double diffraction

When two crystals overlap, which have slightly different lattice parameters to each other, a wave diffracted by the upper crystal is further diffracted by the lower crystal. This phenomenon is called "double diffraction." In addition to the diffraction spots from the upper and lower crystals, spurious spots due to double diffraction appear. Thus, care must be taken to identify a specimen substance. When the upper crystal rotates slightly with respect to the lower crystal (or vice versa) or when the two crystals form twins, many spurious spots also appear due to double diffraction.