1.05.2.3.1 Introduction to atomic layer stacking

Results of numerous neutron and electron irradiation damage studies suggest that two types of interstitial dislocation loops nucleate in a-Al2O3: (1) 1/3 [0001] (0001); and (2) 1/3(10І1){10І0} (see, e. g., the review by Kinoshita and Zinkle4). The first of these involves precipitation on basal planes in the hexagonal a-Al2O3 structure, while the second is due to precip­itation on m-type prism planes. In MgAl2O4, similar studies indicate that primitive interstitial dislocation loops also have two characters: (1) 1/6 (111) {111} and (2) 1/4 (110) {110}.4 Though the crystal struc­ture of spinel is cubic, compared with that of alumina, which is hexagonal, the nature ofthe dislocation loops formed in spinel is similar to those in alumina: {111} spinel loops are analogous to (0001) alumina loops; likewise, {110} spinel loops are analogous to {1010} alumina loops. We will first compare and contrast {111} spinel versus (0001) alumina loops and later discuss {110} spinel versus {1010} alumina loops.

Both spinel (111) {111} and alumina [0001] (0001) interstitial dislocation loops involve insertion of extra atomic layers perpendicular to the (111) and [0001] directions, respectively. These layers are either pure cation or pure anion layers. In both spinel and alumina, anion layers along (111) and [0001] directions, respectively (i. e., along the 3 direction in both structures), are close packed (specifically, they are fully dense, triangular atom nets), while the cation layers contain ‘vacancies,’ which are necessary to accommodate the cation deficiency (compared with anion concentration) in both compounds (these ‘vacancies’ actually are interstices; they are ‘holes’ in the otherwise fully dense triangular atom nets that make up each cation layer). Table 1 shows the arrangement of cation and anion layers in spinel and alumina, along (111) and [0001] directions, respectively.11 Both structures can be described by a 24-layer stacking sequence along these directions. Both spinel and alumina can be thought of as con­sisting ofpseudo-close-packed anion sublattices, with cation layers interleaved between the anion layers. The anion sublattice in spinel is cubic close-packed (ccp) with an ABCABC… layer stacking arrangement, while alumina’s anion sublattice is hexagonal close — packed (hcp) with BCBCBC. .. layer stacking. In both structures, between each pair of anion layers there are three layers of interstices where cations may reside: a tetrahedral (t) interstice layer, followed by an octahedral (o) interstice layer, followed by another t layer. In spinel, Mg cations reside on t layers, while Al cations occupy the o layers. In alumina, all t layers are empty and Al occupies 2/3 of the o layer inter­stices. In spinel, cation interlayers alternate between a pure Al kagome layer and a mixed MgAlMg, three — layer thick slab. In alumina, each interlayer is pure Al in a honeycomb arrangement.