Tables for
Volume D
Physical properties of crystals
Edited by A. Authier

International Tables for Crystallography (2013). Vol. D, ch. 2.3, p. 342

Section [\bf q]-dependent terms

I. Gregoraa*

aInstitute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-18221 Prague 8, Czech Republic
Correspondence e-mail: [\bf q]-dependent terms

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So far, we have not explicitly considered the dependence of the Raman tensor on the magnitude of the scattering wavevector, assuming [{\bf q}\to 0] (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 [{\bf q}=0]. 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 [\Gamma(j)]. General properties of the q-dependent terms can be advantageously discussed in connection with the so-called morphic effects (see Sections 2.3.4[link] and 2.3.5[link]).

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