MgCr8(PO5)4 crystallizes in the monoclinic Pm space group. Mg(1) is bonded to one O(15), one O(9), and two equivalent O(14) atoms to form distorted MgO4 tetrahedra that share  a cornercorner with one Cr(2)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra,  an edgeedge with one Cr(5)O6 octahedra, and  an edgeedge with one P(4)O4 tetrahedra. The corner-sharing octahedral tilt angles are 26°. There are six inequivalent Cr sites. In the first Cr site, Cr(1) is bonded in a 6-coordinate geometry to one O(13), one O(14), one O(15), one O(2), one O(3), and one O(4) atom. In the second Cr site, Cr(2) is bonded to one O(1), one O(15), one O(4), one O(7), and two equivalent O(3) atoms to form distorted CrO6 octahedra that share corners with two equivalent Cr(5)O6 octahedra,  a cornercorner with one Mg(1)O4 tetrahedra,  a cornercorner with one P(2)O4 tetrahedra,  a cornercorner with one P(4)O4 tetrahedra, and corners with two equivalent P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 45-55°. In the third Cr site, Cr(3) is bonded to one O(10), one O(2), one O(5), one O(8), and two equivalent O(6) atoms to form CrO6 octahedra that share  a cornercorner with one P(1)O4 tetrahedra,  a cornercorner with one P(3)O4 tetrahedra, and corners with two equivalent P(2)O4 tetrahedra. In the fourth Cr site, Cr(4) is bonded in a 6-coordinate geometry to one O(10), one O(11), one O(12), one O(5), one O(6), and one O(7) atom. In the fifth Cr site, Cr(5) is bonded to one O(12), one O(15), one O(7), one O(9), and two equivalent O(11) atoms to form CrO6 octahedra that share corners with two equivalent Cr(2)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra,  a cornercorner with one P(4)O4 tetrahedra, corners with two equivalent P(3)O4 tetrahedra, and  an edgeedge with one Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 45-55°. In the sixth Cr site, Cr(6) is bonded in a 6-coordinate geometry to one O(10), one O(13), one O(16), one O(2), and two equivalent O(14) atoms. There are four inequivalent P sites. In the first P site, P(1) is bonded to one O(16), one O(5), and two equivalent O(3) atoms to form PO4 tetrahedra that share  a cornercorner with one Cr(3)O6 octahedra and corners with two equivalent Cr(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 31-41°. In the second P site, P(2) is bonded to one O(4), one O(9), and two equivalent O(6) atoms to form PO4 tetrahedra that share  a cornercorner with one Cr(2)O6 octahedra,  a cornercorner with one Cr(5)O6 octahedra, corners with two equivalent Cr(3)O6 octahedra, and  a cornercorner with one Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 35-44°. In the third P site, P(3) is bonded to one O(13), one O(8), and two equivalent O(11) atoms to form PO4 tetrahedra that share  a cornercorner with one Cr(3)O6 octahedra and corners with two equivalent Cr(5)O6 octahedra. The corner-sharing octahedral tilt angles range from 34-39°. In the fourth P site, P(4) is bonded to one O(1), one O(12), and two equivalent O(14) atoms to form PO4 tetrahedra that share  a cornercorner with one Cr(2)O6 octahedra,  a cornercorner with one Cr(5)O6 octahedra, and  an edgeedge with one Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 36-41°. There are sixteen inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Cr(2) and one P(4) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Cr(3), one Cr(6), and two equivalent Cr(1) atoms. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Cr(1), one Cr(2), and one P(1) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Cr(2), two equivalent Cr(1), and one P(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Cr(3), two equivalent Cr(4), and one P(1) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Cr(3), one Cr(4), and one P(2) atom. In the seventh O site, O(7) is bonded to one Cr(2), one Cr(5), and two equivalent Cr(4) atoms to form distorted OCr4 trigonal pyramids that share corners with two equivalent O(15)MgCr4 trigonal bipyramids and corners with two equivalent O(10)Cr4 trigonal pyramids. In the eighth O site, O(8) is bonded in a bent 150 degrees geometry to one Cr(3) and one P(3) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one Mg(1), one Cr(5), and one P(2) atom. In the tenth O site, O(10) is bonded to one Cr(3), one Cr(6), and two equivalent Cr(4) atoms to form corner-sharing OCr4 trigonal pyramids. In the eleventh O site, O(11) is bonded in a 3-coordinate geometry to one Cr(4), one Cr(5), and one P(3) atom. In the twelfth O site, O(12) is bonded in a 2-coordinate geometry to one Cr(5), two equivalent Cr(4), and one P(4) atom. In the thirteenth O site, O(13) is bonded in a 4-coordinate geometry to one Cr(6), two equivalent Cr(1), and one P(3) atom. In the fourteenth O site, O(14) is bonded in a distorted see-saw-like geometry to one Mg(1), one Cr(1), one Cr(6), and one P(4) atom. In the fifteenth O site, O(15) is bonded to one Mg(1), one Cr(2), one Cr(5), and two equivalent Cr(1) atoms to form distorted corner-sharing OMgCr4 trigonal bipyramids. In the sixteenth O site, O(16) is bonded in a bent 150 degrees geometry to one Cr(6) and one P(1) atom.
MgCr8(PO5)4 crystallizes in the monoclinic Pm space group. Mg(1) is bonded to one O(15), one O(9), and two equivalent O(14) atoms to form distorted MgO4 tetrahedra that share  a cornercorner with one Cr(2)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra,  an edgeedge with one Cr(5)O6 octahedra, and  an edgeedge with one P(4)O4 tetrahedra. The corner-sharing octahedral tilt angles are 26°. The Mg(1)-O(15) bond length is 1.96 Å. The Mg(1)-O(9) bond length is 1.84 Å. Both Mg(1)-O(14) bond lengths are 2.00 Å. There are six inequivalent Cr sites. In the first Cr site, Cr(1) is bonded in a 6-coordinate geometry to one O(13), one O(14), one O(15), one O(2), one O(3), and one O(4) atom. The Cr(1)-O(13) bond length is 2.57 Å. The Cr(1)-O(14) bond length is 2.22 Å. The Cr(1)-O(15) bond length is 2.15 Å. The Cr(1)-O(2) bond length is 1.97 Å. The Cr(1)-O(3) bond length is 2.11 Å. The Cr(1)-O(4) bond length is 2.38 Å. In the second Cr site, Cr(2) is bonded to one O(1), one O(15), one O(4), one O(7), and two equivalent O(3) atoms to form distorted CrO6 octahedra that share corners with two equivalent Cr(5)O6 octahedra,  a cornercorner with one Mg(1)O4 tetrahedra,  a cornercorner with one P(2)O4 tetrahedra,  a cornercorner with one P(4)O4 tetrahedra, and corners with two equivalent P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 45-55°. The Cr(2)-O(1) bond length is 1.95 Å. The Cr(2)-O(15) bond length is 2.47 Å. The Cr(2)-O(4) bond length is 2.11 Å. The Cr(2)-O(7) bond length is 2.00 Å. Both Cr(2)-O(3) bond lengths are 2.08 Å. In the third Cr site, Cr(3) is bonded to one O(10), one O(2), one O(5), one O(8), and two equivalent O(6) atoms to form CrO6 octahedra that share  a cornercorner with one P(1)O4 tetrahedra,  a cornercorner with one P(3)O4 tetrahedra, and corners with two equivalent P(2)O4 tetrahedra. The Cr(3)-O(10) bond length is 2.07 Å. The Cr(3)-O(2) bond length is 1.95 Å. The Cr(3)-O(5) bond length is 2.16 Å. The Cr(3)-O(8) bond length is 1.98 Å. Both Cr(3)-O(6) bond lengths are 2.08 Å. In the fourth Cr site, Cr(4) is bonded in a 6-coordinate geometry to one O(10), one O(11), one O(12), one O(5), one O(6), and one O(7) atom. The Cr(4)-O(10) bond length is 2.04 Å. The Cr(4)-O(11) bond length is 2.17 Å. The Cr(4)-O(12) bond length is 2.70 Å. The Cr(4)-O(5) bond length is 2.41 Å. The Cr(4)-O(6) bond length is 2.19 Å. The Cr(4)-O(7) bond length is 2.05 Å. In the fifth Cr site, Cr(5) is bonded to one O(12), one O(15), one O(7), one O(9), and two equivalent O(11) atoms to form CrO6 octahedra that share corners with two equivalent Cr(2)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra,  a cornercorner with one P(4)O4 tetrahedra, corners with two equivalent P(3)O4 tetrahedra, and  an edgeedge with one Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 45-55°. The Cr(5)-O(12) bond length is 2.24 Å. The Cr(5)-O(15) bond length is 2.18 Å. The Cr(5)-O(7) bond length is 2.15 Å. The Cr(5)-O(9) bond length is 2.34 Å. Both Cr(5)-O(11) bond lengths are 2.09 Å. In the sixth Cr site, Cr(6) is bonded in a 6-coordinate geometry to one O(10), one O(13), one O(16), one O(2), and two equivalent O(14) atoms. The Cr(6)-O(10) bond length is 2.16 Å. The Cr(6)-O(13) bond length is 2.18 Å. The Cr(6)-O(16) bond length is 2.04 Å. The Cr(6)-O(2) bond length is 2.64 Å. Both Cr(6)-O(14) bond lengths are 2.22 Å. There are four inequivalent P sites. In the first P site, P(1) is bonded to one O(16), one O(5), and two equivalent O(3) atoms to form PO4 tetrahedra that share  a cornercorner with one Cr(3)O6 octahedra and corners with two equivalent Cr(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 31-41°. The P(1)-O(16) bond length is 1.52 Å. The P(1)-O(5) bond length is 1.55 Å. Both P(1)-O(3) bond lengths are 1.59 Å. In the second P site, P(2) is bonded to one O(4), one O(9), and two equivalent O(6) atoms to form PO4 tetrahedra that share  a cornercorner with one Cr(2)O6 octahedra,  a cornercorner with one Cr(5)O6 octahedra, corners with two equivalent Cr(3)O6 octahedra, and  a cornercorner with one Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 35-44°. The P(2)-O(4) bond length is 1.55 Å. The P(2)-O(9) bond length is 1.53 Å. Both P(2)-O(6) bond lengths are 1.56 Å. In the third P site, P(3) is bonded to one O(13), one O(8), and two equivalent O(11) atoms to form PO4 tetrahedra that share  a cornercorner with one Cr(3)O6 octahedra and corners with two equivalent Cr(5)O6 octahedra. The corner-sharing octahedral tilt angles range from 34-39°. The P(3)-O(13) bond length is 1.57 Å. The P(3)-O(8) bond length is 1.53 Å. Both P(3)-O(11) bond lengths are 1.57 Å. In the fourth P site, P(4) is bonded to one O(1), one O(12), and two equivalent O(14) atoms to form PO4 tetrahedra that share  a cornercorner with one Cr(2)O6 octahedra,  a cornercorner with one Cr(5)O6 octahedra, and  an edgeedge with one Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 36-41°. The P(4)-O(1) bond length is 1.50 Å. The P(4)-O(12) bond length is 1.53 Å. Both P(4)-O(14) bond lengths are 1.60 Å. There are sixteen inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Cr(2) and one P(4) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Cr(3), one Cr(6), and two equivalent Cr(1) atoms. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Cr(1), one Cr(2), and one P(1) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Cr(2), two equivalent Cr(1), and one P(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Cr(3), two equivalent Cr(4), and one P(1) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Cr(3), one Cr(4), and one P(2) atom. In the seventh O site, O(7) is bonded to one Cr(2), one Cr(5), and two equivalent Cr(4) atoms to form distorted OCr4 trigonal pyramids that share corners with two equivalent O(15)MgCr4 trigonal bipyramids and corners with two equivalent O(10)Cr4 trigonal pyramids. In the eighth O site, O(8) is bonded in a bent 150 degrees geometry to one Cr(3) and one P(3) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one Mg(1), one Cr(5), and one P(2) atom. In the tenth O site, O(10) is bonded to one Cr(3), one Cr(6), and two equivalent Cr(4) atoms to form corner-sharing OCr4 trigonal pyramids. In the eleventh O site, O(11) is bonded in a 3-coordinate geometry to one Cr(4), one Cr(5), and one P(3) atom. In the twelfth O site, O(12) is bonded in a 2-coordinate geometry to one Cr(5), two equivalent Cr(4), and one P(4) atom. In the thirteenth O site, O(13) is bonded in a 4-coordinate geometry to one Cr(6), two equivalent Cr(1), and one P(3) atom. In the fourteenth O site, O(14) is bonded in a distorted see-saw-like geometry to one Mg(1), one Cr(1), one Cr(6), and one P(4) atom. In the fifteenth O site, O(15) is bonded to one Mg(1), one Cr(2), one Cr(5), and two equivalent Cr(1) atoms to form distorted corner-sharing OMgCr4 trigonal bipyramids. In the sixteenth O site, O(16) is bonded in a bent 150 degrees geometry to one Cr(6) and one P(1) atom.