K2MgNd4Nb2O12 crystallizes in the triclinic P1 space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 7-coordinate geometry to one O(10), one O(11), one O(2), one O(4), one O(7), and two equivalent O(9) atoms. In the second K site, K(2) is bonded in a 2-coordinate geometry to one O(1), one O(2), one O(4), and one O(7) atom. Mg(1) is bonded to one O(10), one O(11), one O(5), one O(8), and one O(9) atom to form distorted MgO5 square pyramids that share  a cornercorner with one Nb(1)O5 trigonal bipyramid,  a cornercorner with one Nb(2)O5 trigonal bipyramid, and  a faceface with one Nb(2)O5 trigonal bipyramid. There are four inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a 8-coordinate geometry to one O(1), one O(11), one O(12), one O(3), one O(4), one O(6), one O(7), and one O(8) atom. In the second Nd site, Nd(2) is bonded in a 7-coordinate geometry to one O(11), one O(2), one O(3), one O(5), one O(6), one O(8), and one O(9) atom. In the third Nd site, Nd(3) is bonded in a 7-coordinate geometry to one O(1), one O(10), one O(12), one O(3), one O(4), one O(6), and one O(7) atom. In the fourth Nd site, Nd(4) is bonded in a 7-coordinate geometry to one O(1), one O(12), one O(2), one O(3), one O(5), one O(6), and one O(8) atom. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(12), one O(4), one O(7), and one O(9) atom to form distorted NbO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O5 square pyramid. In the second Nb site, Nb(2) is bonded to one O(10), one O(11), one O(2), one O(5), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O5 square pyramid and  a faceface with one Mg(1)O5 square pyramid. There are twelve inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one K(2), one Nd(1), one Nd(3), one Nd(4), and one Nb(1) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Nd(2), one Nd(4), and one Nb(2) atom. In the third O site, O(3) is bonded to one Nd(1), one Nd(2), one Nd(3), and one Nd(4) atom to form ONd4 tetrahedra that share corners with three equivalent O(12)Nd3Nb tetrahedra and edges with two equivalent O(6)Nd4 tetrahedra. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Nd(1), one Nd(3), and one Nb(1) atom. In the fifth O site, O(5) is bonded in a distorted see-saw-like geometry to one Mg(1), one Nd(2), one Nd(4), and one Nb(2) atom. In the sixth O site, O(6) is bonded to one Nd(1), one Nd(2), one Nd(3), and one Nd(4) atom to form ONd4 tetrahedra that share  a cornercorner with one O(12)Nd3Nb tetrahedra,  an edgeedge with one O(12)Nd3Nb tetrahedra, and edges with two equivalent O(3)Nd4 tetrahedra. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Nd(1), one Nd(3), and one Nb(1) atom. In the eighth O site, O(8) is bonded in a 5-coordinate geometry to one Mg(1), one Nd(1), one Nd(2), one Nd(4), and one Nb(2) atom. In the ninth O site, O(9) is bonded in a 2-coordinate geometry to two equivalent K(1), one Mg(1), one Nd(2), and one Nb(1) atom. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one K(1), one Mg(1), one Nd(3), and one Nb(2) atom. In the eleventh O site, O(11) is bonded in a 5-coordinate geometry to one K(1), one Mg(1), one Nd(1), one Nd(2), and one Nb(2) atom. In the twelfth O site, O(12) is bonded to one Nd(1), one Nd(3), one Nd(4), and one Nb(1) atom to form a mixture of distorted edge and corner-sharing ONd3Nb tetrahedra.
K2MgNd4Nb2O12 crystallizes in the triclinic P1 space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 7-coordinate geometry to one O(10), one O(11), one O(2), one O(4), one O(7), and two equivalent O(9) atoms. The K(1)-O(10) bond length is 2.78 Å. The K(1)-O(11) bond length is 2.82 Å. The K(1)-O(2) bond length is 2.66 Å. The K(1)-O(4) bond length is 2.79 Å. The K(1)-O(7) bond length is 2.84 Å. There is one shorter (3.03 Å) and one longer (3.08 Å) K(1)-O(9) bond length. In the second K site, K(2) is bonded in a 2-coordinate geometry to one O(1), one O(2), one O(4), and one O(7) atom. The K(2)-O(1) bond length is 2.92 Å. The K(2)-O(2) bond length is 3.10 Å. The K(2)-O(4) bond length is 2.59 Å. The K(2)-O(7) bond length is 2.49 Å. Mg(1) is bonded to one O(10), one O(11), one O(5), one O(8), and one O(9) atom to form distorted MgO5 square pyramids that share  a cornercorner with one Nb(1)O5 trigonal bipyramid,  a cornercorner with one Nb(2)O5 trigonal bipyramid, and  a faceface with one Nb(2)O5 trigonal bipyramid. The Mg(1)-O(10) bond length is 2.08 Å. The Mg(1)-O(11) bond length is 2.03 Å. The Mg(1)-O(5) bond length is 2.05 Å. The Mg(1)-O(8) bond length is 2.13 Å. The Mg(1)-O(9) bond length is 2.11 Å. There are four inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a 8-coordinate geometry to one O(1), one O(11), one O(12), one O(3), one O(4), one O(6), one O(7), and one O(8) atom. The Nd(1)-O(1) bond length is 2.71 Å. The Nd(1)-O(11) bond length is 2.44 Å. The Nd(1)-O(12) bond length is 2.69 Å. The Nd(1)-O(3) bond length is 2.44 Å. The Nd(1)-O(4) bond length is 2.66 Å. The Nd(1)-O(6) bond length is 2.42 Å. The Nd(1)-O(7) bond length is 2.47 Å. The Nd(1)-O(8) bond length is 3.07 Å. In the second Nd site, Nd(2) is bonded in a 7-coordinate geometry to one O(11), one O(2), one O(3), one O(5), one O(6), one O(8), and one O(9) atom. The Nd(2)-O(11) bond length is 2.66 Å. The Nd(2)-O(2) bond length is 2.38 Å. The Nd(2)-O(3) bond length is 2.33 Å. The Nd(2)-O(5) bond length is 2.41 Å. The Nd(2)-O(6) bond length is 2.30 Å. The Nd(2)-O(8) bond length is 2.61 Å. The Nd(2)-O(9) bond length is 2.74 Å. In the third Nd site, Nd(3) is bonded in a 7-coordinate geometry to one O(1), one O(10), one O(12), one O(3), one O(4), one O(6), and one O(7) atom. The Nd(3)-O(1) bond length is 2.65 Å. The Nd(3)-O(10) bond length is 2.45 Å. The Nd(3)-O(12) bond length is 2.43 Å. The Nd(3)-O(3) bond length is 2.32 Å. The Nd(3)-O(4) bond length is 2.50 Å. The Nd(3)-O(6) bond length is 2.40 Å. The Nd(3)-O(7) bond length is 2.63 Å. In the fourth Nd site, Nd(4) is bonded in a 7-coordinate geometry to one O(1), one O(12), one O(2), one O(3), one O(5), one O(6), and one O(8) atom. The Nd(4)-O(1) bond length is 2.53 Å. The Nd(4)-O(12) bond length is 2.45 Å. The Nd(4)-O(2) bond length is 2.55 Å. The Nd(4)-O(3) bond length is 2.35 Å. The Nd(4)-O(5) bond length is 2.47 Å. The Nd(4)-O(6) bond length is 2.34 Å. The Nd(4)-O(8) bond length is 2.58 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(12), one O(4), one O(7), and one O(9) atom to form distorted NbO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O5 square pyramid. The Nb(1)-O(1) bond length is 2.05 Å. The Nb(1)-O(12) bond length is 2.04 Å. The Nb(1)-O(4) bond length is 1.92 Å. The Nb(1)-O(7) bond length is 1.94 Å. The Nb(1)-O(9) bond length is 1.91 Å. In the second Nb site, Nb(2) is bonded to one O(10), one O(11), one O(2), one O(5), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O5 square pyramid and  a faceface with one Mg(1)O5 square pyramid. The Nb(2)-O(10) bond length is 2.10 Å. The Nb(2)-O(11) bond length is 2.12 Å. The Nb(2)-O(2) bond length is 1.92 Å. The Nb(2)-O(5) bond length is 2.28 Å. The Nb(2)-O(8) bond length is 2.00 Å. There are twelve inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one K(2), one Nd(1), one Nd(3), one Nd(4), and one Nb(1) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Nd(2), one Nd(4), and one Nb(2) atom. In the third O site, O(3) is bonded to one Nd(1), one Nd(2), one Nd(3), and one Nd(4) atom to form ONd4 tetrahedra that share corners with three equivalent O(12)Nd3Nb tetrahedra and edges with two equivalent O(6)Nd4 tetrahedra. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Nd(1), one Nd(3), and one Nb(1) atom. In the fifth O site, O(5) is bonded in a distorted see-saw-like geometry to one Mg(1), one Nd(2), one Nd(4), and one Nb(2) atom. In the sixth O site, O(6) is bonded to one Nd(1), one Nd(2), one Nd(3), and one Nd(4) atom to form ONd4 tetrahedra that share  a cornercorner with one O(12)Nd3Nb tetrahedra,  an edgeedge with one O(12)Nd3Nb tetrahedra, and edges with two equivalent O(3)Nd4 tetrahedra. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Nd(1), one Nd(3), and one Nb(1) atom. In the eighth O site, O(8) is bonded in a 5-coordinate geometry to one Mg(1), one Nd(1), one Nd(2), one Nd(4), and one Nb(2) atom. In the ninth O site, O(9) is bonded in a 2-coordinate geometry to two equivalent K(1), one Mg(1), one Nd(2), and one Nb(1) atom. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one K(1), one Mg(1), one Nd(3), and one Nb(2) atom. In the eleventh O site, O(11) is bonded in a 5-coordinate geometry to one K(1), one Mg(1), one Nd(1), one Nd(2), and one Nb(2) atom. In the twelfth O site, O(12) is bonded to one Nd(1), one Nd(3), one Nd(4), and one Nb(1) atom to form a mixture of distorted edge and corner-sharing ONd3Nb tetrahedra.