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Figure 3
Examples of the graphics output from ISODISTORT (Stokes et al., 2017BB62). (a) An incommensurate modulation with k vector (0.143,0.143,0) applied to a parent structure in [Pm{\bar 3}m] (e.g. NiTi, Ni red, Ti blue). The basic space group for the distorted structure is Ammm, and the figure shows, as well as the parent cell, the cell corresponding to this basic (average) symmetry. Note that the basic symmetry is orthorhombic. The modulation vector is along the z axis of the Ammm cell, and the period is 1/0.143, i.e. approximately seven (110) planes. (b) and (c) show results obtained from applying a commensurate modulation, k vector (1/7,1/7,0). It can be seen that, though the displacements have a period of seven (110) planes, the atomic arrangement precludes the construction of a simple unit cell with this period. The unit cell in (b) is obtained in orthorhombic symmetry, Amm2, by extending the cell to 14 (110) planes, and the unit cell in (c) by resorting to the monoclinic symmetry P2/m. The symmetries in (b) and (c), and especially the monoclinic symmetry in (c), may be artefacts arising from commensurate choices for the modulation vector k.

ISSN: 2052-5206
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