ErBO3 crystallizes in the monoclinic C2/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form ErO8 hexagonal bipyramids that share corners with two equivalent Er(2)O8 hexagonal bipyramids, corners with two equivalent B(1)O4 tetrahedra, edges with six equivalent Er(2)O8 hexagonal bipyramids, edges with two equivalent B(1)O4 tetrahedra, and edges with two equivalent B(2)O4 tetrahedra. In the second Er site, Er(2) is bonded to one O(4), one O(5), two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form distorted ErO8 hexagonal bipyramids that share  a cornercorner with one Er(1)O8 hexagonal bipyramid,  a cornercorner with one Er(2)O8 hexagonal bipyramid,  a cornercorner with one B(1)O4 tetrahedra,  a cornercorner with one B(2)O4 tetrahedra, edges with three equivalent Er(1)O8 hexagonal bipyramids, edges with three equivalent Er(2)O8 hexagonal bipyramids,  an edgeedge with one B(2)O4 tetrahedra, and edges with three equivalent B(1)O4 tetrahedra. There are two inequivalent B sites. In the first B site, B(1) is bonded to one O(1), one O(2), one O(4), and one O(5) atom to form BO4 tetrahedra that share  a cornercorner with one Er(1)O8 hexagonal bipyramid,  a cornercorner with one Er(2)O8 hexagonal bipyramid,  a cornercorner with one B(1)O4 tetrahedra,  a cornercorner with one B(2)O4 tetrahedra,  an edgeedge with one Er(1)O8 hexagonal bipyramid, and edges with three equivalent Er(2)O8 hexagonal bipyramids. In the second B site, B(2) is bonded to two equivalent O(3) and two equivalent O(4) atoms to form BO4 tetrahedra that share corners with two equivalent Er(2)O8 hexagonal bipyramids, corners with two equivalent B(1)O4 tetrahedra, edges with two equivalent Er(1)O8 hexagonal bipyramids, and edges with two equivalent Er(2)O8 hexagonal bipyramids. There are five inequivalent O sites. In the first O site, O(3) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one B(2) atom. In the second O site, O(4) is bonded in a 4-coordinate geometry to one Er(1), one Er(2), one B(1), and one B(2) atom. In the third O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Er(2) and two equivalent B(1) atoms. In the fourth O site, O(1) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one B(1) atom. In the fifth O site, O(2) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one B(1) atom.
ErBO3 crystallizes in the monoclinic C2/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form ErO8 hexagonal bipyramids that share corners with two equivalent Er(2)O8 hexagonal bipyramids, corners with two equivalent B(1)O4 tetrahedra, edges with six equivalent Er(2)O8 hexagonal bipyramids, edges with two equivalent B(1)O4 tetrahedra, and edges with two equivalent B(2)O4 tetrahedra. Both Er(1)-O(1) bond lengths are 2.22 Å. Both Er(1)-O(2) bond lengths are 2.43 Å. Both Er(1)-O(3) bond lengths are 2.41 Å. Both Er(1)-O(4) bond lengths are 2.33 Å. In the second Er site, Er(2) is bonded to one O(4), one O(5), two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form distorted ErO8 hexagonal bipyramids that share  a cornercorner with one Er(1)O8 hexagonal bipyramid,  a cornercorner with one Er(2)O8 hexagonal bipyramid,  a cornercorner with one B(1)O4 tetrahedra,  a cornercorner with one B(2)O4 tetrahedra, edges with three equivalent Er(1)O8 hexagonal bipyramids, edges with three equivalent Er(2)O8 hexagonal bipyramids,  an edgeedge with one B(2)O4 tetrahedra, and edges with three equivalent B(1)O4 tetrahedra. The Er(2)-O(4) bond length is 2.31 Å. The Er(2)-O(5) bond length is 2.32 Å. Both Er(2)-O(1) bond lengths are 2.43 Å. There is one shorter (2.21 Å) and one longer (2.42 Å) Er(2)-O(2) bond length. There is one shorter (2.20 Å) and one longer (2.42 Å) Er(2)-O(3) bond length. There are two inequivalent B sites. In the first B site, B(1) is bonded to one O(1), one O(2), one O(4), and one O(5) atom to form BO4 tetrahedra that share  a cornercorner with one Er(1)O8 hexagonal bipyramid,  a cornercorner with one Er(2)O8 hexagonal bipyramid,  a cornercorner with one B(1)O4 tetrahedra,  a cornercorner with one B(2)O4 tetrahedra,  an edgeedge with one Er(1)O8 hexagonal bipyramid, and edges with three equivalent Er(2)O8 hexagonal bipyramids. The B(1)-O(1) bond length is 1.45 Å. The B(1)-O(2) bond length is 1.44 Å. The B(1)-O(4) bond length is 1.50 Å. The B(1)-O(5) bond length is 1.50 Å. In the second B site, B(2) is bonded to two equivalent O(3) and two equivalent O(4) atoms to form BO4 tetrahedra that share corners with two equivalent Er(2)O8 hexagonal bipyramids, corners with two equivalent B(1)O4 tetrahedra, edges with two equivalent Er(1)O8 hexagonal bipyramids, and edges with two equivalent Er(2)O8 hexagonal bipyramids. Both B(2)-O(3) bond lengths are 1.44 Å. Both B(2)-O(4) bond lengths are 1.50 Å. There are five inequivalent O sites. In the first O site, O(3) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one B(2) atom. In the second O site, O(4) is bonded in a 4-coordinate geometry to one Er(1), one Er(2), one B(1), and one B(2) atom. In the third O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Er(2) and two equivalent B(1) atoms. In the fourth O site, O(1) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one B(1) atom. In the fifth O site, O(2) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one B(1) atom.