BC7 is beta beryllia-like structured and crystallizes in the tetragonal P-4m2 space group. B(1) is bonded to four equivalent C(3) atoms to form BC4 tetrahedra that share corners with four equivalent B(1)C4 tetrahedra and corners with eight equivalent C(4)C4 tetrahedra. There are four inequivalent C sites. In the first C site, C(1) is bonded to four equivalent C(2) atoms to form corner-sharing CC4 tetrahedra. In the second C site, C(2) is bonded to two equivalent C(1) and two equivalent C(4) atoms to form CC4 tetrahedra that share corners with four equivalent C(3)B2C2 tetrahedra and corners with eight equivalent C(2)C4 tetrahedra. In the third C site, C(3) is bonded to two equivalent B(1) and two equivalent C(4) atoms to form CB2C2 tetrahedra that share corners with four equivalent C(2)C4 tetrahedra and corners with eight equivalent C(3)B2C2 tetrahedra. In the fourth C site, C(4) is bonded to two equivalent C(2) and two equivalent C(3) atoms to form CC4 tetrahedra that share corners with four equivalent B(1)C4 tetrahedra, corners with four equivalent C(1)C4 tetrahedra, and corners with four equivalent C(4)C4 tetrahedra.
BC7 is beta beryllia-like structured and crystallizes in the tetragonal P-4m2 space group. B(1) is bonded to four equivalent C(3) atoms to form BC4 tetrahedra that share corners with four equivalent B(1)C4 tetrahedra and corners with eight equivalent C(4)C4 tetrahedra. All B(1)-C(3) bond lengths are 1.65 Å. There are four inequivalent C sites. In the first C site, C(1) is bonded to four equivalent C(2) atoms to form corner-sharing CC4 tetrahedra. All C(1)-C(2) bond lengths are 1.54 Å. In the second C site, C(2) is bonded to two equivalent C(1) and two equivalent C(4) atoms to form CC4 tetrahedra that share corners with four equivalent C(3)B2C2 tetrahedra and corners with eight equivalent C(2)C4 tetrahedra. Both C(2)-C(4) bond lengths are 1.56 Å. In the third C site, C(3) is bonded to two equivalent B(1) and two equivalent C(4) atoms to form CB2C2 tetrahedra that share corners with four equivalent C(2)C4 tetrahedra and corners with eight equivalent C(3)B2C2 tetrahedra. Both C(3)-C(4) bond lengths are 1.53 Å. In the fourth C site, C(4) is bonded to two equivalent C(2) and two equivalent C(3) atoms to form CC4 tetrahedra that share corners with four equivalent B(1)C4 tetrahedra, corners with four equivalent C(1)C4 tetrahedra, and corners with four equivalent C(4)C4 tetrahedra.