LuNdSe3 crystallizes in the orthorhombic Cmcm space group. Lu(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form a mixture of corner and edge-sharing LuSe6 octahedra. The corner-sharing octahedral tilt angles are 48°. Nd(1) is bonded in a 8-coordinate geometry to two equivalent Se(1) and six equivalent Se(2) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Lu(1) and two equivalent Nd(1) atoms to form distorted SeNd2Lu2 tetrahedra that share corners with four equivalent Se(1)Nd2Lu2 tetrahedra, corners with eight equivalent Se(2)Nd3Lu2 trigonal bipyramids, and edges with six equivalent Se(2)Nd3Lu2 trigonal bipyramids. In the second Se site, Se(2) is bonded to two equivalent Lu(1) and three equivalent Nd(1) atoms to form distorted SeNd3Lu2 trigonal bipyramids that share corners with four equivalent Se(1)Nd2Lu2 tetrahedra, corners with nine equivalent Se(2)Nd3Lu2 trigonal bipyramids, edges with three equivalent Se(1)Nd2Lu2 tetrahedra, and edges with six equivalent Se(2)Nd3Lu2 trigonal bipyramids.
LuNdSe3 crystallizes in the orthorhombic Cmcm space group. Lu(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form a mixture of corner and edge-sharing LuSe6 octahedra. The corner-sharing octahedral tilt angles are 48°. Both Lu(1)-Se(1) bond lengths are 2.72 Å. All Lu(1)-Se(2) bond lengths are 2.81 Å. Nd(1) is bonded in a 8-coordinate geometry to two equivalent Se(1) and six equivalent Se(2) atoms. Both Nd(1)-Se(1) bond lengths are 2.92 Å. There are four shorter (3.09 Å) and two longer (3.39 Å) Nd(1)-Se(2) bond lengths. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Lu(1) and two equivalent Nd(1) atoms to form distorted SeNd2Lu2 tetrahedra that share corners with four equivalent Se(1)Nd2Lu2 tetrahedra, corners with eight equivalent Se(2)Nd3Lu2 trigonal bipyramids, and edges with six equivalent Se(2)Nd3Lu2 trigonal bipyramids. In the second Se site, Se(2) is bonded to two equivalent Lu(1) and three equivalent Nd(1) atoms to form distorted SeNd3Lu2 trigonal bipyramids that share corners with four equivalent Se(1)Nd2Lu2 tetrahedra, corners with nine equivalent Se(2)Nd3Lu2 trigonal bipyramids, edges with three equivalent Se(1)Nd2Lu2 tetrahedra, and edges with six equivalent Se(2)Nd3Lu2 trigonal bipyramids.