Rigid frameworks of zeolite-like compounds of the pharmacosiderite structure-type

The crystal structure of the mineral pharmacosiderite, K[Fe4(OH)4As3O12]·6 to 7H2O, is the basis for most of the more than five dozen crystal structure determinations of related compounds of various chemical compositions, especially those in which both, the octahedrally coordinated Fe3+, and the tetrahedrally coordinated As, are replaced by germanium atoms. The structure of pharmacosiderite is a metastructure of the ReO3-type, where Re is replaced by the cluster Fe4(OH)4O12, which consists of four edge-sharing coordination octahedra and the O atoms are replaced by AsO4-groups. It can also be thought of as a defect structure of the spinel-type in which one half of the oxygen atoms are left out in an ordered way, so that the Fe4(OH)4O12-clusters correspond to M4O16 groups as they are existing inside of spinel- or NaCl- (or MgO)-type structures (where M is any octahedrally coordinated cation). The aristotype of the pharmacosiderite-type crystallizes in space group P 4¯ 3 m. The symmetry reductions to space groups R 3 m, I 2 3, P 4¯ 2 m and P 4¯ b 2, in which some of the compounds crystallizing in the pharmacosiderite-type have been described, are presented and the standardized descriptions of each of them are given. The pharmacosiderite topology defines a rigid framework which cannot become flexible without severe distortions of its individual polyhedra, either by changing the internal shapes of the octahedral clusters or of the coordination tetrahedra. Such distortions would be energetically not favorable. It is thus very different from noncollapsible frameworks, such as the LTA-type, or from collapsible frameworks, such as the RHO-type, which both are much more flexible since their individual coordination tetrahedra can remain stiff while they tilt around their hinges, that is their connecting bridging vertices.

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► The pharmacosiderite-type framework is an ordered defect spinel.
► The framework of the pharmacosiderite-type is rigid, not flexible.
► Relationships between pharmacosiderite in space group P 4¯ 3 m and its subgroups are presented.
► Standardized descriptions in subgroups R 3 m, I 2 3, P 4¯ 2 m and P 4¯ b 2 are given.