GdCe4O9 crystallizes in the triclinic P1 space group. Gd(1) is bonded in a 6-coordinate geometry to one O(2), one O(3), one O(6), one O(7), and two equivalent O(8) atoms. There are four inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one O(1), one O(4), one O(8), two equivalent O(2), and two equivalent O(3) atoms to form a mixture of distorted edge and corner-sharing CeO7 hexagonal pyramids. In the second Ce site, Ce(2) is bonded in a body-centered cubic geometry to one O(1), one O(4), one O(6), one O(7), two equivalent O(5), and two equivalent O(9) atoms. In the third Ce site, Ce(3) is bonded in a body-centered cubic geometry to one O(2), one O(3), one O(5), one O(9), two equivalent O(1), and two equivalent O(4) atoms. In the fourth Ce site, Ce(4) is bonded to one O(5), one O(8), one O(9), two equivalent O(6), and two equivalent O(7) atoms to form a mixture of distorted edge and corner-sharing CeO7 hexagonal pyramids. There are nine inequivalent O sites. In the first O site, O(1) is bonded to one Ce(1), one Ce(2), and two equivalent Ce(3) atoms to form OCe4 tetrahedra that share  a cornercorner with one O(8)Ce2Gd2 tetrahedra,  a cornercorner with one O(7)Ce3Gd tetrahedra, corners with two equivalent O(1)Ce4 tetrahedra, corners with two equivalent O(4)Ce4 tetrahedra, corners with four equivalent O(3)Ce3Gd tetrahedra, corners with four equivalent O(5)Ce4 tetrahedra,  a cornercorner with one O(6)Ce3Gd trigonal pyramid, edges with two equivalent O(4)Ce4 tetrahedra, edges with two equivalent O(9)Ce4 tetrahedra, and edges with two equivalent O(2)Ce3Gd trigonal pyramids. In the second O site, O(2) is bonded to one Gd(1), one Ce(3), and two equivalent Ce(1) atoms to form distorted OCe3Gd trigonal pyramids that share  a cornercorner with one O(7)Ce3Gd tetrahedra,  a cornercorner with one O(5)Ce4 tetrahedra,  a cornercorner with one O(9)Ce4 tetrahedra, corners with two equivalent O(3)Ce3Gd tetrahedra, corners with four equivalent O(8)Ce2Gd2 tetrahedra, corners with four equivalent O(4)Ce4 tetrahedra,  a cornercorner with one O(6)Ce3Gd trigonal pyramid, corners with two equivalent O(2)Ce3Gd trigonal pyramids, edges with two equivalent O(3)Ce3Gd tetrahedra, and edges with two equivalent O(1)Ce4 tetrahedra. In the third O site, O(3) is bonded to one Gd(1), one Ce(3), and two equivalent Ce(1) atoms to form OCe3Gd tetrahedra that share  a cornercorner with one O(7)Ce3Gd tetrahedra,  a cornercorner with one O(5)Ce4 tetrahedra,  a cornercorner with one O(9)Ce4 tetrahedra, corners with two equivalent O(3)Ce3Gd tetrahedra, corners with four equivalent O(1)Ce4 tetrahedra,  a cornercorner with one O(6)Ce3Gd trigonal pyramid, corners with two equivalent O(2)Ce3Gd trigonal pyramids, edges with two equivalent O(8)Ce2Gd2 tetrahedra, edges with two equivalent O(4)Ce4 tetrahedra, and edges with two equivalent O(2)Ce3Gd trigonal pyramids. In the fourth O site, O(4) is bonded to one Ce(1), one Ce(2), and two equivalent Ce(3) atoms to form OCe4 tetrahedra that share  a cornercorner with one O(8)Ce2Gd2 tetrahedra,  a cornercorner with one O(7)Ce3Gd tetrahedra, corners with two equivalent O(1)Ce4 tetrahedra, corners with two equivalent O(4)Ce4 tetrahedra, corners with four equivalent O(9)Ce4 tetrahedra,  a cornercorner with one O(6)Ce3Gd trigonal pyramid, corners with four equivalent O(2)Ce3Gd trigonal pyramids, edges with two equivalent O(3)Ce3Gd tetrahedra, edges with two equivalent O(1)Ce4 tetrahedra, and edges with two equivalent O(5)Ce4 tetrahedra. In the fifth O site, O(5) is bonded to one Ce(3), one Ce(4), and two equivalent Ce(2) atoms to form OCe4 tetrahedra that share  a cornercorner with one O(8)Ce2Gd2 tetrahedra,  a cornercorner with one O(3)Ce3Gd tetrahedra, corners with two equivalent O(5)Ce4 tetrahedra, corners with two equivalent O(9)Ce4 tetrahedra, corners with four equivalent O(7)Ce3Gd tetrahedra, corners with four equivalent O(1)Ce4 tetrahedra,  a cornercorner with one O(2)Ce3Gd trigonal pyramid, edges with two equivalent O(4)Ce4 tetrahedra, edges with two equivalent O(9)Ce4 tetrahedra, and edges with two equivalent O(6)Ce3Gd trigonal pyramids. In the sixth O site, O(6) is bonded to one Gd(1), one Ce(2), and two equivalent Ce(4) atoms to form distorted OCe3Gd trigonal pyramids that share  a cornercorner with one O(3)Ce3Gd tetrahedra,  a cornercorner with one O(1)Ce4 tetrahedra,  a cornercorner with one O(4)Ce4 tetrahedra, corners with two equivalent O(7)Ce3Gd tetrahedra, corners with four equivalent O(8)Ce2Gd2 tetrahedra, corners with four equivalent O(9)Ce4 tetrahedra,  a cornercorner with one O(2)Ce3Gd trigonal pyramid, corners with two equivalent O(6)Ce3Gd trigonal pyramids, edges with two equivalent O(7)Ce3Gd tetrahedra, and edges with two equivalent O(5)Ce4 tetrahedra. In the seventh O site, O(7) is bonded to one Gd(1), one Ce(2), and two equivalent Ce(4) atoms to form OCe3Gd tetrahedra that share  a cornercorner with one O(3)Ce3Gd tetrahedra,  a cornercorner with one O(1)Ce4 tetrahedra,  a cornercorner with one O(4)Ce4 tetrahedra, corners with two equivalent O(7)Ce3Gd tetrahedra, corners with four equivalent O(5)Ce4 tetrahedra,  a cornercorner with one O(2)Ce3Gd trigonal pyramid, corners with two equivalent O(6)Ce3Gd trigonal pyramids, edges with two equivalent O(8)Ce2Gd2 tetrahedra, edges with two equivalent O(9)Ce4 tetrahedra, and edges with two equivalent O(6)Ce3Gd trigonal pyramids. In the eighth O site, O(8) is bonded to two equivalent Gd(1), one Ce(1), and one Ce(4) atom to form OCe2Gd2 tetrahedra that share  a cornercorner with one O(1)Ce4 tetrahedra,  a cornercorner with one O(4)Ce4 tetrahedra,  a cornercorner with one O(5)Ce4 tetrahedra,  a cornercorner with one O(9)Ce4 tetrahedra, corners with two equivalent O(8)Ce2Gd2 tetrahedra, corners with four equivalent O(2)Ce3Gd trigonal pyramids, corners with four equivalent O(6)Ce3Gd trigonal pyramids, edges with two equivalent O(3)Ce3Gd tetrahedra, and edges with two equivalent O(7)Ce3Gd tetrahedra. In the ninth O site, O(9) is bonded to one Ce(3), one Ce(4), and two equivalent Ce(2) atoms to form OCe4 tetrahedra that share  a cornercorner with one O(8)Ce2Gd2 tetrahedra,  a cornercorner with one O(3)Ce3Gd tetrahedra, corners with two equivalent O(5)Ce4 tetrahedra, corners with two equivalent O(9)Ce4 tetrahedra, corners with four equivalent O(4)Ce4 tetrahedra,  a cornercorner with one O(2)Ce3Gd trigonal pyramid, corners with four equivalent O(6)Ce3Gd trigonal pyramids, edges with two equivalent O(7)Ce3Gd tetrahedra, edges with two equivalent O(1)Ce4 tetrahedra, and edges with two equivalent O(5)Ce4 tetrahedra.
GdCe4O9 crystallizes in the triclinic P1 space group. Gd(1) is bonded in a 6-coordinate geometry to one O(2), one O(3), one O(6), one O(7), and two equivalent O(8) atoms. The Gd(1)-O(2) bond length is 2.35 Å. The Gd(1)-O(3) bond length is 2.33 Å. The Gd(1)-O(6) bond length is 2.36 Å. The Gd(1)-O(7) bond length is 2.34 Å. Both Gd(1)-O(8) bond lengths are 2.26 Å. There are four inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one O(1), one O(4), one O(8), two equivalent O(2), and two equivalent O(3) atoms to form a mixture of distorted edge and corner-sharing CeO7 hexagonal pyramids. The Ce(1)-O(1) bond length is 2.27 Å. The Ce(1)-O(4) bond length is 2.31 Å. The Ce(1)-O(8) bond length is 2.42 Å. There is one shorter (2.41 Å) and one longer (2.42 Å) Ce(1)-O(2) bond length. There is one shorter (2.32 Å) and one longer (2.33 Å) Ce(1)-O(3) bond length. In the second Ce site, Ce(2) is bonded in a body-centered cubic geometry to one O(1), one O(4), one O(6), one O(7), two equivalent O(5), and two equivalent O(9) atoms. The Ce(2)-O(1) bond length is 2.48 Å. The Ce(2)-O(4) bond length is 2.38 Å. The Ce(2)-O(6) bond length is 2.62 Å. The Ce(2)-O(7) bond length is 2.33 Å. There is one shorter (2.36 Å) and one longer (2.37 Å) Ce(2)-O(5) bond length. There is one shorter (2.34 Å) and one longer (2.35 Å) Ce(2)-O(9) bond length. In the third Ce site, Ce(3) is bonded in a body-centered cubic geometry to one O(2), one O(3), one O(5), one O(9), two equivalent O(1), and two equivalent O(4) atoms. The Ce(3)-O(2) bond length is 2.68 Å. The Ce(3)-O(3) bond length is 2.32 Å. The Ce(3)-O(5) bond length is 2.45 Å. The Ce(3)-O(9) bond length is 2.40 Å. There is one shorter (2.36 Å) and one longer (2.37 Å) Ce(3)-O(1) bond length. Both Ce(3)-O(4) bond lengths are 2.36 Å. In the fourth Ce site, Ce(4) is bonded to one O(5), one O(8), one O(9), two equivalent O(6), and two equivalent O(7) atoms to form a mixture of distorted edge and corner-sharing CeO7 hexagonal pyramids. The Ce(4)-O(5) bond length is 2.25 Å. The Ce(4)-O(8) bond length is 2.42 Å. The Ce(4)-O(9) bond length is 2.32 Å. There is one shorter (2.42 Å) and one longer (2.43 Å) Ce(4)-O(6) bond length. Both Ce(4)-O(7) bond lengths are 2.31 Å. There are nine inequivalent O sites. In the first O site, O(1) is bonded to one Ce(1), one Ce(2), and two equivalent Ce(3) atoms to form OCe4 tetrahedra that share  a cornercorner with one O(8)Ce2Gd2 tetrahedra,  a cornercorner with one O(7)Ce3Gd tetrahedra, corners with two equivalent O(1)Ce4 tetrahedra, corners with two equivalent O(4)Ce4 tetrahedra, corners with four equivalent O(3)Ce3Gd tetrahedra, corners with four equivalent O(5)Ce4 tetrahedra,  a cornercorner with one O(6)Ce3Gd trigonal pyramid, edges with two equivalent O(4)Ce4 tetrahedra, edges with two equivalent O(9)Ce4 tetrahedra, and edges with two equivalent O(2)Ce3Gd trigonal pyramids. In the second O site, O(2) is bonded to one Gd(1), one Ce(3), and two equivalent Ce(1) atoms to form distorted OCe3Gd trigonal pyramids that share  a cornercorner with one O(7)Ce3Gd tetrahedra,  a cornercorner with one O(5)Ce4 tetrahedra,  a cornercorner with one O(9)Ce4 tetrahedra, corners with two equivalent O(3)Ce3Gd tetrahedra, corners with four equivalent O(8)Ce2Gd2 tetrahedra, corners with four equivalent O(4)Ce4 tetrahedra,  a cornercorner with one O(6)Ce3Gd trigonal pyramid, corners with two equivalent O(2)Ce3Gd trigonal pyramids, edges with two equivalent O(3)Ce3Gd tetrahedra, and edges with two equivalent O(1)Ce4 tetrahedra. In the third O site, O(3) is bonded to one Gd(1), one Ce(3), and two equivalent Ce(1) atoms to form OCe3Gd tetrahedra that share  a cornercorner with one O(7)Ce3Gd tetrahedra,  a cornercorner with one O(5)Ce4 tetrahedra,  a cornercorner with one O(9)Ce4 tetrahedra, corners with two equivalent O(3)Ce3Gd tetrahedra, corners with four equivalent O(1)Ce4 tetrahedra,  a cornercorner with one O(6)Ce3Gd trigonal pyramid, corners with two equivalent O(2)Ce3Gd trigonal pyramids, edges with two equivalent O(8)Ce2Gd2 tetrahedra, edges with two equivalent O(4)Ce4 tetrahedra, and edges with two equivalent O(2)Ce3Gd trigonal pyramids. In the fourth O site, O(4) is bonded to one Ce(1), one Ce(2), and two equivalent Ce(3) atoms to form OCe4 tetrahedra that share  a cornercorner with one O(8)Ce2Gd2 tetrahedra,  a cornercorner with one O(7)Ce3Gd tetrahedra, corners with two equivalent O(1)Ce4 tetrahedra, corners with two equivalent O(4)Ce4 tetrahedra, corners with four equivalent O(9)Ce4 tetrahedra,  a cornercorner with one O(6)Ce3Gd trigonal pyramid, corners with four equivalent O(2)Ce3Gd trigonal pyramids, edges with two equivalent O(3)Ce3Gd tetrahedra, edges with two equivalent O(1)Ce4 tetrahedra, and edges with two equivalent O(5)Ce4 tetrahedra. In the fifth O site, O(5) is bonded to one Ce(3), one Ce(4), and two equivalent Ce(2) atoms to form OCe4 tetrahedra that share  a cornercorner with one O(8)Ce2Gd2 tetrahedra,  a cornercorner with one O(3)Ce3Gd tetrahedra, corners with two equivalent O(5)Ce4 tetrahedra, corners with two equivalent O(9)Ce4 tetrahedra, corners with four equivalent O(7)Ce3Gd tetrahedra, corners with four equivalent O(1)Ce4 tetrahedra,  a cornercorner with one O(2)Ce3Gd trigonal pyramid, edges with two equivalent O(4)Ce4 tetrahedra, edges with two equivalent O(9)Ce4 tetrahedra, and edges with two equivalent O(6)Ce3Gd trigonal pyramids. In the sixth O site, O(6) is bonded to one Gd(1), one Ce(2), and two equivalent Ce(4) atoms to form distorted OCe3Gd trigonal pyramids that share  a cornercorner with one O(3)Ce3Gd tetrahedra,  a cornercorner with one O(1)Ce4 tetrahedra,  a cornercorner with one O(4)Ce4 tetrahedra, corners with two equivalent O(7)Ce3Gd tetrahedra, corners with four equivalent O(8)Ce2Gd2 tetrahedra, corners with four equivalent O(9)Ce4 tetrahedra,  a cornercorner with one O(2)Ce3Gd trigonal pyramid, corners with two equivalent O(6)Ce3Gd trigonal pyramids, edges with two equivalent O(7)Ce3Gd tetrahedra, and edges with two equivalent O(5)Ce4 tetrahedra. In the seventh O site, O(7) is bonded to one Gd(1), one Ce(2), and two equivalent Ce(4) atoms to form OCe3Gd tetrahedra that share  a cornercorner with one O(3)Ce3Gd tetrahedra,  a cornercorner with one O(1)Ce4 tetrahedra,  a cornercorner with one O(4)Ce4 tetrahedra, corners with two equivalent O(7)Ce3Gd tetrahedra, corners with four equivalent O(5)Ce4 tetrahedra,  a cornercorner with one O(2)Ce3Gd trigonal pyramid, corners with two equivalent O(6)Ce3Gd trigonal pyramids, edges with two equivalent O(8)Ce2Gd2 tetrahedra, edges with two equivalent O(9)Ce4 tetrahedra, and edges with two equivalent O(6)Ce3Gd trigonal pyramids. In the eighth O site, O(8) is bonded to two equivalent Gd(1), one Ce(1), and one Ce(4) atom to form OCe2Gd2 tetrahedra that share  a cornercorner with one O(1)Ce4 tetrahedra,  a cornercorner with one O(4)Ce4 tetrahedra,  a cornercorner with one O(5)Ce4 tetrahedra,  a cornercorner with one O(9)Ce4 tetrahedra, corners with two equivalent O(8)Ce2Gd2 tetrahedra, corners with four equivalent O(2)Ce3Gd trigonal pyramids, corners with four equivalent O(6)Ce3Gd trigonal pyramids, edges with two equivalent O(3)Ce3Gd tetrahedra, and edges with two equivalent O(7)Ce3Gd tetrahedra. In the ninth O site, O(9) is bonded to one Ce(3), one Ce(4), and two equivalent Ce(2) atoms to form OCe4 tetrahedra that share  a cornercorner with one O(8)Ce2Gd2 tetrahedra,  a cornercorner with one O(3)Ce3Gd tetrahedra, corners with two equivalent O(5)Ce4 tetrahedra, corners with two equivalent O(9)Ce4 tetrahedra, corners with four equivalent O(4)Ce4 tetrahedra,  a cornercorner with one O(2)Ce3Gd trigonal pyramid, corners with four equivalent O(6)Ce3Gd trigonal pyramids, edges with two equivalent O(7)Ce3Gd tetrahedra, edges with two equivalent O(1)Ce4 tetrahedra, and edges with two equivalent O(5)Ce4 tetrahedra.