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AcSe3 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Ac3+ is bonded to twelve equivalent Se1- atoms to form a mixture of face and corner-sharing AcSe12 cuboctahedra. There are six shorter (3.13 Å) and six longer (3.51 Å) Ac–Se bond lengths. Se1- is bonded in a 2-coordinate geometry to four equivalent Ac3+ atoms.
AcSe3 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Ac3+ is bonded to twelve Se1- atoms to form AcSe12 cuboctahedra that share corners with four equivalent AcSe12 cuboctahedra, edges with eight equivalent AcSe12 cuboctahedra, edges with sixteen equivalent SeAc4Se8 cuboctahedra, faces with four equivalent AcSe12 cuboctahedra, and faces with eight equivalent SeAc4Se8 cuboctahedra. There are four shorter (3.08 Å) and eight longer (3.48 Å) Ac–Se bond lengths. There are two inequivalent Se1- sites. In the first Se1- site, Se1- is bonded to four equivalent Ac3+ and eight Se1- atoms to form distorted SeAc4Se8 cuboctahedra that share corners with twelve equivalent SeAc4Se8 cuboctahedra, edges with eight equivalent AcSe12 cuboctahedra, edges with eight equivalent SeAc4Se8 cuboctahedra, faces with four equivalent AcSe12 cuboctahedra, and faces with ten equivalent SeAc4Se8 cuboctahedra. There are four shorter (3.08 Å) and four longer (3.48 Å) Se–Se bond lengths. In the second Se1- site, Se1- is bonded in a square co-planar geometry to four equivalent Ac3+ and eight equivalent Se1- atoms.