International
Tables for Crystallography Volume D Physical properties of crystals Edited by A. Authier © International Union of Crystallography 2013 |
International Tables for Crystallography (2013). Vol. D, ch. 2.3, p. 342
Section 2.3.3.6. -dependent terms^{a}Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-18221 Prague 8, Czech Republic |
So far, we have not explicitly considered the dependence of the Raman tensor on the magnitude of the scattering wavevector, assuming (the effects of directional dispersion in the case of scattering by polar modes were briefly mentioned in the preceding section). In some cases, however, the Raman tensors vanish in this limit, or q-dependent corrections to the scattering may appear. Formally, we may expand the susceptibility in a Taylor series in q. The coefficients in this expansion are higher-order susceptibility derivatives taken at . The symmetry-restricted form of these tensorial coefficients may be determined in the same way as that of the zero-order term, i.e. by decomposing the reducible representation of the third-, fourth- and higher-order polar Cartesian tensors into irreducible components . General properties of the q-dependent terms can be advantageously discussed in connection with the so-called morphic effects (see Sections 2.3.4 and 2.3.5).