peaks above 14 eV due to transitions between the valence and conduction band. The results are compared with the absorption spectrum associated with the 2P core levels. Several optical" and electron-energy-loss' measurements have been performed on solid Ar in order to study its electronic transitions from the valence band. These investigations were confined to the spectral region below 14 eV. They were made with limited resolution so that the existence of an exciton series converging to the band gap could not clearly be proved. Thus these measurements led to some uncertainty in the determination of the series limit. 4 1160