EuNd2Se4 crystallizes in the tetragonal I-42d space group. Eu(1) is bonded to eight equivalent Se(1) atoms to form distorted EuSe8 hexagonal bipyramids that share corners with eight equivalent Nd(1)Se8 hexagonal bipyramids, edges with four equivalent Eu(1)Se8 hexagonal bipyramids, and faces with eight equivalent Nd(1)Se8 hexagonal bipyramids. Nd(1) is bonded to eight equivalent Se(1) atoms to form distorted NdSe8 hexagonal bipyramids that share corners with four equivalent Eu(1)Se8 hexagonal bipyramids, corners with four equivalent Nd(1)Se8 hexagonal bipyramids, edges with four equivalent Nd(1)Se8 hexagonal bipyramids, faces with four equivalent Eu(1)Se8 hexagonal bipyramids, and faces with four equivalent Nd(1)Se8 hexagonal bipyramids. Se(1) is bonded to two equivalent Eu(1) and four equivalent Nd(1) atoms to form a mixture of distorted face, corner, and edge-sharing SeNd4Eu2 octahedra. The corner-sharing octahedral tilt angles range from 17-50°.
EuNd2Se4 crystallizes in the tetragonal I-42d space group. Eu(1) is bonded to eight equivalent Se(1) atoms to form distorted EuSe8 hexagonal bipyramids that share corners with eight equivalent Nd(1)Se8 hexagonal bipyramids, edges with four equivalent Eu(1)Se8 hexagonal bipyramids, and faces with eight equivalent Nd(1)Se8 hexagonal bipyramids. There are four shorter (3.04 Å) and four longer (3.24 Å) Eu(1)-Se(1) bond lengths. Nd(1) is bonded to eight equivalent Se(1) atoms to form distorted NdSe8 hexagonal bipyramids that share corners with four equivalent Eu(1)Se8 hexagonal bipyramids, corners with four equivalent Nd(1)Se8 hexagonal bipyramids, edges with four equivalent Nd(1)Se8 hexagonal bipyramids, faces with four equivalent Eu(1)Se8 hexagonal bipyramids, and faces with four equivalent Nd(1)Se8 hexagonal bipyramids. There are a spread of Nd(1)-Se(1) bond distances ranging from 2.98-3.22 Å. Se(1) is bonded to two equivalent Eu(1) and four equivalent Nd(1) atoms to form a mixture of distorted face, corner, and edge-sharing SeNd4Eu2 octahedra. The corner-sharing octahedral tilt angles range from 17-50°.