BC7 is beta beryllia-like structured and crystallizes in the trigonal R3m space group. B(1) is bonded to one C(2) and three equivalent C(4) atoms to form BC4 tetrahedra that share corners with three equivalent C(3)C4 tetrahedra, corners with three equivalent C(5)C4 tetrahedra, and corners with six equivalent B(1)C4 tetrahedra. There are seven inequivalent C sites. In the first C site, C(1) is bonded to one C(7) and three equivalent C(3) atoms to form CC4 tetrahedra that share corners with three equivalent C(4)B3C tetrahedra, corners with three equivalent C(6)C4 tetrahedra, and corners with six equivalent C(1)C4 tetrahedra. In the second C site, C(2) is bonded to one B(1) and three equivalent C(5) atoms to form CBC3 tetrahedra that share corners with three equivalent C(4)B3C tetrahedra, corners with three equivalent C(6)C4 tetrahedra, and corners with six equivalent C(2)BC3 tetrahedra. In the third C site, C(3) is bonded to one C(4) and three equivalent C(1) atoms to form CC4 tetrahedra that share corners with three equivalent B(1)C4 tetrahedra, corners with three equivalent C(7)C4 tetrahedra, and corners with six equivalent C(3)C4 tetrahedra. In the fourth C site, C(4) is bonded to three equivalent B(1) and one C(3) atom to form distorted CB3C tetrahedra that share corners with three equivalent C(2)BC3 tetrahedra, corners with three equivalent C(1)C4 tetrahedra, and corners with six equivalent C(4)B3C tetrahedra. In the fifth C site, C(5) is bonded to one C(6) and three equivalent C(2) atoms to form CC4 tetrahedra that share corners with three equivalent B(1)C4 tetrahedra, corners with three equivalent C(7)C4 tetrahedra, and corners with six equivalent C(5)C4 tetrahedra. In the sixth C site, C(6) is bonded to one C(5) and three equivalent C(7) atoms to form CC4 tetrahedra that share corners with three equivalent C(2)BC3 tetrahedra, corners with three equivalent C(1)C4 tetrahedra, and corners with six equivalent C(6)C4 tetrahedra. In the seventh C site, C(7) is bonded to one C(1) and three equivalent C(6) atoms to form corner-sharing CC4 tetrahedra.
BC7 is beta beryllia-like structured and crystallizes in the trigonal R3m space group. B(1) is bonded to one C(2) and three equivalent C(4) atoms to form BC4 tetrahedra that share corners with three equivalent C(3)C4 tetrahedra, corners with three equivalent C(5)C4 tetrahedra, and corners with six equivalent B(1)C4 tetrahedra. The B(1)-C(2) bond length is 1.61 Å. All B(1)-C(4) bond lengths are 1.63 Å. There are seven inequivalent C sites. In the first C site, C(1) is bonded to one C(7) and three equivalent C(3) atoms to form CC4 tetrahedra that share corners with three equivalent C(4)B3C tetrahedra, corners with three equivalent C(6)C4 tetrahedra, and corners with six equivalent C(1)C4 tetrahedra. The C(1)-C(7) bond length is 1.54 Å. All C(1)-C(3) bond lengths are 1.56 Å. In the second C site, C(2) is bonded to one B(1) and three equivalent C(5) atoms to form CBC3 tetrahedra that share corners with three equivalent C(4)B3C tetrahedra, corners with three equivalent C(6)C4 tetrahedra, and corners with six equivalent C(2)BC3 tetrahedra. All C(2)-C(5) bond lengths are 1.56 Å. In the third C site, C(3) is bonded to one C(4) and three equivalent C(1) atoms to form CC4 tetrahedra that share corners with three equivalent B(1)C4 tetrahedra, corners with three equivalent C(7)C4 tetrahedra, and corners with six equivalent C(3)C4 tetrahedra. The C(3)-C(4) bond length is 1.51 Å. In the fourth C site, C(4) is bonded to three equivalent B(1) and one C(3) atom to form distorted CB3C tetrahedra that share corners with three equivalent C(2)BC3 tetrahedra, corners with three equivalent C(1)C4 tetrahedra, and corners with six equivalent C(4)B3C tetrahedra. In the fifth C site, C(5) is bonded to one C(6) and three equivalent C(2) atoms to form CC4 tetrahedra that share corners with three equivalent B(1)C4 tetrahedra, corners with three equivalent C(7)C4 tetrahedra, and corners with six equivalent C(5)C4 tetrahedra. The C(5)-C(6) bond length is 1.55 Å. In the sixth C site, C(6) is bonded to one C(5) and three equivalent C(7) atoms to form CC4 tetrahedra that share corners with three equivalent C(2)BC3 tetrahedra, corners with three equivalent C(1)C4 tetrahedra, and corners with six equivalent C(6)C4 tetrahedra. All C(6)-C(7) bond lengths are 1.55 Å. In the seventh C site, C(7) is bonded to one C(1) and three equivalent C(6) atoms to form corner-sharing CC4 tetrahedra.