Polymorphism, polytypism and modular aspect of compounds with the general formula A2M3(TO4)4 (A = Na, Rb, Cs, Ca; M = Mg, Mn, Fe3+, Cu2+; T = S6+, P5+): order-disorder, topological description and DFT calculations.

Abstract

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 (L2n+1 type) and P(b)cm (L2n 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-xMgx(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, S1 and S2 modules:…|AS1AS2AS1AS2|… or (AS1AS2), together with MDO1 (AS1AS1) and MDO2 (AS2AS2). The crystal structures of itelmenite, NaCaFe3+3(PO4)4, and Ca2MgFe3+2(PO4)4 are crystal-chemical isotypic to Na2Mn3-xMgx(SO4)4 and should be considered as (A*S1A*S2) derivatives of the (AS1AS2)-type structure.