Er2B4O9 crystallizes in the triclinic P-1 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 9-coordinate geometry to one O(2), one O(3), one O(5), one O(7), one O(9), two equivalent O(1), and two equivalent O(4) atoms. In the second Er site, Er(2) is bonded in a 10-coordinate geometry to one O(1), one O(2), one O(4), one O(5), one O(6), one O(9), two equivalent O(7), and two equivalent O(8) atoms. There are four inequivalent B sites. In the first B site, B(1) is bonded in a trigonal planar geometry to one O(1), one O(2), and one O(6) atom. In the second B site, B(2) is bonded to one O(2), one O(3), one O(5), and one O(8) atom to form corner-sharing BO4 tetrahedra. In the third B site, B(3) is bonded to one O(4), one O(5), one O(7), and one O(9) atom to form corner-sharing BO4 tetrahedra. In the fourth B site, B(4) is bonded to one O(3), one O(6), one O(8), and one O(9) atom to form corner-sharing BO4 tetrahedra. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Er(2), two equivalent Er(1), and one B(1) atom. In the second O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Er(1), one Er(2), one B(1), and one B(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Er(1), one B(2), and one B(4) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to one Er(2), two equivalent Er(1), and one B(3) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Er(1), one Er(2), one B(2), and one B(3) atom. In the sixth O site, O(6) is bonded in a distorted bent 120 degrees geometry to one Er(2), one B(1), and one B(4) atom. In the seventh O site, O(7) is bonded to one Er(1), two equivalent Er(2), and one B(3) atom to form distorted edge-sharing OEr3B tetrahedra. In the eighth O site, O(8) is bonded in a distorted bent 120 degrees geometry to two equivalent Er(2), one B(2), and one B(4) atom. In the ninth O site, O(9) is bonded in a bent 120 degrees geometry to one Er(1), one Er(2), one B(3), and one B(4) atom.
Er2B4O9 crystallizes in the triclinic P-1 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 9-coordinate geometry to one O(2), one O(3), one O(5), one O(7), one O(9), two equivalent O(1), and two equivalent O(4) atoms. The Er(1)-O(2) bond length is 2.65 Å. The Er(1)-O(3) bond length is 2.29 Å. The Er(1)-O(5) bond length is 2.53 Å. The Er(1)-O(7) bond length is 2.37 Å. The Er(1)-O(9) bond length is 2.63 Å. There is one shorter (2.39 Å) and one longer (2.58 Å) Er(1)-O(1) bond length. There is one shorter (2.34 Å) and one longer (2.36 Å) Er(1)-O(4) bond length. In the second Er site, Er(2) is bonded in a 10-coordinate geometry to one O(1), one O(2), one O(4), one O(5), one O(6), one O(9), two equivalent O(7), and two equivalent O(8) atoms. The Er(2)-O(1) bond length is 2.42 Å. The Er(2)-O(2) bond length is 2.53 Å. The Er(2)-O(4) bond length is 2.40 Å. The Er(2)-O(5) bond length is 2.55 Å. The Er(2)-O(6) bond length is 2.51 Å. The Er(2)-O(9) bond length is 2.62 Å. There is one shorter (2.24 Å) and one longer (2.34 Å) Er(2)-O(7) bond length. There is one shorter (2.39 Å) and one longer (2.59 Å) Er(2)-O(8) bond length. There are four inequivalent B sites. In the first B site, B(1) is bonded in a trigonal planar geometry to one O(1), one O(2), and one O(6) atom. The B(1)-O(1) bond length is 1.37 Å. The B(1)-O(2) bond length is 1.40 Å. The B(1)-O(6) bond length is 1.37 Å. In the second B site, B(2) is bonded to one O(2), one O(3), one O(5), and one O(8) atom to form corner-sharing BO4 tetrahedra. The B(2)-O(2) bond length is 1.51 Å. The B(2)-O(3) bond length is 1.46 Å. The B(2)-O(5) bond length is 1.47 Å. The B(2)-O(8) bond length is 1.50 Å. In the third B site, B(3) is bonded to one O(4), one O(5), one O(7), and one O(9) atom to form corner-sharing BO4 tetrahedra. The B(3)-O(4) bond length is 1.47 Å. The B(3)-O(5) bond length is 1.56 Å. The B(3)-O(7) bond length is 1.49 Å. The B(3)-O(9) bond length is 1.47 Å. In the fourth B site, B(4) is bonded to one O(3), one O(6), one O(8), and one O(9) atom to form corner-sharing BO4 tetrahedra. The B(4)-O(3) bond length is 1.44 Å. The B(4)-O(6) bond length is 1.49 Å. The B(4)-O(8) bond length is 1.47 Å. The B(4)-O(9) bond length is 1.46 Å. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Er(2), two equivalent Er(1), and one B(1) atom. In the second O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Er(1), one Er(2), one B(1), and one B(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Er(1), one B(2), and one B(4) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to one Er(2), two equivalent Er(1), and one B(3) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Er(1), one Er(2), one B(2), and one B(3) atom. In the sixth O site, O(6) is bonded in a distorted bent 120 degrees geometry to one Er(2), one B(1), and one B(4) atom. In the seventh O site, O(7) is bonded to one Er(1), two equivalent Er(2), and one B(3) atom to form distorted edge-sharing OEr3B tetrahedra. In the eighth O site, O(8) is bonded in a distorted bent 120 degrees geometry to two equivalent Er(2), one B(2), and one B(4) atom. In the ninth O site, O(9) is bonded in a bent 120 degrees geometry to one Er(1), one Er(2), one B(3), and one B(4) atom.