The Al-Rich Part of the System CaO-Al2O3-MgO: PART II. 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 stoìchìometrìc 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 (M₂S)_n for CAM-I (c = 79.810 Å) and (MS)_n for CAM-II (c = 31.288 Å). In the terminology of hexagonal ferrite structure system, CAM-I is of the "X-type structure" with space group R m and CAM-II is of the "W-type structure" with P m 2 symmetry.