β-alumina-14H and β-alumina-21R: Two chromic Na2-δ(Al,Mg,Cr)17O25 polysomes observed in slags from the production of low-carbon ferrochromium

The crystal structures of unknown phases found in slags from the production of low-carbon ferrochromium were studied by powder and single-crystal X-ray diffraction. Two phases of Na2−δ(Al, Mg, Cr)17O25 composition were found to be composed of an alternating stacking of a spinel-type and a Na-hosting block. Similar structures are known for β-alumina and β”-alumina, NaAl11O17. However, the spinel-type block in Na2-δ(Al, Mg, Cr)17O25 is composed of five cation layers in contrast to three cation layers in the β-alumina spinel-block. The two new phases, β-alumina-14H, P63/mmc, a=5.6467(2), c=31.9111(12) Å, and β-alumina-21R, R  3̅m, a=5.6515(3), c=48.068(3) Å have a 14-layer and 21-layer stacking with a 2 × (cccccch) and a 3 × (ccccccc) repeat sequence of oxygen layers in cubic and hexagonal close packing, respectively.

Two new phases related to the structures of β- and β´´-alumina with a composition of Na2-δ(Al, Mg, Cr)17O25 were found to be composed of an alternating stacking of a spinel-type and a Na-hosting block. In contrast to the structures of hitherto known β-aluminas the spinel-type block in N2-δ(Al, Mg, Cr)17O25 is composed of five cation layers rather than three cation layers.Figure optionsDownload as PowerPoint slide