INDEX cation stacking

The unit cell of as derived from the indexing of the powder pattern, is a 9-fold repeat of the basic half-unit-cell of the A-type structure, and has been described in terms of the cation layer sequence —ABABCBCAC— the complete layer sequence can be read from fig. 28.53. An alternative description is that of an ordered sequence of cation stacking faults along [001]a (the anion layer sequence remains unaltered), and it is tempting to propose a sequence of fully-ordered cation layers (as Sibieude (1973) did) in which the ratio Th La is 1 2. However this leads to an incorrect composition (66.7 mole % LaOi j instead of the observed 75 mole % LaOi.j), so the implication is that the... 

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