The ionic distribution in Mn²⁺ and Zn²⁺ β″-alumina

Abstract

The ionic distributions in Mn²⁺ and Zn²⁺ β″-alumina (idealized formula: $\text{X}{}_{\mathrm{<mtext>5</mtext><mtext>6</mtext>}}\text{Mg}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{Al}{}_{\mathrm{<mtext>31</mtext><mtext>3</mtext>}}\text{O}{}_{17}\text{;}\text{X}\text{=}\text{Mn}\text{and}\text{Zn}\text{)}$ at 296 K are reported from single-crystal X-ray diffraction studies. Mn²⁺ β″-alumina exhibits the shortest c-axis found so far in any divalent β″-alumina: 33.141(3) Å;Zn²⁺ 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 X²⁺ ion arrangement by way of departures from the centrosymmetric space-group $\text{R}\text{3}\text{m}$ of the β″-skeleton. Both 6c end-sites and 9d mO-sites ($\text{R}\text{3}\text{m}$ notation) are occupied for both ion-types, a significantly larger occupation (60% compared to 28% of the total) lying at or near the 9d mid-oxygen sites in Mn²⁺ β″-alumina compared to Zn²⁺ β″-alumina. Disorder is also found in the column-oxygen O(5) in both cases; the O(5) displacement from the 3b site in Mn²⁺ 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 X²⁺ ions refined, and assuming charge compensation through Mg substitution alone, are: Mn_0.79Mg_0.57Al_10.43O₁₇ and Zn_0.87Mg_0.74Al_10.26O₁₇.