Li2FeSiO4 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form LiO4 tetrahedra that share corners with two equivalent Li(1)O4 tetrahedra, corners with four equivalent Si(1)O4 tetrahedra, and corners with two equivalent Fe(1)O4 trigonal pyramids. In the second Li site, Li(2) is bonded in a see-saw-like geometry to one O(1), one O(2), one O(3), and one O(4) atom. Fe(1) is bonded to one O(2), one O(3), and two equivalent O(4) atoms to form distorted FeO4 trigonal pyramids that share corners with two equivalent Li(1)O4 tetrahedra, corners with four equivalent Si(1)O4 tetrahedra, and corners with two equivalent Fe(1)O4 trigonal pyramids. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share corners with four equivalent Li(1)O4 tetrahedra and corners with four equivalent Fe(1)O4 trigonal pyramids. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Li(2), two equivalent Li(1), and one Si(1) atom to form distorted OLi3Si tetrahedra that share corners with two equivalent O(1)Li3Si tetrahedra, corners with two equivalent O(4)LiFe2Si tetrahedra, corners with two equivalent O(2)Li2FeSi trigonal pyramids, corners with two equivalent O(3)Li2FeSi trigonal pyramids,  an edgeedge with one O(2)Li2FeSi trigonal pyramid, and  an edgeedge with one O(3)Li2FeSi trigonal pyramid. In the second O site, O(2) is bonded to one Li(1), one Li(2), one Fe(1), and one Si(1) atom to form distorted OLi2FeSi trigonal pyramids that share corners with two equivalent O(1)Li3Si tetrahedra, corners with two equivalent O(4)LiFe2Si tetrahedra, corners with four equivalent O(3)Li2FeSi trigonal pyramids,  an edgeedge with one O(1)Li3Si tetrahedra, and  an edgeedge with one O(4)LiFe2Si tetrahedra. In the third O site, O(3) is bonded to one Li(1), one Li(2), one Fe(1), and one Si(1) atom to form distorted OLi2FeSi trigonal pyramids that share corners with two equivalent O(1)Li3Si tetrahedra, corners with two equivalent O(4)LiFe2Si tetrahedra, corners with four equivalent O(2)Li2FeSi trigonal pyramids,  an edgeedge with one O(1)Li3Si tetrahedra, and  an edgeedge with one O(4)LiFe2Si tetrahedra. In the fourth O site, O(4) is bonded to one Li(2), two equivalent Fe(1), and one Si(1) atom to form distorted OLiFe2Si tetrahedra that share corners with two equivalent O(1)Li3Si tetrahedra, corners with two equivalent O(4)LiFe2Si tetrahedra, corners with two equivalent O(2)Li2FeSi trigonal pyramids, corners with two equivalent O(3)Li2FeSi trigonal pyramids,  an edgeedge with one O(2)Li2FeSi trigonal pyramid, and  an edgeedge with one O(3)Li2FeSi trigonal pyramid.
Li2FeSiO4 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form LiO4 tetrahedra that share corners with two equivalent Li(1)O4 tetrahedra, corners with four equivalent Si(1)O4 tetrahedra, and corners with two equivalent Fe(1)O4 trigonal pyramids. The Li(1)-O(2) bond length is 1.98 Å. The Li(1)-O(3) bond length is 2.01 Å. Both Li(1)-O(1) bond lengths are 1.99 Å. In the second Li site, Li(2) is bonded in a see-saw-like geometry to one O(1), one O(2), one O(3), and one O(4) atom. The Li(2)-O(1) bond length is 1.91 Å. The Li(2)-O(2) bond length is 2.24 Å. The Li(2)-O(3) bond length is 2.03 Å. The Li(2)-O(4) bond length is 2.27 Å. Fe(1) is bonded to one O(2), one O(3), and two equivalent O(4) atoms to form distorted FeO4 trigonal pyramids that share corners with two equivalent Li(1)O4 tetrahedra, corners with four equivalent Si(1)O4 tetrahedra, and corners with two equivalent Fe(1)O4 trigonal pyramids. The Fe(1)-O(2) bond length is 1.97 Å. The Fe(1)-O(3) bond length is 1.98 Å. There is one shorter (2.11 Å) and one longer (2.12 Å) Fe(1)-O(4) bond length. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share corners with four equivalent Li(1)O4 tetrahedra and corners with four equivalent Fe(1)O4 trigonal pyramids. The Si(1)-O(1) bond length is 1.61 Å. The Si(1)-O(2) bond length is 1.67 Å. The Si(1)-O(3) bond length is 1.66 Å. The Si(1)-O(4) bond length is 1.68 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Li(2), two equivalent Li(1), and one Si(1) atom to form distorted OLi3Si tetrahedra that share corners with two equivalent O(1)Li3Si tetrahedra, corners with two equivalent O(4)LiFe2Si tetrahedra, corners with two equivalent O(2)Li2FeSi trigonal pyramids, corners with two equivalent O(3)Li2FeSi trigonal pyramids,  an edgeedge with one O(2)Li2FeSi trigonal pyramid, and  an edgeedge with one O(3)Li2FeSi trigonal pyramid. In the second O site, O(2) is bonded to one Li(1), one Li(2), one Fe(1), and one Si(1) atom to form distorted OLi2FeSi trigonal pyramids that share corners with two equivalent O(1)Li3Si tetrahedra, corners with two equivalent O(4)LiFe2Si tetrahedra, corners with four equivalent O(3)Li2FeSi trigonal pyramids,  an edgeedge with one O(1)Li3Si tetrahedra, and  an edgeedge with one O(4)LiFe2Si tetrahedra. In the third O site, O(3) is bonded to one Li(1), one Li(2), one Fe(1), and one Si(1) atom to form distorted OLi2FeSi trigonal pyramids that share corners with two equivalent O(1)Li3Si tetrahedra, corners with two equivalent O(4)LiFe2Si tetrahedra, corners with four equivalent O(2)Li2FeSi trigonal pyramids,  an edgeedge with one O(1)Li3Si tetrahedra, and  an edgeedge with one O(4)LiFe2Si tetrahedra. In the fourth O site, O(4) is bonded to one Li(2), two equivalent Fe(1), and one Si(1) atom to form distorted OLiFe2Si tetrahedra that share corners with two equivalent O(1)Li3Si tetrahedra, corners with two equivalent O(4)LiFe2Si tetrahedra, corners with two equivalent O(2)Li2FeSi trigonal pyramids, corners with two equivalent O(3)Li2FeSi trigonal pyramids,  an edgeedge with one O(2)Li2FeSi trigonal pyramid, and  an edgeedge with one O(3)Li2FeSi trigonal pyramid.