Rare-earth (RE) transition-metal (T) carbides RE_xT_yC_z have been intensively studied during the last thirty years with respect to their interesting crystal chemistry. Since the pioneering theoretical studies by Burdett, Whangbo, and Hoffmann on YCoC (Figure 1a) these species are considered as organometallic T_y,C_z~(δ-) polyanions embedded in an ionic matrix provided by the rare-earth-metal atoms. Within the zero-, one-, two, or three-dimensional polyanionic networks, the carbon atoms can be isolated (carbometallates) or they can form C2 pairs or C3 units.
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