the difference of their ionic valences causes defects in the structure or sometimes modification of the structure (7).
A new phase in the system SrO-Al 2 O 3 -MgO, which was analyzed by EPMA as Sr 2 MgAl 22 O 36 , was recently found. It is The possibility to incorporate Mg in these structures conlocated in the middle of the join connecting strontium hexataining Ca-, Sr-, or Ba-oxides is twofold: aluminate (SrAl 12 O 19 ) of magnetoplumbite structure and strontium magnesium hexaaluminate (SrMgAl 10 O 17 ) of -alumina (i) Mg replaces Al in the Al-spinel block, inducing a structure. The crystals were grown by the floating zone method valence deficiency. A charge compensation with a posiand its crystal structure was analyzed by using the single crystal tively charged conduction layer is required.
X-ray was analyzed by using the single crystal X-ray diffraction
(ii) A spinel unit (Mg 2 Al 4 O 8 ) will be inserted in the Almethod. The space group was P 6m2 with lattice parameters spinel blocks. a ؍ 5.583 A ˚and c ؍ 22.225 A ˚. The observed c-axis length is intermediate between those of SrAl 12 O 19 (c ؍ 22.00 A ˚) and For ͱ-alumina structures (i) is preferred and (ii) is possi-SrMgAl 10 O 17 (c ؍ 22.399 A ˚). The structure refinement result ble; on the other hand, for magnetoplumbite structures has shown that it had the alternate stacking of the conduction only (ii) is possible as shown, for instance, in the Al-rich layers of magnetoplumbite-type (SrAlO 3 ) and -alumina-type part of the system CaO-Al 2 O 3 -MgO (8) for the (SrO) separated by a spinel block ((Al, Mg) 11 O 16 ). This is the compounds CAM-I (Ca 2 Mg 2 Al 28 O 46 ) and CAM-II first time that such a ''mixed-layer'' structure of -alumina (CaMg 2 Al 16 O 27 ) (9). These phases are composed of two and magnetoplumbite is found to exist in the hexaaluminates types of structure units, M (CaAl 12 O 19 ; magnetoplumbite containing one species of large cation.