Explanation

Coupling vibration between atoms in crystal. Generally speaking, a normal vibration does not mean the vibration of a certain atom, but the vibration in which all atoms of the whole crystal participate, and their vibration frequencies are the same. The common vibration of all atoms in the system represented by normal coordinates is often called a crystal. Because the lattice is periodic, the atomic vibration is not isolated, but propagates in the crystal in the form of waves, so the lattice has the form of waves, called lattice waves. A lattice wave represents a vibrational mode in which all atoms in a crystal participate. Gebo can be divided into two types: acoustic wave and optical wave, with one or two modes. They each have corresponding frequency ranges, with different excitation frequencies describing different atomic vibration states. The quantum of lattice wave energy is called phonon. Accordingly, phonon can be divided into acoustic wave phonon and optical wave phonon. Acoustic wave phonon, adjacent atoms are vibrating in the same direction. For long acoustic waves, the displacement of adjacent atoms is the same, and different atoms in the original cell move as a whole with the same amplitude and phase. Therefore, long acoustic waves represent the motion of the centroid of the cell and reflect the overall vibration of the lattice. For optical wave phonon, the vibration direction of adjacent atoms is opposite. Long optical waves, the centroid of the original cell remains stationary. Long optical waves represent the relative vibrations of two atoms in the primary cell. Acoustic waves describe the relative motion between different primitive cells. Optical waves describe the relative motion between atoms within the same cell.

Classification