Eu2Si2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded in a 8-coordinate geometry to one O(4), one O(5), one O(6), one O(7), two equivalent O(2), and two equivalent O(3) atoms. In the second Eu site, Eu(2) is bonded in a 9-coordinate geometry to one O(1), one O(2), one O(3), one O(5), one O(7), two equivalent O(4), and two equivalent O(6) atoms. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(5), and one O(6) atom to form corner-sharing SiO4 tetrahedra. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(4), and one O(7) atom to form corner-sharing SiO4 tetrahedra. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Eu(1), one Eu(2), and one Si(1) atom. In the second O site, O(6) is bonded in a 1-coordinate geometry to one Eu(1), two equivalent Eu(2), and one Si(1) atom. In the third O site, O(7) is bonded in a distorted trigonal planar geometry to one Eu(1), one Eu(2), and one Si(2) atom. In the fourth O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Eu(2), one Si(1), and one Si(2) atom. In the fifth O site, O(2) is bonded to one Eu(2), two equivalent Eu(1), and one Si(1) atom to form distorted edge-sharing OEu3Si tetrahedra. In the sixth O site, O(3) is bonded in a 4-coordinate geometry to one Eu(2), two equivalent Eu(1), and one Si(2) atom. In the seventh O site, O(4) is bonded in a distorted single-bond geometry to one Eu(1), two equivalent Eu(2), and one Si(2) atom.
Eu2Si2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded in a 8-coordinate geometry to one O(4), one O(5), one O(6), one O(7), two equivalent O(2), and two equivalent O(3) atoms. The Eu(1)-O(4) bond length is 2.55 Å. The Eu(1)-O(5) bond length is 2.41 Å. The Eu(1)-O(6) bond length is 2.61 Å. The Eu(1)-O(7) bond length is 2.36 Å. There is one shorter (2.45 Å) and one longer (2.48 Å) Eu(1)-O(2) bond length. There is one shorter (2.60 Å) and one longer (2.64 Å) Eu(1)-O(3) bond length. In the second Eu site, Eu(2) is bonded in a 9-coordinate geometry to one O(1), one O(2), one O(3), one O(5), one O(7), two equivalent O(4), and two equivalent O(6) atoms. The Eu(2)-O(1) bond length is 2.72 Å. The Eu(2)-O(2) bond length is 2.53 Å. The Eu(2)-O(3) bond length is 2.42 Å. The Eu(2)-O(5) bond length is 2.43 Å. The Eu(2)-O(7) bond length is 2.47 Å. There is one shorter (2.51 Å) and one longer (3.04 Å) Eu(2)-O(4) bond length. There is one shorter (2.39 Å) and one longer (3.07 Å) Eu(2)-O(6) bond length. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(5), and one O(6) atom to form corner-sharing SiO4 tetrahedra. The Si(1)-O(1) bond length is 1.67 Å. The Si(1)-O(2) bond length is 1.65 Å. The Si(1)-O(5) bond length is 1.62 Å. The Si(1)-O(6) bond length is 1.64 Å. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(4), and one O(7) atom to form corner-sharing SiO4 tetrahedra. The Si(2)-O(1) bond length is 1.67 Å. The Si(2)-O(3) bond length is 1.65 Å. The Si(2)-O(4) bond length is 1.64 Å. The Si(2)-O(7) bond length is 1.63 Å. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Eu(1), one Eu(2), and one Si(1) atom. In the second O site, O(6) is bonded in a 1-coordinate geometry to one Eu(1), two equivalent Eu(2), and one Si(1) atom. In the third O site, O(7) is bonded in a distorted trigonal planar geometry to one Eu(1), one Eu(2), and one Si(2) atom. In the fourth O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Eu(2), one Si(1), and one Si(2) atom. In the fifth O site, O(2) is bonded to one Eu(2), two equivalent Eu(1), and one Si(1) atom to form distorted edge-sharing OEu3Si tetrahedra. In the sixth O site, O(3) is bonded in a 4-coordinate geometry to one Eu(2), two equivalent Eu(1), and one Si(2) atom. In the seventh O site, O(4) is bonded in a distorted single-bond geometry to one Eu(1), two equivalent Eu(2), and one Si(2) atom.