Crystal is a solid with a regular geometric shape formed through a crystallization process. The atoms or molecules in the crystal are periodically and repeatedly arranged in space according to a certain rule. The so-called single crystal means that the particles inside the crystal are arranged regularly and periodically in the three-dimensional space, or the whole crystal is composed of the same spatial lattice in the three-dimensional direction, and the particles in the whole crystal are arranged in a long-range order in space.