Ba2As2Se5 crystallizes in the monoclinic P2_1 space group. There are four inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to one Se(1), one Se(10), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), one Se(8), and one Se(9) atom. In the second Ba site, Ba(2) is bonded in a 8-coordinate geometry to one Se(1), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), one Se(8), and one Se(9) atom. In the third Ba site, Ba(3) is bonded in a 9-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), one Se(8), one Se(9), and two equivalent Se(2) atoms. In the fourth Ba site, Ba(4) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), one Se(8), and two equivalent Se(10) atoms. There are four inequivalent As sites. In the first As site, As(1) is bonded in a 3-coordinate geometry to one Se(1), one Se(2), and one Se(7) atom. In the second As site, As(2) is bonded in a distorted trigonal non-coplanar geometry to one Se(10), one Se(4), and one Se(5) atom. In the third As site, As(3) is bonded in a distorted trigonal non-coplanar geometry to one Se(1), one Se(2), and one Se(8) atom. In the fourth As site, As(4) is bonded in a distorted trigonal non-coplanar geometry to one Se(3), one Se(6), and one Se(9) atom. There are ten inequivalent Se sites. In the first Se site, Se(9) is bonded in a distorted rectangular see-saw-like geometry to one Ba(1), one Ba(2), one Ba(3), and one As(4) atom. In the second Se site, Se(10) is bonded in a distorted rectangular see-saw-like geometry to one Ba(1), two equivalent Ba(4), and one As(2) atom. In the third Se site, Se(1) is bonded to one Ba(1), one Ba(2), one As(1), and one As(3) atom to form a mixture of distorted edge and corner-sharing SeBa2As2 trigonal pyramids. In the fourth Se site, Se(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ba(3), one As(1), and one As(3) atom. In the fifth Se site, Se(3) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(4) atom to form distorted SeBa4As square pyramids that share corners with two equivalent Se(4)Ba4As square pyramids, corners with four equivalent Se(5)Ba4As square pyramids, corners with two equivalent Se(1)Ba2As2 trigonal pyramids,  an edgeedge with one Se(4)Ba4As square pyramid,  an edgeedge with one Se(6)Ba4As square pyramid, edges with two equivalent Se(8)Ba4As trigonal bipyramids, and  a faceface with one Se(6)Ba4As square pyramid. In the sixth Se site, Se(4) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(2) atom to form SeBa4As square pyramids that share corners with two equivalent Se(3)Ba4As square pyramids, corners with two equivalent Se(5)Ba4As square pyramids, corners with two equivalent Se(6)Ba4As square pyramids, corners with two equivalent Se(8)Ba4As trigonal bipyramids, corners with two equivalent Se(1)Ba2As2 trigonal pyramids,  an edgeedge with one Se(3)Ba4As square pyramid,  an edgeedge with one Se(6)Ba4As square pyramid,  an edgeedge with one Se(8)Ba4As trigonal bipyramid, and  a faceface with one Se(5)Ba4As square pyramid. In the seventh Se site, Se(5) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(2) atom to form distorted SeBa4As square pyramids that share corners with two equivalent Se(4)Ba4As square pyramids, corners with four equivalent Se(3)Ba4As square pyramids, corners with two equivalent Se(1)Ba2As2 trigonal pyramids, edges with two equivalent Se(6)Ba4As square pyramids, edges with two equivalent Se(8)Ba4As trigonal bipyramids, and  a faceface with one Se(4)Ba4As square pyramid. In the eighth Se site, Se(6) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(4) atom to form distorted SeBa4As square pyramids that share corners with two equivalent Se(4)Ba4As square pyramids, corners with four equivalent Se(8)Ba4As trigonal bipyramids, corners with two equivalent Se(1)Ba2As2 trigonal pyramids,  an edgeedge with one Se(3)Ba4As square pyramid,  an edgeedge with one Se(4)Ba4As square pyramid, edges with two equivalent Se(5)Ba4As square pyramids, and  a faceface with one Se(3)Ba4As square pyramid. In the ninth Se site, Se(7) is bonded in a 5-coordinate geometry to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(1) atom. In the tenth Se site, Se(8) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(3) atom to form distorted SeBa4As trigonal bipyramids that share corners with two equivalent Se(4)Ba4As square pyramids, corners with four equivalent Se(6)Ba4As square pyramids,  a cornercorner with one Se(1)Ba2As2 trigonal pyramid,  an edgeedge with one Se(4)Ba4As square pyramid, edges with two equivalent Se(3)Ba4As square pyramids, edges with two equivalent Se(5)Ba4As square pyramids, and  an edgeedge with one Se(1)Ba2As2 trigonal pyramid.
Ba2As2Se5 crystallizes in the monoclinic P2_1 space group. There are four inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to one Se(1), one Se(10), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), one Se(8), and one Se(9) atom. The Ba(1)-Se(1) bond length is 3.61 Å. The Ba(1)-Se(10) bond length is 3.47 Å. The Ba(1)-Se(3) bond length is 3.45 Å. The Ba(1)-Se(4) bond length is 3.45 Å. The Ba(1)-Se(5) bond length is 3.39 Å. The Ba(1)-Se(6) bond length is 3.32 Å. The Ba(1)-Se(7) bond length is 3.81 Å. The Ba(1)-Se(8) bond length is 3.53 Å. The Ba(1)-Se(9) bond length is 3.33 Å. In the second Ba site, Ba(2) is bonded in a 8-coordinate geometry to one Se(1), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), one Se(8), and one Se(9) atom. The Ba(2)-Se(1) bond length is 3.48 Å. The Ba(2)-Se(3) bond length is 3.41 Å. The Ba(2)-Se(4) bond length is 3.54 Å. The Ba(2)-Se(5) bond length is 3.32 Å. The Ba(2)-Se(6) bond length is 3.40 Å. The Ba(2)-Se(7) bond length is 3.39 Å. The Ba(2)-Se(8) bond length is 3.45 Å. The Ba(2)-Se(9) bond length is 3.49 Å. In the third Ba site, Ba(3) is bonded in a 9-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), one Se(8), one Se(9), and two equivalent Se(2) atoms. The Ba(3)-Se(3) bond length is 3.47 Å. The Ba(3)-Se(4) bond length is 3.39 Å. The Ba(3)-Se(5) bond length is 3.34 Å. The Ba(3)-Se(6) bond length is 3.41 Å. The Ba(3)-Se(7) bond length is 3.46 Å. The Ba(3)-Se(8) bond length is 3.43 Å. The Ba(3)-Se(9) bond length is 3.53 Å. There is one shorter (3.54 Å) and one longer (3.59 Å) Ba(3)-Se(2) bond length. In the fourth Ba site, Ba(4) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), one Se(8), and two equivalent Se(10) atoms. The Ba(4)-Se(3) bond length is 3.57 Å. The Ba(4)-Se(4) bond length is 3.38 Å. The Ba(4)-Se(5) bond length is 3.30 Å. The Ba(4)-Se(6) bond length is 3.39 Å. The Ba(4)-Se(7) bond length is 3.70 Å. The Ba(4)-Se(8) bond length is 3.58 Å. There is one shorter (3.32 Å) and one longer (3.49 Å) Ba(4)-Se(10) bond length. There are four inequivalent As sites. In the first As site, As(1) is bonded in a 3-coordinate geometry to one Se(1), one Se(2), and one Se(7) atom. The As(1)-Se(1) bond length is 2.51 Å. The As(1)-Se(2) bond length is 2.56 Å. The As(1)-Se(7) bond length is 2.39 Å. In the second As site, As(2) is bonded in a distorted trigonal non-coplanar geometry to one Se(10), one Se(4), and one Se(5) atom. The As(2)-Se(10) bond length is 2.37 Å. The As(2)-Se(4) bond length is 2.41 Å. The As(2)-Se(5) bond length is 2.40 Å. In the third As site, As(3) is bonded in a distorted trigonal non-coplanar geometry to one Se(1), one Se(2), and one Se(8) atom. The As(3)-Se(1) bond length is 2.51 Å. The As(3)-Se(2) bond length is 2.47 Å. The As(3)-Se(8) bond length is 2.39 Å. In the fourth As site, As(4) is bonded in a distorted trigonal non-coplanar geometry to one Se(3), one Se(6), and one Se(9) atom. The As(4)-Se(3) bond length is 2.43 Å. The As(4)-Se(6) bond length is 2.37 Å. The As(4)-Se(9) bond length is 2.35 Å. There are ten inequivalent Se sites. In the first Se site, Se(9) is bonded in a distorted rectangular see-saw-like geometry to one Ba(1), one Ba(2), one Ba(3), and one As(4) atom. In the second Se site, Se(10) is bonded in a distorted rectangular see-saw-like geometry to one Ba(1), two equivalent Ba(4), and one As(2) atom. In the third Se site, Se(1) is bonded to one Ba(1), one Ba(2), one As(1), and one As(3) atom to form a mixture of distorted edge and corner-sharing SeBa2As2 trigonal pyramids. In the fourth Se site, Se(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ba(3), one As(1), and one As(3) atom. In the fifth Se site, Se(3) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(4) atom to form distorted SeBa4As square pyramids that share corners with two equivalent Se(4)Ba4As square pyramids, corners with four equivalent Se(5)Ba4As square pyramids, corners with two equivalent Se(1)Ba2As2 trigonal pyramids,  an edgeedge with one Se(4)Ba4As square pyramid,  an edgeedge with one Se(6)Ba4As square pyramid, edges with two equivalent Se(8)Ba4As trigonal bipyramids, and  a faceface with one Se(6)Ba4As square pyramid. In the sixth Se site, Se(4) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(2) atom to form SeBa4As square pyramids that share corners with two equivalent Se(3)Ba4As square pyramids, corners with two equivalent Se(5)Ba4As square pyramids, corners with two equivalent Se(6)Ba4As square pyramids, corners with two equivalent Se(8)Ba4As trigonal bipyramids, corners with two equivalent Se(1)Ba2As2 trigonal pyramids,  an edgeedge with one Se(3)Ba4As square pyramid,  an edgeedge with one Se(6)Ba4As square pyramid,  an edgeedge with one Se(8)Ba4As trigonal bipyramid, and  a faceface with one Se(5)Ba4As square pyramid. In the seventh Se site, Se(5) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(2) atom to form distorted SeBa4As square pyramids that share corners with two equivalent Se(4)Ba4As square pyramids, corners with four equivalent Se(3)Ba4As square pyramids, corners with two equivalent Se(1)Ba2As2 trigonal pyramids, edges with two equivalent Se(6)Ba4As square pyramids, edges with two equivalent Se(8)Ba4As trigonal bipyramids, and  a faceface with one Se(4)Ba4As square pyramid. In the eighth Se site, Se(6) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(4) atom to form distorted SeBa4As square pyramids that share corners with two equivalent Se(4)Ba4As square pyramids, corners with four equivalent Se(8)Ba4As trigonal bipyramids, corners with two equivalent Se(1)Ba2As2 trigonal pyramids,  an edgeedge with one Se(3)Ba4As square pyramid,  an edgeedge with one Se(4)Ba4As square pyramid, edges with two equivalent Se(5)Ba4As square pyramids, and  a faceface with one Se(3)Ba4As square pyramid. In the ninth Se site, Se(7) is bonded in a 5-coordinate geometry to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(1) atom. In the tenth Se site, Se(8) is bonded to one Ba(1), one Ba(2), one Ba(3), one Ba(4), and one As(3) atom to form distorted SeBa4As trigonal bipyramids that share corners with two equivalent Se(4)Ba4As square pyramids, corners with four equivalent Se(6)Ba4As square pyramids,  a cornercorner with one Se(1)Ba2As2 trigonal pyramid,  an edgeedge with one Se(4)Ba4As square pyramid, edges with two equivalent Se(3)Ba4As square pyramids, edges with two equivalent Se(5)Ba4As square pyramids, and  an edgeedge with one Se(1)Ba2As2 trigonal pyramid.