Internal Strains and Raman‐Active Optical Phonons

The connection between internal strains and Raman‐active optical phonons in crystals is discussed. In crystals with more than one atom per unit cell, there is usually a number of atomic position parameters which cannot be uniquely defined by crystal symmetry arguments. Within the present scheme, it is shown that those parameters correspond to the number of fully symmetric (Γ 1 , A 1 ) optical phonons at q = 0. Besides lowering the symmetry of the crystal, an external stress can introduce new atomic position parameters (internal strains). The criteria are given for determining the type of internal strain which may be produced by an external stress. The internal strain is looked upon as a “frozen” Raman‐active phonon, both belonging to the same irreducible representation of the crystal point group. It is shown that the well‐known matrices of the Raman tensor can be used directly to determine the selection rules for the stress‐induced internal strains in all crystals, regardless of symmetry. Related unit cell crystal parameters can be determined with X‐ray diffraction techniques.