CRYSTAL-STRUCTURES OF TRANSITION-METAL CARBIDES

The factors contributing to the formation of carbides of complex crystal structure by transition metals such as CI are discussed in terms of the cells of metal (M) atoms around central carbon (C) atoms. The octahedral cell is favoured by M-M repulsion, which occurs particularly in high C compounds of the group IVA and VA metals. The main alternative, the trigonal prismatic cell, is favoured by a better angular separation of M-M and C-M bonds, which is important in WC; and also favoured by stronger M-M attraction, as occurs in low C compounds of group VIA and later metals, and by smaller M atomic sizes as in the first long period metals. Low C content provides an opportunity, in trigonal prismatic structures, for the formation of complex structures which have the advantage of providing a C atom with more than six M neighbours. This advantage also appears strongly in the M(23)C(6) structure where the cell is a square antiprism with eight M neighbours around its central C atom.