Framework structures derived from a novel type of trigonal secondary building unit with on-axis T atoms: the tridymite group

An infinite series of secondary building units generated around a trigonal axis, with the top and/or bottom T atom coinciding with the symmetry axis allows the systematic enumeration of a novel class of (4; 2)-connected 3D framework structures. Known members of this group are the framework topologies of tridymite, cristobalite, MAPSO-46 (AFS), CoAPO-50 (AFY) and beryllophosphate-H (BPH). Adjacent T atoms on a trigonal axis with a T-O-T bond angle of 180° (T₂ unit) present in all structures confer some specific properties on this class of compounds.