Na15Sn4 crystallizes in the cubic I-43d space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to eight equivalent Na(2) and four equivalent Sn(1) atoms to form NaNa8Sn4 cuboctahedra that share corners with eight equivalent Sn(1)Na12 cuboctahedra, edges with eight equivalent Na(1)Na8Sn4 cuboctahedra, and faces with four equivalent Sn(1)Na12 cuboctahedra. In the second Na site, Na(2) is bonded in a 3-coordinate geometry to two equivalent Na(1) and three equivalent Sn(1) atoms. Sn(1) is bonded to three equivalent Na(1) and nine equivalent Na(2) atoms to form distorted SnNa12 cuboctahedra that share corners with six equivalent Na(1)Na8Sn4 cuboctahedra, edges with six equivalent Sn(1)Na12 cuboctahedra, faces with three equivalent Na(1)Na8Sn4 cuboctahedra, and faces with five equivalent Sn(1)Na12 cuboctahedra.
Na15Sn4 crystallizes in the cubic I-43d space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to eight equivalent Na(2) and four equivalent Sn(1) atoms to form NaNa8Sn4 cuboctahedra that share corners with eight equivalent Sn(1)Na12 cuboctahedra, edges with eight equivalent Na(1)Na8Sn4 cuboctahedra, and faces with four equivalent Sn(1)Na12 cuboctahedra. There are four shorter (3.36 Å) and four longer (3.41 Å) Na(1)-Na(2) bond lengths. All Na(1)-Sn(1) bond lengths are 3.52 Å. In the second Na site, Na(2) is bonded in a 3-coordinate geometry to two equivalent Na(1) and three equivalent Sn(1) atoms. There are a spread of Na(2)-Sn(1) bond distances ranging from 3.20-3.48 Å. Sn(1) is bonded to three equivalent Na(1) and nine equivalent Na(2) atoms to form distorted SnNa12 cuboctahedra that share corners with six equivalent Na(1)Na8Sn4 cuboctahedra, edges with six equivalent Sn(1)Na12 cuboctahedra, faces with three equivalent Na(1)Na8Sn4 cuboctahedra, and faces with five equivalent Sn(1)Na12 cuboctahedra.