ELECTRONIC-STRUCTURE OF LAYERED MB2C RARE-EARTH BOROCARBIDE COMPOUNDS

The electronic structure and bonding properties of the layered borocar bide compounds with the ThB2C, alpha-UB2C, and YB2C structures are ana lyzed and compared by means of extended Huckel tight-binding calculati ons. These materials contain alternatively metallic and boron-carbon s heets. The results show that an ionic picture between the metallic and the boron-carbon sublattices is a good starting point to explain the arrangement observed in the different nonmetal networks. Thus, a forma l charge of 4- per B2C unit accounts for the structure of the B-C net in ThB2C and alpha-UB2C, whereas a formal charge of 3- must be attribu ted to the one in YB2C. An alternative B2C two-dimensional arrangement is proposed for the charge of 4- per B2C. The semimetallic nature obs erved for all the anionic two-dimensional boron-carbon layers is discu ssed. In the three-dimensional materials, the metal-nonmetal bonding o ccurs primarily through electron donation from the anionic sublattice toward the metallic elements. Metallic behavior is expected for all th e materials.