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Five basic compounds, characterized by a cluster with the composition, [Fe 3  3+ O(SO 4 ) 6 (H 2 O) 3 ] 5−  are reconsidered. A crystal chemical survey allows to explain these structures on the basis of some simple architectural principles and to assert that the structures built on chains or chains and layers of [MFe 3  3+ O(SO 4 ) 6  · (H 2 O) 3 ] 4−  composition (M stands for Na or K) cannot form polytypes. On the contrary those based on sandwich sheets with [Na 2 K 2 Fe 6  3+ O 2 (SO 4 ) 12 (H 2 O) 6 ] 6−  composition can originate them. In the latter case three experimental and one theoretical standard polytypes constitute the bases from which more complex stacking sequences, not yet synthesized, can be derived. The influence of the ionic substitution of K for Na on the structural topology of related compounds is discussed. Excluding the Na-salt, Na can replace K up to 40% in all the compounds analyzed here. However in the salts built up on double sheets a larger amount of Na (20% more) enters the structure, since it is required in the construction of the sandwich sheets.

zeitschrift für kristallographie - crystalline materials

A survey of (Na, H 3 O + , K) 5 Fe 3 O(SO 4 ) 6  · nH 2 O compounds: architectural principles and influence of the Na–K replacement on their structures