Na3Cr11Se20 crystallizes in the monoclinic Cm space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a 6-coordinate geometry to one Se(12), one Se(5), two equivalent Se(16), and two equivalent Se(9) atoms. In the second Na site, Na(2) is bonded in a 6-coordinate geometry to one Se(16), one Se(9), two equivalent Se(13), and two equivalent Se(18) atoms. In the third Na site, Na(3) is bonded in a 6-coordinate geometry to one Se(13), one Se(18), two equivalent Se(1), and two equivalent Se(20) atoms. There are eleven inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one Se(19), one Se(3), two equivalent Se(1), and two equivalent Se(6) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 50°. In the second Cr site, Cr(2) is bonded to one Se(20), one Se(4), two equivalent Se(2), and two equivalent Se(8) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 49°. In the third Cr site, Cr(3) is bonded to one Se(5), one Se(6), two equivalent Se(10), and two equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 49°. In the fourth Cr site, Cr(4) is bonded to one Se(6), one Se(7), two equivalent Se(10), and two equivalent Se(4) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 49-50°. In the fifth Cr site, Cr(5) is bonded to one Se(7), one Se(8), two equivalent Se(12), and two equivalent Se(4) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 49°. In the sixth Cr site, Cr(6) is bonded to one Se(10), one Se(9), two equivalent Se(14), and two equivalent Se(5) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 48-50°. In the seventh Cr site, Cr(7) is bonded to one Se(11), one Se(12), two equivalent Se(16), and two equivalent Se(7) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. In the eighth Cr site, Cr(8) is bonded to one Se(13), one Se(14), two equivalent Se(17), and two equivalent Se(9) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 47°. In the ninth Cr site, Cr(9) is bonded to one Se(14), one Se(15), two equivalent Se(11), and two equivalent Se(17) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-55°. In the tenth Cr site, Cr(10) is bonded to one Se(1), one Se(17), two equivalent Se(13), and two equivalent Se(19) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 49°. In the eleventh Cr site, Cr(11) is bonded to one Se(18), one Se(2), two equivalent Se(15), and two equivalent Se(20) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 55°. There are twenty inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Na(3), one Cr(10), and two equivalent Cr(1) atoms to form SeNa2Cr3 square pyramids that share  a cornercorner with one Se(17)Cr5 trigonal bipyramid, corners with two equivalent Se(20)Na2Cr3 trigonal bipyramids, edges with two equivalent Se(13)Na3Cr3 octahedra, edges with two equivalent Se(1)Na2Cr3 square pyramids, and  an edgeedge with one Se(20)Na2Cr3 trigonal bipyramid. In the second Se site, Se(2) is bonded in a distorted T-shaped geometry to one Cr(11) and two equivalent Cr(2) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Cr(1) and two equivalent Cr(3) atoms. In the fourth Se site, Se(4) is bonded to one Cr(2), two equivalent Cr(4), and two equivalent Cr(5) atoms to form distorted SeCr5 square pyramids that share corners with two equivalent Se(10)Cr5 square pyramids,  a cornercorner with one Se(20)Na2Cr3 trigonal bipyramid, corners with two equivalent Se(12)NaCr3 trigonal pyramids,  an edgeedge with one Se(10)Cr5 square pyramid, edges with two equivalent Se(4)Cr5 square pyramids, and  an edgeedge with one Se(12)NaCr3 trigonal pyramid. In the fifth Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one Na(1), one Cr(3), and two equivalent Cr(6) atoms. In the sixth Se site, Se(6) is bonded in a rectangular see-saw-like geometry to one Cr(3), one Cr(4), and two equivalent Cr(1) atoms. In the seventh Se site, Se(7) is bonded in a rectangular see-saw-like geometry to one Cr(4), one Cr(5), and two equivalent Cr(7) atoms. In the eighth Se site, Se(8) is bonded in a distorted T-shaped geometry to one Cr(5) and two equivalent Cr(2) atoms. In the ninth Se site, Se(9) is bonded to one Na(2), two equivalent Na(1), one Cr(6), and two equivalent Cr(8) atoms to form SeNa3Cr3 octahedra that share  a cornercorner with one Se(10)Cr5 square pyramid, corners with two equivalent Se(17)Cr5 trigonal bipyramids, corners with two equivalent Se(12)NaCr3 trigonal pyramids, edges with two equivalent Se(13)Na3Cr3 octahedra, edges with two equivalent Se(9)Na3Cr3 octahedra, and  an edgeedge with one Se(17)Cr5 trigonal bipyramid. In the tenth Se site, Se(10) is bonded to one Cr(6), two equivalent Cr(3), and two equivalent Cr(4) atoms to form distorted SeCr5 square pyramids that share  a cornercorner with one Se(9)Na3Cr3 octahedra, corners with two equivalent Se(4)Cr5 square pyramids,  an edgeedge with one Se(4)Cr5 square pyramid, and edges with two equivalent Se(10)Cr5 square pyramids. The corner-sharing octahedral tilt angles are 4°. In the eleventh Se site, Se(11) is bonded in a distorted trigonal planar geometry to one Cr(7) and two equivalent Cr(9) atoms. In the twelfth Se site, Se(12) is bonded to one Na(1), one Cr(7), and two equivalent Cr(5) atoms to form SeNaCr3 trigonal pyramids that share corners with two equivalent Se(9)Na3Cr3 octahedra, corners with two equivalent Se(4)Cr5 square pyramids, corners with two equivalent Se(12)NaCr3 trigonal pyramids, and  an edgeedge with one Se(4)Cr5 square pyramid. The corner-sharing octahedral tilt angles are 40°. In the thirteenth Se site, Se(13) is bonded to one Na(3), two equivalent Na(2), one Cr(8), and two equivalent Cr(10) atoms to form SeNa3Cr3 octahedra that share corners with two equivalent Se(20)Na2Cr3 trigonal bipyramids, edges with two equivalent Se(13)Na3Cr3 octahedra, edges with two equivalent Se(9)Na3Cr3 octahedra, edges with two equivalent Se(1)Na2Cr3 square pyramids, and edges with two equivalent Se(17)Cr5 trigonal bipyramids. In the fourteenth Se site, Se(14) is bonded in a rectangular see-saw-like geometry to one Cr(8), one Cr(9), and two equivalent Cr(6) atoms. In the fifteenth Se site, Se(15) is bonded in a trigonal planar geometry to one Cr(9) and two equivalent Cr(11) atoms. In the sixteenth Se site, Se(16) is bonded in a 5-coordinate geometry to one Na(2), two equivalent Na(1), and two equivalent Cr(7) atoms. In the seventeenth Se site, Se(17) is bonded to one Cr(10), two equivalent Cr(8), and two equivalent Cr(9) atoms to form distorted SeCr5 trigonal bipyramids that share corners with two equivalent Se(9)Na3Cr3 octahedra,  a cornercorner with one Se(1)Na2Cr3 square pyramid,  an edgeedge with one Se(9)Na3Cr3 octahedra, edges with two equivalent Se(13)Na3Cr3 octahedra, and edges with two equivalent Se(17)Cr5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 3°. In the eighteenth Se site, Se(18) is bonded in a 4-coordinate geometry to one Na(3), two equivalent Na(2), and one Cr(11) atom. In the nineteenth Se site, Se(19) is bonded in a distorted T-shaped geometry to one Cr(1) and two equivalent Cr(10) atoms. In the twentieth Se site, Se(20) is bonded to two equivalent Na(3), one Cr(2), and two equivalent Cr(11) atoms to form distorted SeNa2Cr3 trigonal bipyramids that share corners with two equivalent Se(13)Na3Cr3 octahedra,  a cornercorner with one Se(4)Cr5 square pyramid, corners with two equivalent Se(1)Na2Cr3 square pyramids,  an edgeedge with one Se(1)Na2Cr3 square pyramid, and edges with two equivalent Se(20)Na2Cr3 trigonal bipyramids. The corner-sharing octahedral tilt angles are 43°.
Na3Cr11Se20 crystallizes in the monoclinic Cm space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a 6-coordinate geometry to one Se(12), one Se(5), two equivalent Se(16), and two equivalent Se(9) atoms. The Na(1)-Se(12) bond length is 2.85 Å. The Na(1)-Se(5) bond length is 3.03 Å. Both Na(1)-Se(16) bond lengths are 3.00 Å. Both Na(1)-Se(9) bond lengths are 3.04 Å. In the second Na site, Na(2) is bonded in a 6-coordinate geometry to one Se(16), one Se(9), two equivalent Se(13), and two equivalent Se(18) atoms. The Na(2)-Se(16) bond length is 3.00 Å. The Na(2)-Se(9) bond length is 3.12 Å. Both Na(2)-Se(13) bond lengths are 3.11 Å. Both Na(2)-Se(18) bond lengths are 3.00 Å. In the third Na site, Na(3) is bonded in a 6-coordinate geometry to one Se(13), one Se(18), two equivalent Se(1), and two equivalent Se(20) atoms. The Na(3)-Se(13) bond length is 3.11 Å. The Na(3)-Se(18) bond length is 3.06 Å. Both Na(3)-Se(1) bond lengths are 3.03 Å. Both Na(3)-Se(20) bond lengths are 2.98 Å. There are eleven inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one Se(19), one Se(3), two equivalent Se(1), and two equivalent Se(6) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 50°. The Cr(1)-Se(19) bond length is 2.58 Å. The Cr(1)-Se(3) bond length is 2.53 Å. Both Cr(1)-Se(1) bond lengths are 2.53 Å. Both Cr(1)-Se(6) bond lengths are 2.53 Å. In the second Cr site, Cr(2) is bonded to one Se(20), one Se(4), two equivalent Se(2), and two equivalent Se(8) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 49°. The Cr(2)-Se(20) bond length is 2.45 Å. The Cr(2)-Se(4) bond length is 2.63 Å. Both Cr(2)-Se(2) bond lengths are 2.51 Å. Both Cr(2)-Se(8) bond lengths are 2.55 Å. In the third Cr site, Cr(3) is bonded to one Se(5), one Se(6), two equivalent Se(10), and two equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 49°. The Cr(3)-Se(5) bond length is 2.51 Å. The Cr(3)-Se(6) bond length is 2.55 Å. Both Cr(3)-Se(10) bond lengths are 2.71 Å. Both Cr(3)-Se(3) bond lengths are 2.45 Å. In the fourth Cr site, Cr(4) is bonded to one Se(6), one Se(7), two equivalent Se(10), and two equivalent Se(4) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 49-50°. The Cr(4)-Se(6) bond length is 2.57 Å. The Cr(4)-Se(7) bond length is 2.54 Å. Both Cr(4)-Se(10) bond lengths are 2.55 Å. Both Cr(4)-Se(4) bond lengths are 2.56 Å. In the fifth Cr site, Cr(5) is bonded to one Se(7), one Se(8), two equivalent Se(12), and two equivalent Se(4) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 49°. The Cr(5)-Se(7) bond length is 2.60 Å. The Cr(5)-Se(8) bond length is 2.46 Å. Both Cr(5)-Se(12) bond lengths are 2.44 Å. Both Cr(5)-Se(4) bond lengths are 2.67 Å. In the sixth Cr site, Cr(6) is bonded to one Se(10), one Se(9), two equivalent Se(14), and two equivalent Se(5) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 48-50°. The Cr(6)-Se(10) bond length is 2.58 Å. The Cr(6)-Se(9) bond length is 2.53 Å. Both Cr(6)-Se(14) bond lengths are 2.54 Å. Both Cr(6)-Se(5) bond lengths are 2.54 Å. In the seventh Cr site, Cr(7) is bonded to one Se(11), one Se(12), two equivalent Se(16), and two equivalent Se(7) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. The Cr(7)-Se(11) bond length is 2.62 Å. The Cr(7)-Se(12) bond length is 2.53 Å. Both Cr(7)-Se(16) bond lengths are 2.50 Å. Both Cr(7)-Se(7) bond lengths are 2.53 Å. In the eighth Cr site, Cr(8) is bonded to one Se(13), one Se(14), two equivalent Se(17), and two equivalent Se(9) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 47°. The Cr(8)-Se(13) bond length is 2.54 Å. The Cr(8)-Se(14) bond length is 2.52 Å. Both Cr(8)-Se(17) bond lengths are 2.61 Å. Both Cr(8)-Se(9) bond lengths are 2.51 Å. In the ninth Cr site, Cr(9) is bonded to one Se(14), one Se(15), two equivalent Se(11), and two equivalent Se(17) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-55°. The Cr(9)-Se(14) bond length is 2.61 Å. The Cr(9)-Se(15) bond length is 2.47 Å. Both Cr(9)-Se(11) bond lengths are 2.44 Å. Both Cr(9)-Se(17) bond lengths are 2.69 Å. In the tenth Cr site, Cr(10) is bonded to one Se(1), one Se(17), two equivalent Se(13), and two equivalent Se(19) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 49°. The Cr(10)-Se(1) bond length is 2.50 Å. The Cr(10)-Se(17) bond length is 2.63 Å. Both Cr(10)-Se(13) bond lengths are 2.60 Å. Both Cr(10)-Se(19) bond lengths are 2.48 Å. In the eleventh Cr site, Cr(11) is bonded to one Se(18), one Se(2), two equivalent Se(15), and two equivalent Se(20) atoms to form a mixture of edge and corner-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles are 55°. The Cr(11)-Se(18) bond length is 2.38 Å. The Cr(11)-Se(2) bond length is 2.54 Å. Both Cr(11)-Se(15) bond lengths are 2.49 Å. Both Cr(11)-Se(20) bond lengths are 2.56 Å. There are twenty inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Na(3), one Cr(10), and two equivalent Cr(1) atoms to form SeNa2Cr3 square pyramids that share  a cornercorner with one Se(17)Cr5 trigonal bipyramid, corners with two equivalent Se(20)Na2Cr3 trigonal bipyramids, edges with two equivalent Se(13)Na3Cr3 octahedra, edges with two equivalent Se(1)Na2Cr3 square pyramids, and  an edgeedge with one Se(20)Na2Cr3 trigonal bipyramid. In the second Se site, Se(2) is bonded in a distorted T-shaped geometry to one Cr(11) and two equivalent Cr(2) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Cr(1) and two equivalent Cr(3) atoms. In the fourth Se site, Se(4) is bonded to one Cr(2), two equivalent Cr(4), and two equivalent Cr(5) atoms to form distorted SeCr5 square pyramids that share corners with two equivalent Se(10)Cr5 square pyramids,  a cornercorner with one Se(20)Na2Cr3 trigonal bipyramid, corners with two equivalent Se(12)NaCr3 trigonal pyramids,  an edgeedge with one Se(10)Cr5 square pyramid, edges with two equivalent Se(4)Cr5 square pyramids, and  an edgeedge with one Se(12)NaCr3 trigonal pyramid. In the fifth Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one Na(1), one Cr(3), and two equivalent Cr(6) atoms. In the sixth Se site, Se(6) is bonded in a rectangular see-saw-like geometry to one Cr(3), one Cr(4), and two equivalent Cr(1) atoms. In the seventh Se site, Se(7) is bonded in a rectangular see-saw-like geometry to one Cr(4), one Cr(5), and two equivalent Cr(7) atoms. In the eighth Se site, Se(8) is bonded in a distorted T-shaped geometry to one Cr(5) and two equivalent Cr(2) atoms. In the ninth Se site, Se(9) is bonded to one Na(2), two equivalent Na(1), one Cr(6), and two equivalent Cr(8) atoms to form SeNa3Cr3 octahedra that share  a cornercorner with one Se(10)Cr5 square pyramid, corners with two equivalent Se(17)Cr5 trigonal bipyramids, corners with two equivalent Se(12)NaCr3 trigonal pyramids, edges with two equivalent Se(13)Na3Cr3 octahedra, edges with two equivalent Se(9)Na3Cr3 octahedra, and  an edgeedge with one Se(17)Cr5 trigonal bipyramid. In the tenth Se site, Se(10) is bonded to one Cr(6), two equivalent Cr(3), and two equivalent Cr(4) atoms to form distorted SeCr5 square pyramids that share  a cornercorner with one Se(9)Na3Cr3 octahedra, corners with two equivalent Se(4)Cr5 square pyramids,  an edgeedge with one Se(4)Cr5 square pyramid, and edges with two equivalent Se(10)Cr5 square pyramids. The corner-sharing octahedral tilt angles are 4°. In the eleventh Se site, Se(11) is bonded in a distorted trigonal planar geometry to one Cr(7) and two equivalent Cr(9) atoms. In the twelfth Se site, Se(12) is bonded to one Na(1), one Cr(7), and two equivalent Cr(5) atoms to form SeNaCr3 trigonal pyramids that share corners with two equivalent Se(9)Na3Cr3 octahedra, corners with two equivalent Se(4)Cr5 square pyramids, corners with two equivalent Se(12)NaCr3 trigonal pyramids, and  an edgeedge with one Se(4)Cr5 square pyramid. The corner-sharing octahedral tilt angles are 40°. In the thirteenth Se site, Se(13) is bonded to one Na(3), two equivalent Na(2), one Cr(8), and two equivalent Cr(10) atoms to form SeNa3Cr3 octahedra that share corners with two equivalent Se(20)Na2Cr3 trigonal bipyramids, edges with two equivalent Se(13)Na3Cr3 octahedra, edges with two equivalent Se(9)Na3Cr3 octahedra, edges with two equivalent Se(1)Na2Cr3 square pyramids, and edges with two equivalent Se(17)Cr5 trigonal bipyramids. In the fourteenth Se site, Se(14) is bonded in a rectangular see-saw-like geometry to one Cr(8), one Cr(9), and two equivalent Cr(6) atoms. In the fifteenth Se site, Se(15) is bonded in a trigonal planar geometry to one Cr(9) and two equivalent Cr(11) atoms. In the sixteenth Se site, Se(16) is bonded in a 5-coordinate geometry to one Na(2), two equivalent Na(1), and two equivalent Cr(7) atoms. In the seventeenth Se site, Se(17) is bonded to one Cr(10), two equivalent Cr(8), and two equivalent Cr(9) atoms to form distorted SeCr5 trigonal bipyramids that share corners with two equivalent Se(9)Na3Cr3 octahedra,  a cornercorner with one Se(1)Na2Cr3 square pyramid,  an edgeedge with one Se(9)Na3Cr3 octahedra, edges with two equivalent Se(13)Na3Cr3 octahedra, and edges with two equivalent Se(17)Cr5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 3°. In the eighteenth Se site, Se(18) is bonded in a 4-coordinate geometry to one Na(3), two equivalent Na(2), and one Cr(11) atom. In the nineteenth Se site, Se(19) is bonded in a distorted T-shaped geometry to one Cr(1) and two equivalent Cr(10) atoms. In the twentieth Se site, Se(20) is bonded to two equivalent Na(3), one Cr(2), and two equivalent Cr(11) atoms to form distorted SeNa2Cr3 trigonal bipyramids that share corners with two equivalent Se(13)Na3Cr3 octahedra,  a cornercorner with one Se(4)Cr5 square pyramid, corners with two equivalent Se(1)Na2Cr3 square pyramids,  an edgeedge with one Se(1)Na2Cr3 square pyramid, and edges with two equivalent Se(20)Na2Cr3 trigonal bipyramids. The corner-sharing octahedral tilt angles are 43°.