Hf2Al4C5 crystallizes in the trigonal R-3m space group. Hf(1) is bonded to three equivalent C(1) and three equivalent C(2) atoms to form HfC6 octahedra that share corners with three equivalent Hf(1)C6 octahedra, corners with three equivalent Al(1)C4 tetrahedra, edges with nine equivalent Hf(1)C6 octahedra, and edges with three equivalent Al(1)C4 tetrahedra. The corner-sharing octahedra are not tilted. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one C(3) and three equivalent C(1) atoms to form AlC4 tetrahedra that share corners with three equivalent Hf(1)C6 octahedra, corners with six equivalent Al(1)C4 tetrahedra, corners with four equivalent Al(2)C4 trigonal pyramids, and edges with three equivalent Hf(1)C6 octahedra. The corner-sharing octahedral tilt angles are 10°. In the second Al site, Al(2) is bonded to four equivalent C(3) atoms to form a mixture of distorted corner and edge-sharing AlC4 trigonal pyramids. There are three inequivalent C sites. In the first C site, C(1) is bonded to three equivalent Hf(1) and three equivalent Al(1) atoms to form CHf3Al3 octahedra that share corners with three equivalent C(2)Hf6 octahedra, corners with three equivalent C(3)Al5 trigonal bipyramids, edges with three equivalent C(2)Hf6 octahedra, and edges with six equivalent C(1)Hf3Al3 octahedra. The corner-sharing octahedral tilt angles are 1°. In the second C site, C(2) is bonded to six equivalent Hf(1) atoms to form CHf6 octahedra that share corners with six equivalent C(1)Hf3Al3 octahedra, edges with six equivalent C(1)Hf3Al3 octahedra, and edges with six equivalent C(2)Hf6 octahedra. The corner-sharing octahedral tilt angles are 1°. In the third C site, C(3) is bonded to one Al(1) and four equivalent Al(2) atoms to form CAl5 trigonal bipyramids that share corners with three equivalent C(1)Hf3Al3 octahedra, corners with six equivalent C(3)Al5 trigonal bipyramids, and edges with three equivalent C(3)Al5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 64°.
Hf2Al4C5 crystallizes in the trigonal R-3m space group. Hf(1) is bonded to three equivalent C(1) and three equivalent C(2) atoms to form HfC6 octahedra that share corners with three equivalent Hf(1)C6 octahedra, corners with three equivalent Al(1)C4 tetrahedra, edges with nine equivalent Hf(1)C6 octahedra, and edges with three equivalent Al(1)C4 tetrahedra. The corner-sharing octahedra are not tilted. All Hf(1)-C(1) bond lengths are 2.36 Å. All Hf(1)-C(2) bond lengths are 2.34 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one C(3) and three equivalent C(1) atoms to form AlC4 tetrahedra that share corners with three equivalent Hf(1)C6 octahedra, corners with six equivalent Al(1)C4 tetrahedra, corners with four equivalent Al(2)C4 trigonal pyramids, and edges with three equivalent Hf(1)C6 octahedra. The corner-sharing octahedral tilt angles are 10°. The Al(1)-C(3) bond length is 1.94 Å. All Al(1)-C(1) bond lengths are 2.14 Å. In the second Al site, Al(2) is bonded to four equivalent C(3) atoms to form a mixture of distorted corner and edge-sharing AlC4 trigonal pyramids. There are three shorter (1.95 Å) and one longer (2.18 Å) Al(2)-C(3) bond length. There are three inequivalent C sites. In the first C site, C(1) is bonded to three equivalent Hf(1) and three equivalent Al(1) atoms to form CHf3Al3 octahedra that share corners with three equivalent C(2)Hf6 octahedra, corners with three equivalent C(3)Al5 trigonal bipyramids, edges with three equivalent C(2)Hf6 octahedra, and edges with six equivalent C(1)Hf3Al3 octahedra. The corner-sharing octahedral tilt angles are 1°. In the second C site, C(2) is bonded to six equivalent Hf(1) atoms to form CHf6 octahedra that share corners with six equivalent C(1)Hf3Al3 octahedra, edges with six equivalent C(1)Hf3Al3 octahedra, and edges with six equivalent C(2)Hf6 octahedra. The corner-sharing octahedral tilt angles are 1°. In the third C site, C(3) is bonded to one Al(1) and four equivalent Al(2) atoms to form CAl5 trigonal bipyramids that share corners with three equivalent C(1)Hf3Al3 octahedra, corners with six equivalent C(3)Al5 trigonal bipyramids, and edges with three equivalent C(3)Al5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 64°.