Frank—Kasper tetrahedron

FRANK-KASPER polyhedra in MgCu2. The polyhedron around an Mg atom (c.n. 16) is composed of 12 Cu atoms and of four Mg atoms that form a tetrahedron by themselves the Cu atoms form four triangles that are opposed to the Mg atoms. The polyhedron around a Cu atom (c.n. 12) is an icosahedron in which two opposite faces are occupied by Cu atoms... 

We call the Frank-Kasper ( 5, >, 3)-map any ( 5, b, 3)-map that is bRo (in chemistry and crystallography Frank-Kasper polyhedra are just four polyhedra, dual to all four ( 5,6, 3)-polyhedra that are 6Rq). Note that the only oriented ( a, b], 3)-maps with a < 4 that is bRo are Prismb, Cube, Tetrahedron, and Bundle3. 

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