Sentence:  If we allow the coordination figure to be a <WSD>quasiregular</WSD> polyhedron (a polyhedron with one kind of vertex and edge, but two kinds of face) there is just one possibility compatible with translational symmetry – a cuboctahedron. 
Meaning of the word marked with <WSD> tags:  That is semiregular with regular faces of precisely two types that alternate around each vertex.