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. 1.11, pp. 276-277
Section 1.11.6.2. Tensor atomic factors (non-magnetic case)aA. V. Shubnikov Institute of Crystallography, Leninsky pr. 59, Moscow 119333, Russia,bSteinmann Institut der Universität Bonn, Poppelsdorfer Schloss, Bonn, D-53115, Germany, and cFaculty of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia |
In time-reversal invariant systems, equation (1.11.6.3) can be rewritten as
where
corresponds to the symmetric part of the dipole–dipole contribution,
and
mean the symmetric and antisymmetric parts of the third-rank tensor describing the dipole–quadrupole term, and
denotes a symmetric quadrupole–quadrupole contribution. From the physical point of view, it is useful to separate the dipole–quadrupole term into
and
, because in conventional optics, where
, only
is relevant.
The tensors contributing to the atomic factor in (1.11.6.16),
,
,
,
, are of different ranks and must obey the site symmetry of the atomic position. Generally, the tensors can be different, even for crystallographically equivalent positions, but all tensors of the same rank can be related to one of them, because all are connected through the symmetry operations of the crystal space group. In contrast, the scattering amplitude tensor
does not necessarily comply with the point symmetry of the atomic position, because this symmetry is usually violated considering the arbitrary directions of the radiation wavevectors
and
.
Equation (1.11.6.16) is also frequently considered as a phenomenological expression of the tensor atomic factor where each tensor possesses internal symmetry (with respect to index permutations) and external symmetry (with respect to the atomic environment of the resonant atom). For instance, the tensor
is symmetric, the rank-3 tensor has a symmetric and a antisymmetric part, and the rank-4 tensor is symmetric with respect to the permutation of each pair of indices. The external symmetry of
coincides with the symmetry of the dielectric susceptibility tensor (Chapter 1.6
). Correspondingly, the third-rank tensors
and
are similar to the gyration susceptibility and electro-optic tensors (Chapter 1.6
), and
has the same tensor form as that for elastic constants (Chapter 1.3
). The symmetry restrictions on these tensors (determining the number of independent elements and relationships between tensor elements) are very important and widely used in practical work on resonant X-ray scattering. Since they can be found in Chapters 1.3
and 1.6
or in textbooks (Sirotin & Shaskolskaya, 1982
; Nye, 1985
), we do not discuss all possible symmetry cases in the following, but consider in the next section one specific example for X-ray scattering when the symmetries of the tensors given by expression (1.11.6.3)
do not coincide with the most general external symmetry that is dictated by the atomic environment.
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