Rhombohedral phases with a space group of R¯3m, and x-values ranging from 0.05 to 0.25, were found in cubic Co2Pr (Fd¯3m) after adding C. The rhombohedral crystals were heavily twinned on the {110} and {211} planes. The twinning mechanism was studied by analyzing the reduction in crystal symmetry that resulted from the cubic-rhombohedral phase transformation. The origin of the twinning, and the formation of 4 twin variants, were attributed to the insertion of C interstitials into Co4 tetrahedra along the ¯3 axis in the rhombohedral lattice. This corresponded to one of the 4 equivalent <111> axes of the parent cubic lattice.

L.Wu, Y.Zhu: Philosophical Magazine A, 1997, 76[2], 481-92