Pr-Co permanent magnets have attracted much attention recently due to their excellent magnetic properties and low cost; these properties can be further improved by adding small amount of carbon. In this paper, we describe the mechanism of structural transformation resulting from carbon doping in PrCo2 crystals.

We identified the heavily twinned PrCo2Cx (x=0.05-0.25) as a rhombohedral phase with space group Rm. Fig.1 shows its typical morphology of the (110) and (211) primary twins and (110)(211) secondary twins together with the corresponding diffraction patterns. We note that the Bragg spots split along the [hh0]* direction, and that the splitting increases with distance from the [110]* row. The splitting (or α angle of the rhombohedral lattice) also varies with carbon concentration.

Because PrCo2 is an untwinned FCC phase (Fdm), the formation of a twin in PrCo2Cx caused by adding carbon atoms apparently is driven by a minimization in lattice strain, which also is the origin of the cubic-rhombohedral phase-transformation.