In the rock-salt or sodium chloride ( halite ) structure, each of the two atom types forms a separate face-centered cubic lattice, with the two lattices interpenetrating so as to form a 3D checkerboard pattern . ( See picture here . ) Alternately, one could view this structure as a face-centered cubic structure with secondary atoms in its octahedral holes.
42.
The atomic packing factor of the diamond cubic structure ( the proportion of space that would be filled by spheres that are centered on the vertices of the structure and are as large as possible without overlapping ) is | 16 } } H " 0.34, significantly smaller ( indicating a less dense structure ) than the packing factors for the face-centered and body-centered cubic lattices.
43.
At room temperature, protactinium crystallizes in body-centered tetragonal structure which can be regarded as distorted body-centered cubic lattice; this structure does not change upon compression up to 53 GPa . The structure changes to face-centered cubic ( " fcc " ) upon cooling from high temperature, at about 1200 �C . The thermal expansion coefficient of the tetragonal phase between room temperature and 700 �C is 9.9 / �C.
