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Figure 8
Comparison of the distribution of vacancies (red hexagons). (a) Vacancy distribution within one layer derived from the simplest model in superspace group Pbam(a0½)0ss with q = (0.3 0 0.5) and a vacancy concentration of 0.4 at coordinates with site symmetry 2/m, which is either vacant (red hexagon) or not vacant. This distribution can be derived without any knowledge of the average structure or chemistry of mullite. Average unit-cell borders (black) and octahedra (grey) are included for visual orientation. (b) Vacancy distribution with the structural model of the ordered superspace model. The subsequent layer, which is equal to the first layer shifted by 5a, is also shown with reduced opacity to indicate the relative location of vacancies in layers that are separated by 1c. (c) The hexagon size represents the probability that this site is vacant in the disordered SSM. In the case of SA1, this probability varies between 11.3% (smaller hexagons) and 31.5% (larger hexagons).

IUCrJ
Volume 5| Part 4| July 2018| Pages 497-509
ISSN: 2052-2525
https://doi.org/10.1107/S2052252518007467