5d Orbital and electronic localization in rare earth diiodides and praseodymium halides, Pr2X5 (X = Br, I)
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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. 1993, American Chemical Society. All rights reserved.
