The insertion of two sets of four-fold (4i) and (4j) occupations in the same space group leads to the C 9 stoichiometry. The geometry optimization down to the ground state energies through successive calculation cycles with increasing precision of the tetragonal Brillouin zone led to the lattice parameters and energies given in Table , 2 nd column. In Fig. , the structure of C 9 in both the ball-and-stick and tetrahedral representations shows similar, though larger, tetragonal C 4 with tetrahedra at corner site (shown as brown and white spheres) at the 8 corners connected by C2(4j) (white spheres). C3(4i) at z = ½ forming the central square (green spheres) are connected to C2 to form two edge-sharing tetrahedra (Table ). As the original occurrence, the structure then consists of both corner and edge sharing tetrahedra. Note that the tetrahedra are significantly distorted, as can be seen from the angles.