International
Tables for
Crystallography
Volume B
Reciprocal space
Edited by U. Shmueli

International Tables for Crystallography (2010). Vol. B, ch. 2.5, p. 349   | 1 | 2 |

Figure 2.5.3.19 

M. Tanakaf
[Figure 2.5.3.19]
Figure 2.5.3.19

CBED patterns of the incommensurate phase of Sr2Nb2O7 taken at 60 kV. (a) [010] incidence: fundamental reflections show a mirror symmetry perpendicular to the a* axis but incommensurate reflections do not [symmetry ([_{\,1}^m ])]. (b) [201] incidence: incommensurate reflections show mirror symmetry perpendicular to the b* axis [symmetry ([_{\,\bar 1}^m ])]. The wave number vector of the modulation is k = (½ − δ)a*.