Figure 1
The schematic diffraction pattern of a hexagonal crystal in the P6 space group in two possible orientations within its hexagonal lattice of P622 (strictly P6/mmm) symmetry, related by the twofold rotation around the a* axis. Since this twofold rotation does not belong to the crystal symmetry, reflections with the same indices have different intensities in the two orientations (top). In the perfect twin, both orientations superimpose, and the diffraction pattern acquires P622 symmetry (bottom). |