K2MnSnSe4 crystallizes in the tetragonal P-4n2 space group. There are three inequivalent K sites. In the first K site, K(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. In the second K site, K(2) is bonded in a distorted square co-planar geometry to four equivalent Se(2) atoms. In the third K site, K(3) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to four equivalent Se(1) atoms to form distorted MnSe4 trigonal pyramids that share corners with four equivalent Sn(1)Se4 tetrahedra. In the second Mn site, Mn(2) is bonded to four equivalent Se(2) atoms to form distorted MnSe4 tetrahedra that share edges with two equivalent Sn(1)Se4 tetrahedra. Sn(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form SnSe4 tetrahedra that share corners with two equivalent Mn(1)Se4 trigonal pyramids and  an edgeedge with one Mn(2)Se4 tetrahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 5-coordinate geometry to one K(3), two equivalent K(1), one Mn(1), and one Sn(1) atom. In the second Se site, Se(2) is bonded in a 6-coordinate geometry to one K(2), one K(3), two equivalent K(1), one Mn(2), and one Sn(1) atom.
K2MnSnSe4 crystallizes in the tetragonal P-4n2 space group. There are three inequivalent K sites. In the first K site, K(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. There are two shorter (3.58 Å) and two longer (3.61 Å) K(1)-Se(1) bond lengths. There are two shorter (3.41 Å) and two longer (3.59 Å) K(1)-Se(2) bond lengths. In the second K site, K(2) is bonded in a distorted square co-planar geometry to four equivalent Se(2) atoms. All K(2)-Se(2) bond lengths are 3.31 Å. In the third K site, K(3) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. All K(3)-Se(1) bond lengths are 3.37 Å. All K(3)-Se(2) bond lengths are 3.41 Å. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to four equivalent Se(1) atoms to form distorted MnSe4 trigonal pyramids that share corners with four equivalent Sn(1)Se4 tetrahedra. All Mn(1)-Se(1) bond lengths are 2.54 Å. In the second Mn site, Mn(2) is bonded to four equivalent Se(2) atoms to form distorted MnSe4 tetrahedra that share edges with two equivalent Sn(1)Se4 tetrahedra. All Mn(2)-Se(2) bond lengths are 2.58 Å. Sn(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form SnSe4 tetrahedra that share corners with two equivalent Mn(1)Se4 trigonal pyramids and  an edgeedge with one Mn(2)Se4 tetrahedra. Both Sn(1)-Se(1) bond lengths are 2.55 Å. Both Sn(1)-Se(2) bond lengths are 2.55 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 5-coordinate geometry to one K(3), two equivalent K(1), one Mn(1), and one Sn(1) atom. In the second Se site, Se(2) is bonded in a 6-coordinate geometry to one K(2), one K(3), two equivalent K(1), one Mn(2), and one Sn(1) atom.