International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2010 
International Tables for Crystallography (2010). Vol. B, ch. 2.5, p. 349
Figure 2.5.3.19
M. Tanaka^{f}


Figure 2.5.3.19
CBED patterns of the incommensurate phase of Sr_{2}Nb_{2}O_{7} taken at 60 kV. (a) [010] incidence: fundamental reflections show a mirror symmetry perpendicular to the a* axis but incommensurate reflections do not [symmetry ()]. (b) [201] incidence: incommensurate reflections show mirror symmetry perpendicular to the b* axis [symmetry ()]. The wave number vector of the modulation is k = (½ − δ)a*. 