Edge-sharing Tetrahedra

Fig. 11.5 Typical bonding types encountered in Ge-Se glasses corner-sharing (CS) tetrahedra, edge-sharing (ES) tetrahedra, and ethane-like (ET) units involving a homopolar Ge-Ge bond...

ONO == 115.4°) the nitrogen atom can be described as a tetrahedron with two corners defining an edge shared with one oxygen atom and a third corner occupied by the other oxygen atom the double bond hence... 

Nature of sites in crystal structure Vastly different, e.g., cube-octahedron octahedron-tetrahedron Similar, e.g., octahedra with slightly different bond lengths or ligands Identical, finite clusters, e.g., edge-shared octahedral dimers Identical, continuous chains, e.g., chains of edge-shared octahedra... 

The most numerous and most important tetrahedral structures are those in which only vertices are shared. The sharing of faces of tetrahedral groups AX4 would result in very close approach of the A atoms (to 0-67 AX or OAl XX, where XX is the length of the tetrahedron edge) and a very small A-X-A angle (38° 56 for regular tetrahedra) and for these reasons need not be considered. 

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