On hexagonal perovskites with cationic vacancies. 22

In the system Srsub(4-x)Basub(x)NiRe2vacantO12 two different stacking polytypes of the rhombohedral 12 L type are present. Both crystallize in the space group R-3m with the same sequence of the close packed AO3 sheets: (3)(1) = (hhcc)3. Accordingly the octahedral net consists of groups of three face connected octahedra which are linked by another through single corner sharing octahedra. In the strontium compound, Sr4NiRe2vacantO12, the occupation of the octahedral holes is the same as in other rhombohedral 12 L perovskites of type A4(B, M)3vacantO12 (The two outer octahedra are occupied by rhenium, the center is vacant and the nickel is located in the single octahedron). In the barium stacking polytype, Ba4NiRe2vacantO12, the distribution is completely altered: The rhenium atoms occupy the central position in the block of three face connected octahedra and the single octahedron, the nickel atom and the vacancy are distributed statistically over the two outer positions in the block of three octahedra. Intensity calculations on powder data gave the intensity related R' value of 7.2% for Sr4NiRe2vacantO12 and 6.2% for Ba4NiRe2vacantO12. (author)