The monoclinic structure has two unit cells. The primative cell (P) has fractional lattice points at the corners of the cell, and the base-centered cell (C) has additional fractional lattice points at ...

This figure will allow you to see the FCC unit cell. First, rotate the lattice straight up until you align the blue, green, and pink atoms at the far left. One of the faces of the unit cell should be ...

However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase9,10,11, has to date remained obscure.