Y5Al3C4 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Y sites. In the first Y site, Y(1) is bonded in a 3-coordinate geometry to two equivalent Al(2), one C(1), and two equivalent C(2) atoms. In the second Y site, Y(2) is bonded to two equivalent C(2) and four equivalent C(1) atoms to form edge-sharing YC6 octahedra. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded in a trigonal planar geometry to one C(2) and two equivalent C(1) atoms. In the second Al site, Al(2) is bonded in a body-centered cubic geometry to eight equivalent Y(1) atoms. There are two inequivalent C sites. In the first C site, C(1) is bonded to two equivalent Y(1), two equivalent Y(2), and two equivalent Al(1) atoms to form distorted CY4Al2 octahedra that share corners with two equivalent C(1)Y4Al2 octahedra, corners with two equivalent C(2)Y5Al octahedra, edges with three equivalent C(1)Y4Al2 octahedra, and edges with four equivalent C(2)Y5Al octahedra. The corner-sharing octahedral tilt angles range from 0-58°. In the second C site, C(2) is bonded to one Y(2), four equivalent Y(1), and one Al(1) atom to form CY5Al octahedra that share  a cornercorner with one C(2)Y5Al octahedra, corners with two equivalent C(1)Y4Al2 octahedra, edges with two equivalent C(2)Y5Al octahedra, and edges with four equivalent C(1)Y4Al2 octahedra. The corner-sharing octahedral tilt angles range from 0-58°.
Y5Al3C4 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Y sites. In the first Y site, Y(1) is bonded in a 3-coordinate geometry to two equivalent Al(2), one C(1), and two equivalent C(2) atoms. Both Y(1)-Al(2) bond lengths are 3.11 Å. The Y(1)-C(1) bond length is 2.51 Å. Both Y(1)-C(2) bond lengths are 2.45 Å. In the second Y site, Y(2) is bonded to two equivalent C(2) and four equivalent C(1) atoms to form edge-sharing YC6 octahedra. Both Y(2)-C(2) bond lengths are 2.51 Å. All Y(2)-C(1) bond lengths are 2.70 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded in a trigonal planar geometry to one C(2) and two equivalent C(1) atoms. The Al(1)-C(2) bond length is 2.07 Å. Both Al(1)-C(1) bond lengths are 2.07 Å. In the second Al site, Al(2) is bonded in a body-centered cubic geometry to eight equivalent Y(1) atoms. There are two inequivalent C sites. In the first C site, C(1) is bonded to two equivalent Y(1), two equivalent Y(2), and two equivalent Al(1) atoms to form distorted CY4Al2 octahedra that share corners with two equivalent C(1)Y4Al2 octahedra, corners with two equivalent C(2)Y5Al octahedra, edges with three equivalent C(1)Y4Al2 octahedra, and edges with four equivalent C(2)Y5Al octahedra. The corner-sharing octahedral tilt angles range from 0-58°. In the second C site, C(2) is bonded to one Y(2), four equivalent Y(1), and one Al(1) atom to form CY5Al octahedra that share  a cornercorner with one C(2)Y5Al octahedra, corners with two equivalent C(1)Y4Al2 octahedra, edges with two equivalent C(2)Y5Al octahedra, and edges with four equivalent C(1)Y4Al2 octahedra. The corner-sharing octahedral tilt angles range from 0-58°.