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Figure 8
The ideal tridymite parent structure with (a) the P63/mmc setting and in a closely related (c), (e) orthorhombic Cmcm setting, along with three q = 0 RUMs of this ideal parent tridymite structure type involving coupled rotations around the parent hexagonal (b) ch = c0, (d) ah − bh = a0 and (f) ah + bh = b0 directions. Only two tetrahedral layer-thick projections are shown in each case, for clarity. Note that rotation around a0 and b0 automatically lowers the parent hexagonal symmetry to orthorhombic (hence the different setting used). In the case of (b), again only two (001)h layers are shown. In this case, however, there need be no correlation from one such two-layer slab to the next, which thus gives rise to a line of RUMs along c. For q = 0, if the other two (001)h layers rotate in phase the resultant space group symmetry is [P{\overline 6}2c]. Alternatively, if they rotate out of phase a P6322 space group results.

IUCrJ
Volume 2| Part 1| January 2015| Pages 74-84
ISSN: 2052-2525
doi:10.1107/S2052252514022556