The Al-rich part of the system CaO-Al2O3-MgO .2. Structure refinement of two new magnetoplumbite-related phases

Crystals of two new phases, which were recently found in the system CaO-Al2O3-MgO lying on the join connecting calcium hexaaluminate (CaAl12O19) of the magnetoplumbite structure and spinel (MgAl2O4), were grown by the floating zone method. The stoichiometric formulas for these two phases can be given as Ca2Mg2Al28O46 (CAM-I) and CaMg2Al16O27 (CAM-II). On the basis of the stacking sequence revealed by high-resolution electron microscopy, the structure models were made, and further, the structure refinement was conducted by using single crystal X-ray diffraction data. It was shown that both phases have magnetoplumbite-related structures composed of two types of structure units, M(CaAl12O19; magnetoplumbite unit) and S(Mg2Al4O8; spinel unit), and that the stacking sequences are (M(2)S)(n) for CAM-I (c = 79.810 Angstrom) and (MS)(n) for CAM-II (c = 31.288 Angstrom). In the terminology of hexagonal ferrite structure system, CAM-I is of the ''X-type structure'' with space group R(3) over barm$ and CAM-II is of the ''W-type structure'' with <P(6)over bar m2> symmetry. (C) 1995 Academic Press, Inc.