Next, we consider 1/4 (110) {110} loops in spinel. Spinel {110} planes alternate in composition, (AlO2)~-(MgAlO2)+…, such that each layer is a mixed cation/anion layer. To insert a charge-neutral interstitial slab along (110) in spinel requires that we insert a {110} double-layer block, (AlO2)~-(MgAlO2)+, that is, a stoichiometric MgAl2O4 unit. The thickness of this slab is a/4 (110), where a is the spinel cubic lattice parameter. Along the (110) direction normal to the traces of the {110} planes, the registry of the {110} planes varies between adjacent planes, analogous to the registry shifts that occur between adjacent {111} planes in spinel (discussed earlier). The O atom patterns are identical in all {110} planes, but the registry of the O atom patterns between adjacent {110} planes alternates every other layer, analogous to the BCBC… stacking described earlier. The Mg atom patterns are identical in each (MgAlO2)+ layer, while the registry of the Mg atom patterns alternates every other (MgAlO2)+ layer. We denote the Mg stacking sequence by aj a2 aj a2 …. There are two Al atom patterns along (110): (1) the first occurs in each (AlO2)_ layer with no change in registry between layers (we denote this Al pattern by p0); and (2) the second occurs in each (MgAlO2)+ layer, and the registry of these Al atom patterns alternates every other (MgAlO2)+ layer (we denote this Al stacking sequence by p1 p2 p1 p2 .. .). Combining all these considerations, we can write the {110} planar stacking sequence in spinel as fol­lows: (p0B) (a1p1C) (p0B) (a2p2C).

Now, as with the spinel {111} case described earlier, when an extra 1/4 (110) two-layer block, (AlO2)~-(MgAlO2)+, is inserted into the spinel {110} stacking sequence, (p0 B) (a1p1C) (p0B) (a2p 2C), astack — ing fault occurs as follows:

(p0B) («1p1 C) (p0B) («2p2c) (p0B) (a^C)

(p0B) (a2p2C) (before)

(p0B) («1p1 C) (p0B) («2p2C) (p0B) («1p1C)

(p0B) («1p1 C) (p0B) (a2p2 C) (after)

(p0B) («1p1 C) (p0B) («2p2C) (p0B) («1p1C) I (p0B) 1 (a1p1C) (p0B) (a2p2C) (after, showing stacking fault positions) [6]

Notice in eqn [6] that after block insertion, the anion sublattice is not faulted (BCBC… layer stacking is preserved), whereas the cation sublattice is faulted, specifically at the positions of the red vertical lines in the last sequence (the left-hand red line corresponds to the cation fault position for cation planar registries moving from right to left; likewise, the right-hand red line corresponds to the cation fault position for cation planar registries moving from left to right). Thus, the dislocation loop formed by 1/4 (110) two — layer block insertion in spinel is an extrinsic, cation — faulted, sessile interstitial Frank loop.

Figure 3 shows an example of 1/4 (110) interstitial dislocation loops in spinel, produced by neutron irra — diation.12 The alternating black-white fringe contrast within the loops is an indication of the presence of a stacking fault within the perimeter of each loop. The character of the {110} loops was determined by Hobbs and Clinard using the TEM imaging methods of Groves and Kelly,14,15 with attention to the pre­cautions outlined by Maher and Eyre.16 These loops were determined to be extrinsic, faulted 1/4 (110) {110} interstitial dislocation loops. It is evident in Figure 3 that the extrinsic fault associated with these loops is not removed by internal shear, even when the loops grow to significant sizes (> 1 pm diameter). This is the subject of our next topic of discussion, namely, the unfaulting of faulted Frank loops.