Superstructure in SmCo7 phase.

We have performed a systematic investigation on the microstructural features of SmCo(7-x)Cu(x) (x = 0, 3.5) alloys. Transmission electron microscopy observations suggest that the SmCo7 is essentially a superstructure phase with a modulation wave vector q = (a* + b*)/3 + c*/2. The superstructure can be well interpreted by the partial ordered substitution of Co-pairs for Sm atoms within the basic structure of SmCo5. In situ cooling and heating observations indicated that the superstructure phase is stable below 670 K. Additional substitution of Cu for Co in SmCo3.5Cu3.5 does not result in evident changes of crystal structure, but makes the superstructure phase unstable at temperatures >480 K.