Hexagonal close-packed structure anion stacking

MgO has the NaCl crystal structure 23,24 each magnesium ion is coordinated by six O2 ions and each 02 by six Mg2+. The structure of a-Al203 consists of close-packed sheets of 02 ions stacked in the sequence A-B-A-B, forming a hexagonal close-packed array of anions.25 The cations are located in two-thirds of the octahedral sites. The structure of a-Al203 results in coordination numbers of 6 and 4 for the cation and the anion, respectively.25... 

When the temperature is below 193 K, the orthorhomic form further transforms to the hexagonal system in space group D h - P63/mmc. The O2- and Ba2+ ions together form close packed layers and stack in the sequenceABCACB and the Ti4+ ions occupy the octahedral interstices composed only of O2-anions. In this layer stacked structure, the A layers have horizontal mirror symmetry as shown in Fig. 10.4.2(d). Two-thirds of the TiC>6 octahedra share faces to form the Ti2C>9 group, in which the Ti-Ti distance is 267 pm, as represented by a line. This centrosymmetrical crystal is non-ferroelectric. 

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