view article
Figure 17
OD theory. (a) The symmetry elements of an isolated layer (first layer) are shown in black. The symmetry elements used to generate the second layer are shown in blue. The basis vectors of an isolated layer are a0, b0 and c0. The view is along c0. The figure displays the situation for a shift of c/4; in the case of a shift of −c/4, the z positions of all symmetry elements change from z to −z, the 21/2 axis changes to 2−1/2 with a translation of −c0/4, and the n1/2,2 glide plane converts to n−1/2,2 with a translation of a0 − c0/4. (b) Consecutive application of the shift of −c/4 leads to the ordered model structure MDO1 in P12/a1, which is the main structural motif of denisovite. All depicted symmetry elements (shown in red) are crystallographic ones. All other symmetry elements from panel (a) are still present, but they are only local symmetries within one layer (black) or between neighbouring layers (blue). The view is along [001]. Note that the a axis in P12/a1 is not parallel to a0, but a = 2a0 − c0/2. Thereby the n−1/2,2 glide plane changes to a proper a-glide plane. The subscript p in ap denotes the projection of the a axis.

IUCrJ
Volume 4| Part 3| May 2017| Pages 223-242
ISSN: 2052-2525
https://doi.org/10.1107/S2052252517002585