5D-ORBITAL AND ELECTRONIC LOCALIZATION IN RARE-EARTH DIIODIDES AND PR2X5 (X = BR, I)

An analysis of the origin of the valence and conduction band gaps and bandwidths for the LnI2 (Ln = La, Ce, Pr, Gd) compounds is presented. The PrI2 system is analyzed in detail with a focus on the 3-center bonding present in these compounds, as was found for the isostructural MoS2 system. Despite the long Ln-Ln distances found in these compounds, appreciable band gaps arise that are traceable to the splitting between the bonding and antibonding orbitals of the 3-center bond systems. We argue that the narrowness of the valence band is not due to the weakness of the metal-metal interactions. Comparison with the Pr2X5 (X = Br, I) systems treated by Meyer and Hoffmann shows the close analogies between the electronic structure of these compounds and the LnI2 materials. A hybridization scheme for the low-lying d levels of trigonal prismatic centers clearly reveals these relationships. Since the Pr2X5 compounds are magnetically ordered with localized 5d electrons, questions about the properties of the LnI2 compounds are raised.