So far we have considered Coulombic charge and fault­ing for 1/3 [0001] (0001) loops in alumina and 1/6 (111) {111} loops in spinel. Now, we must repeat these considerations for 1/3(1011){1010} prismatic loops in alumina and 1/4 (110) {110} loops in spinel. We begin with alumina prismatic loops. Alumina {1010} prism planes contain both Al and O in the ratio 2:3, that is, identical to the Al2O3 compound stoichiometry. Along the (1010) direction normal to the traces of the {1010} planes, the registry of the {1010} planes varies between adjacent planes, analo­gous to the registry shifts that occur between adjacent (0001) basal planes in alumina (discussed earlier). However, the patterns of Al atoms in all {1010} planes are identical. Similarly, the O atom patterns are identi­cal in all {1010} planes. The registry of the O atom patterns between adjacent {1010} planes alternates every other layer, analogous to the BCBC… stacking of oxygen basal planes (Table 1, eqns [2-4]). On the other hand, the registry of the Al cation patterns is distinct from the O pattern registries (B and C), and the registry ofthe Al patterns only repeats every fourth layer. In other words, the stacking sequence of {1010} plane Al atom patterns can be described using the same nomenclature as in Table 1 and eqn [2] for (0001) alumina planes, that is, ax a2 a3 ax a2 a3…. Putting the anion and cations together, we can write the {1010} stacking sequence in alumina as (a1B) (a2C) (a3B) (axC) (a2B) (a3C).

Now, as with the basal plane story described earlier, when an extra 1/3(1010) two-layer block, (Al2O3)x-(Al2O3)x, is inserted into the stacking sequence, (ajB) (a2C) (a3B) (axC) (a2B) (a3C), a stack­ing fault occurs as follows:

(a1B) (a2C) (a3B) (a1 C) (a2B) (a3C) (a1B)

(a2C) (a3 B) (a1 C) (a2B) (a3 C) (before)

(a1B) (a2C) (a3B) (a1 C) (a2B) (a3C) (a1B) (a2C)

(a1B) (a2 C) (a3B) (a1 C) (a2 B) (a3C) (after)

(a1B) (a2C) (a3B) (a1 C) (a2B) (a3C) (a1B) (a2C) | (a1B) (a2 C) (a3B) (a1 C) (a2 B) (a3C)

(after, showing stacking fault position) [5]

Notice in eqn [5] that after block insertion, the anion sublattice is not faulted (BCBC… layer stacking is preserved), whereas the cation sublattice is faulted, specifically at the position of the red vertical line in the last sequence. Similar to the case of basal plane interstitial loop formation in alumina (discussed ear­lier), the dislocation loop formed by 1/3(1010) block insertion in alumina is an intrinsic, cation-faulted, sessile interstitial Frank loop.