Stacking fault energy defined

This should be the case for the A site and C site because they have the same energy level with respect to gravity, so RHCP would be favored in colloidal sedimentation. In order to quantitatively define the amount of the FCC phase versus the HCP phase, an overall stacking parameter, a, is used. This stacking parameter is defined such that a = 0 for the HCP, a = 1 for the FCC phase, and a RHCP structure would have an a = 0.5. This has been used to describe colloidal crystal structure and stacking fault density. ... 

---