The ionic distributions in Mn
2+ and Zn
2+ β″-alumina (idealized formula:
at 296 K are reported from single-crystal X-ray diffraction studies. Mn
2+ β″-alumina exhibits the shortest
c-axis found so far in any divalent β″-alumina: 33.141(3) Å;Zn
2+ β″-alumina, which involves the smallest divalent ion studied in the frame-work, has a considerably longer
c-axis: 33.517(3) Å. Both compounds show clear evidence of short-range correlation effects in the X
2+ ion arrangement by way of departures from the centrosymmetric space-group
of the β″-skeleton. Both 6
c end-sites and 9
d mO-sites (
notation) are occupied for both ion-types, a significantly larger occupation (60% compared to 28% of the total) lying at or near the 9
d mid-oxygen sites in Mn
2+ β″-alumina compared to Zn
2+ β″-alumina. Disorder is also found in the column-oxygen O(5) in both cases; the O(5) displacement from the 3
b site in Mn
2+ β″-alumina (0.59 Å) is the largest found in any divalent β″-alumina. The formula-unit/cell-layer, deduced on the basis of the amounts of X
2+ ions refined, and assuming charge compensation through Mg substitution alone, are: Mn
0.79Mg
0.57Al
10.43O
17 and Zn
0.87Mg
0.74Al
10.26O
17.
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