Chemical fourling on the unit cell level as a structure-building operation in the solid state

Chemical fourling on the unit-cell level is introduced as a structure-building operation in the solid state. A chemical fourling structure is achieved when two twin planes are perpendicular to each other. Each fourling unit has infinite extension in only one direction and can be related to a well-known structure type or packing of atoms. The structures of the tetragonal tungsten bronzes M_xWO₃ and Pd(NH₃)₄Cl · H₂O are presented as examples of chemical fourlings of cubic close packing. The structures of SeO₂, Mn₂Hg₅, and Zr₂F₇O are shown to be a sequence of chemical fourlings of primitive cubic packing. Chemical fourling units of hexagonal close packing are found in the structures of tetragonal Ti₃Sb, α-V₃S and Sb₆O₇(SO₄)₂. The structures of CuAl₂, SeTl, NbTe₄, Ti₃Sb, and MnU₆ all have the same chemical fourling unit. Cr₂₃C₆ is described as a bounded chemical fourling of cubic close packing.