Ionic bonding of transition-metal halides: A spectroscopic approach.

Abstract

The dielectric theory of the chemical bond has been applied to crystals with either Cd(OH${)}_{2}$ or ${\mathrm{CdCl}}_{2}$ structure, namely to layered Mn, Fe, Co, and Ni dihalides (${\mathrm{MX}}_{2}$) with octahedral coordination, in order to evaluate the fractional ionic character ${f}_{i}$ for this class of insulators. The crystalline spectroscopic energy gap ${E}_{g}$ has been measured via the optical data, related to the dominant exciton peaks \ensuremath{\Gamma}, and then evaluated either through the Phillips model (${E}_{g}^{\mathrm{Phillips}}$) or the measured dielectric constant ${\ensuremath{\epsilon}}_{1}$(0) in the framework of the Penn model (${E}_{g}^{\mathrm{Penn}}$). The obtained scales of ionicities, ${f}_{i}$ or ${f}_{i}^{\mathrm{DT}}$, ranging from ${f}_{i}$\ensuremath{\simeq}0.72 of ${\mathrm{NiI}}_{2}$ to ${f}_{i}$\ensuremath{\simeq}0.80 of ${\mathrm{MnCl}}_{2}$ are then compared to the ionicity scale ${f}_{i}^{\mathrm{XPS}}$ based on x-ray photoelectron spectroscopy. For transition-metal chlorides, for which photoemission spectra are available, the different ionicity scales are in good agreement. Furthermore, the ionicity parameters scale rather well with the ionicity trend given by the fitted values of the net charge Z, the electrostatic parameter for dealing with crystals not completely ionic. The overall agreement between the spectroscopically determined ionicity, the structural, thermochemical, and electronic properties of these compounds seems to indicate that the dielectric theory of Phillips and Van Vechten can be successfully applied to layered materials with reduced ionicity and open d-shell configuration.