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Figure 3
Vacancy positions on the f.c.c. lattice as seen from above. (a) The possible vacancy positions. Gray circles mark sites with integer z coordinates and gray triangles mark sites with half-integer z coordinates. The a and b axis of the unit cell are along the diagonal as shown by the red square. (b) One possible ordering of vacancies for x = 1/4. In this ordering there are no vacancies with half-integer z coordinates. This ordering has no nearest-neighbor vacancies, and each vacancy has four next-nearest neighbor vacancies. (c) The vacancy ordering with x = 1/5 suggested by Zeier et al. (2017BB42) giving a tetragonal structure. Note that every second layer is identical. This ordering has no nearest-neighbor vacancies, but every vacancy has two next-nearest neighbor vacancies. (d) One of several possible vacancy orderings for x = 1/6 where there are no nearest and no next-nearest neighbor vacancies. Other possible orderings are shown in the Supporting information.

Volume 7| Part 4| July 2020| Pages 673-680
ISSN: 2052-2525