Carbide bromides are mixed anion compounds containing bromide and carbide anions. Many carbide bromides are cluster compounds, containing on, two or more carbon atoms in a core, surrounded by a layer of metal atoms, encased in a shell of bromide ions. These ions may be shared between clusters to form chains, double chains or layers.
The great majority of these carbide bromide compounds contain rare earth elements. Since these elements have similar properties, similar structures can be made by substituting the elements. R<sub>2</sub>CBr<sub>2</sub> forms a structure with layers of R<sub>6</sub>C clusters that contain one carbon atom. Each layer has bromide coating the top and bottom. Very similar is R<sub>2</sub>CBr<sub>2</sub> which has layers of R<sub>6</sub>C<sub>2</sub> clusters containing pairs of carbon atoms. This dicarbon is an (C<sub>2</sub><sup>4âÂÂ</sup>), and contains a double bond. Layers have bromide on both sides, and so they are only weakly held together by van der Waals forces. If these layers are aligned with each other a 1T- form results with a small c measurement on the unit cell. In some compounds the layers are not quite aligned, but repeat after three layers giving a 3R form, with a larger c unit cell height. Where the layers align, the crystal system is trigonal. But if the layers never quite align at any height, a monoclinic crystal results. The C<sub>2</sub> unit sits at an angle to the layers, and thus reduces symmetry compared to compounds with single carbon atoms in the cluster.
In R<sub>2</sub>CBr there are layers of R<sub>6</sub>C that share bromide between layers.