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 m, 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.