The abc packing is identical to FCC packing and we have already seen FCC structures in the previous lecture. You should actually use ping pong balls to construct abcabc layer structures and convince yourself that this is FCC. No amount of explanation or visualization on the screen is equal or equivalent to a hands on experience.
Before we consider the calculation of packing fractions in these structures, let us consider what kind of voids or holes can be created in the above packing process. On the first layer of (three) spheres, when a single sphere is placed, it creates a tetrahedral hole near the location of the base of this (fourth) sphere. The center of the tetrahedral hole is midway between the two layers a and b and the tetrahedral hole is equidistant from these four spheres.
