Voids in The Unit Cell
A simple cubic unit cell has a single cubic void in the center.
A body-centered cubic unit cell has six octahedral voids located at the center of each face of the unit cell, for a total of three net octahedral voids. Additionally, there are 36 tetrahedral voids located in an octahedral spacing around each octahedral void, for a total of eighteen net tetrahedral voids. These tetrahedral voids are not local maxima and are not technically voids, but they do occasionally appear in multi-atom unit cells.
A face-centered cubic unit cell has eight tetrahedral voids located slightly towards the center from each corner of the unit cell, for a total of eight net tetrahedral voids. Additionally, there are twelve octahedral voids located at the center of edge of the unit cell as well as one octahedral hole in the very center, for a total of four net octahedral voids.
Read more about this topic: Cubic Crystal System
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