ErTl2P2Se7 crystallizes in the monoclinic C2/c space group. Er(1) is bonded in a 7-coordinate geometry to one Se(2), one Se(3), one Se(5), one Se(6), one Se(7), and two equivalent Se(4) atoms. There are two inequivalent Tl sites. In the first Tl site, Tl(1) is bonded in a 3-coordinate geometry to one Se(1), one Se(3), and one Se(6) atom. In the second Tl site, Tl(2) is bonded in a distorted single-bond geometry to one Se(6) atom. There are two inequivalent P sites. In the first P site, P(1) is bonded in a tetrahedral geometry to one Se(3), one Se(4), one Se(5), and one Se(6) atom. In the second P site, P(2) is bonded in a distorted tetrahedral geometry to one P(2), one Se(1), one Se(2), and one Se(7) atom. There are seven inequivalent Se sites. In the first Se site, Se(7) is bonded in an L-shaped geometry to one Er(1) and one P(2) atom. In the second Se site, Se(1) is bonded in a distorted L-shaped geometry to one Tl(1) and one P(2) atom. In the third Se site, Se(2) is bonded in an L-shaped geometry to one Er(1) and one P(2) atom. In the fourth Se site, Se(6) is bonded in a 4-coordinate geometry to one Er(1), one Tl(1), one Tl(2), and one P(1) atom. In the fifth Se site, Se(3) is bonded in a 3-coordinate geometry to one Er(1), one Tl(1), and one P(1) atom. In the sixth Se site, Se(4) is bonded in a distorted T-shaped geometry to two equivalent Er(1) and one P(1) atom. In the seventh Se site, Se(5) is bonded in an L-shaped geometry to one Er(1) and one P(1) atom.
ErTl2P2Se7 crystallizes in the monoclinic C2/c space group. Er(1) is bonded in a 7-coordinate geometry to one Se(2), one Se(3), one Se(5), one Se(6), one Se(7), and two equivalent Se(4) atoms. The Er(1)-Se(2) bond length is 2.88 Å. The Er(1)-Se(3) bond length is 2.93 Å. The Er(1)-Se(5) bond length is 2.88 Å. The Er(1)-Se(6) bond length is 2.94 Å. The Er(1)-Se(7) bond length is 2.93 Å. There is one shorter (2.95 Å) and one longer (3.09 Å) Er(1)-Se(4) bond length. There are two inequivalent Tl sites. In the first Tl site, Tl(1) is bonded in a 3-coordinate geometry to one Se(1), one Se(3), and one Se(6) atom. The Tl(1)-Se(1) bond length is 3.16 Å. The Tl(1)-Se(3) bond length is 3.22 Å. The Tl(1)-Se(6) bond length is 3.27 Å. In the second Tl site, Tl(2) is bonded in a distorted single-bond geometry to one Se(6) atom. The Tl(2)-Se(6) bond length is 3.31 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded in a tetrahedral geometry to one Se(3), one Se(4), one Se(5), and one Se(6) atom. The P(1)-Se(3) bond length is 2.21 Å. The P(1)-Se(4) bond length is 2.22 Å. The P(1)-Se(5) bond length is 2.19 Å. The P(1)-Se(6) bond length is 2.21 Å. In the second P site, P(2) is bonded in a distorted tetrahedral geometry to one P(2), one Se(1), one Se(2), and one Se(7) atom. The P(2)-P(2) bond length is 2.24 Å. The P(2)-Se(1) bond length is 2.15 Å. The P(2)-Se(2) bond length is 2.19 Å. The P(2)-Se(7) bond length is 2.18 Å. There are seven inequivalent Se sites. In the first Se site, Se(7) is bonded in an L-shaped geometry to one Er(1) and one P(2) atom. In the second Se site, Se(1) is bonded in a distorted L-shaped geometry to one Tl(1) and one P(2) atom. In the third Se site, Se(2) is bonded in an L-shaped geometry to one Er(1) and one P(2) atom. In the fourth Se site, Se(6) is bonded in a 4-coordinate geometry to one Er(1), one Tl(1), one Tl(2), and one P(1) atom. In the fifth Se site, Se(3) is bonded in a 3-coordinate geometry to one Er(1), one Tl(1), and one P(1) atom. In the sixth Se site, Se(4) is bonded in a distorted T-shaped geometry to two equivalent Er(1) and one P(1) atom. In the seventh Se site, Se(5) is bonded in an L-shaped geometry to one Er(1) and one P(1) atom.