Ga4B2O9 crystallizes in the monoclinic C2/m space group. There are four inequivalent Ga sites. In the first Ga site, Ga(1) is bonded to one O(4), two equivalent O(1), and two equivalent O(7) atoms to form GaO5 trigonal bipyramids that share corners with two equivalent Ga(2)O6 octahedra, corners with two equivalent Ga(3)O6 octahedra, corners with two equivalent Ga(4)O4 tetrahedra, and  an edgeedge with one Ga(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 51-69°. In the second Ga site, Ga(2) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(6) atoms to form GaO6 octahedra that share corners with two equivalent Ga(4)O4 tetrahedra, corners with two equivalent Ga(1)O5 trigonal bipyramids, and edges with two equivalent Ga(2)O6 octahedra. In the third Ga site, Ga(3) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form GaO6 octahedra that share corners with two equivalent Ga(4)O4 tetrahedra, corners with two equivalent Ga(1)O5 trigonal bipyramids, and edges with two equivalent Ga(3)O6 octahedra. In the fourth Ga site, Ga(4) is bonded to one O(3), one O(7), and two equivalent O(2) atoms to form distorted GaO4 tetrahedra that share corners with two equivalent Ga(2)O6 octahedra, corners with two equivalent Ga(3)O6 octahedra, and corners with two equivalent Ga(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 57-80°. There are two inequivalent B sites. In the first B site, B(1) is bonded in a trigonal planar geometry to one O(6) and two equivalent O(2) atoms. In the second B site, B(2) is bonded in a trigonal planar geometry to one O(5) and two equivalent O(1) atoms. There are seven inequivalent O sites. In the first O site, O(3) is bonded in a trigonal planar geometry to one Ga(4) and two equivalent Ga(2) atoms. In the second O site, O(4) is bonded in a 3-coordinate geometry to one Ga(1) and two equivalent Ga(3) atoms. In the third O site, O(5) is bonded in a distorted T-shaped geometry to two equivalent Ga(3) and one B(2) atom. In the fourth O site, O(6) is bonded in a distorted T-shaped geometry to two equivalent Ga(2) and one B(1) atom. In the fifth O site, O(7) is bonded in a trigonal planar geometry to one Ga(4) and two equivalent Ga(1) atoms. In the sixth O site, O(1) is bonded in a distorted trigonal planar geometry to one Ga(1), one Ga(2), and one B(2) atom. In the seventh O site, O(2) is bonded in a distorted trigonal planar geometry to one Ga(3), one Ga(4), and one B(1) atom.
Ga4B2O9 crystallizes in the monoclinic C2/m space group. There are four inequivalent Ga sites. In the first Ga site, Ga(1) is bonded to one O(4), two equivalent O(1), and two equivalent O(7) atoms to form GaO5 trigonal bipyramids that share corners with two equivalent Ga(2)O6 octahedra, corners with two equivalent Ga(3)O6 octahedra, corners with two equivalent Ga(4)O4 tetrahedra, and  an edgeedge with one Ga(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 51-69°. The Ga(1)-O(4) bond length is 1.97 Å. Both Ga(1)-O(1) bond lengths are 1.94 Å. There is one shorter (1.90 Å) and one longer (1.91 Å) Ga(1)-O(7) bond length. In the second Ga site, Ga(2) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(6) atoms to form GaO6 octahedra that share corners with two equivalent Ga(4)O4 tetrahedra, corners with two equivalent Ga(1)O5 trigonal bipyramids, and edges with two equivalent Ga(2)O6 octahedra. Both Ga(2)-O(1) bond lengths are 2.27 Å. Both Ga(2)-O(3) bond lengths are 1.83 Å. Both Ga(2)-O(6) bond lengths are 2.06 Å. In the third Ga site, Ga(3) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form GaO6 octahedra that share corners with two equivalent Ga(4)O4 tetrahedra, corners with two equivalent Ga(1)O5 trigonal bipyramids, and edges with two equivalent Ga(3)O6 octahedra. Both Ga(3)-O(2) bond lengths are 2.23 Å. Both Ga(3)-O(4) bond lengths are 1.84 Å. Both Ga(3)-O(5) bond lengths are 2.04 Å. In the fourth Ga site, Ga(4) is bonded to one O(3), one O(7), and two equivalent O(2) atoms to form distorted GaO4 tetrahedra that share corners with two equivalent Ga(2)O6 octahedra, corners with two equivalent Ga(3)O6 octahedra, and corners with two equivalent Ga(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 57-80°. The Ga(4)-O(3) bond length is 1.85 Å. The Ga(4)-O(7) bond length is 1.79 Å. Both Ga(4)-O(2) bond lengths are 1.87 Å. There are two inequivalent B sites. In the first B site, B(1) is bonded in a trigonal planar geometry to one O(6) and two equivalent O(2) atoms. The B(1)-O(6) bond length is 1.39 Å. Both B(1)-O(2) bond lengths are 1.40 Å. In the second B site, B(2) is bonded in a trigonal planar geometry to one O(5) and two equivalent O(1) atoms. The B(2)-O(5) bond length is 1.40 Å. Both B(2)-O(1) bond lengths are 1.40 Å. There are seven inequivalent O sites. In the first O site, O(3) is bonded in a trigonal planar geometry to one Ga(4) and two equivalent Ga(2) atoms. In the second O site, O(4) is bonded in a 3-coordinate geometry to one Ga(1) and two equivalent Ga(3) atoms. In the third O site, O(5) is bonded in a distorted T-shaped geometry to two equivalent Ga(3) and one B(2) atom. In the fourth O site, O(6) is bonded in a distorted T-shaped geometry to two equivalent Ga(2) and one B(1) atom. In the fifth O site, O(7) is bonded in a trigonal planar geometry to one Ga(4) and two equivalent Ga(1) atoms. In the sixth O site, O(1) is bonded in a distorted trigonal planar geometry to one Ga(1), one Ga(2), and one B(2) atom. In the seventh O site, O(2) is bonded in a distorted trigonal planar geometry to one Ga(3), one Ga(4), and one B(1) atom.