| Popis: |
The crystal structure of Na2Mn3(SO4)4 [unit-cell parameters a = 14.8307 (18), b = 9.9107 (18), c = 8.6845 (12) Å, space group Cmc21] displays order–disorder (OD) character and can be described using the OD groupoid family, more precisely a family of OD structures built up by two types of non-polar layers, with layer symmetry P(m)c21 (L 2n+1 type) and P(b)cm (L 2n type) (category IV). A new hypothetical MDO2 polytype has been proposed and the geometry optimization demonstrates its reasonability as another possible stable polytype. Compounds Na2Mn3–x Mg x (SO4)4 with the unit-cell parameters a ∼ 29.2–29.7 Å, b ∼ 9.5–9.9 Å, c ∼ 8.7 Å and space group Pbca can be described in terms of modularity as a sequence of A, S 1 and S 2 modules:…|AS 1 AS 2 AS 1 AS 2|… or (AS 1 AS 2), together with MDO1 (AS 1 AS 1) and MDO2 (AS 2 AS 2). The crystal structures of itelmenite, NaCaFe3+ 3(PO4)4, and Ca2MgFe3+ 2(PO4)4 are crystal-chemical isotypic to Na2Mn3–x Mg x (SO4)4 and should be considered as (A*S 1 A*S 2) derivatives of the (AS 1 AS 2)-type structure. |
