Crystal structure of the new magnetoplumbite-related compound in the system SrO-Al2O3-MgO

A new phase in the system SrO-Al2O3-MgO, which was analyzed by EPMA as Sr2MgAl22O36, was recently found. It is located in the middle of the join connecting strontium hexaaluminate (SrA(12)O(19)) of magnetoplumbite structure and strontium magnesium hexaaluminate (SrMgAl10O17) of beta-alumina structure. The crystals were grown by the floating zone method and its crystal structure was analyzed by using the single crystal X-ray was analyzed by using the single crystal X-ray diffraction method. The space group was <P(6)over bar m2> with lattice parameters a = 5.583 Angstrom and c = 22.225 Angstrom. The observed c-axis length is intermediate between those of SrAl12O19 (C = 22.00;Angstrom) and SrMgAl10O17 (c = 22.399 Angstrom). The structure refinement result has shown that it had the alternate stacking of the conduction layers of magnetoplumbite-type (SrAlO3) and beta-alumina-type (SrO) separated by a spinel block ((Al, Mg)(11)O-16). This is the first time that such a ''mixed-layer'' structure of beta-alumina and magnetoplumbite is found to exist in the hexaaluminates containing one species of large cation. (C) 1996 Academic Press, Inc.