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 3-coordinate geometry to one O(10), one O(2), and one O(5) atom. In the second K site, K(2) is bonded in a distorted water-like geometry to one O(10) and one O(9) atom. Mg(1) is bonded in an L-shaped geometry to one O(4) and one O(7) atom. There are four inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a body-centered cubic 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 6-coordinate geometry to one O(11), one O(2), one O(3), one O(5), one O(6), and one O(8) atom. In the third Nd site, Nd(3) is bonded in a 6-coordinate geometry to one O(1), 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 body-centered cubic geometry to one O(1), one O(11), 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 in a distorted trigonal bipyramidal geometry to one O(1), one O(12), one O(4), one O(7), and one O(9) atom. In the second Nb site, Nb(2) is bonded in a distorted trigonal bipyramidal geometry to one O(10), one O(11), one O(2), one O(5), and one O(8) atom. There are twelve inequivalent O sites. In the first O site, O(1) is bonded to one Nd(1), one Nd(3), one Nd(4), and one Nb(1) atom to form distorted ONd3Nb tetrahedra that share  a cornercorner with one O(2)KNd2Nb tetrahedra,  a cornercorner with one O(5)KNd2Nb tetrahedra,  a cornercorner with one O(4)Nd2MgNb tetrahedra,  a cornercorner with one O(3)Nd4 tetrahedra, corners with two equivalent O(11)Nd3Nb tetrahedra, corners with three equivalent O(6)Nd4 tetrahedra, corners with three equivalent O(7)Nd2MgNb trigonal pyramids,  an edgeedge with one O(4)Nd2MgNb tetrahedra,  an edgeedge with one O(8)Nd3Nb tetrahedra,  an edgeedge with one O(3)Nd4 tetrahedra, and edges with two equivalent O(12)Nd3Nb tetrahedra. In the second O site, O(2) is bonded to one K(1), one Nd(2), one Nd(4), and one Nb(2) atom to form distorted OKNd2Nb tetrahedra that share  a cornercorner with one O(1)Nd3Nb tetrahedra,  a cornercorner with one O(12)Nd3Nb tetrahedra,  a cornercorner with one O(8)Nd3Nb tetrahedra, corners with two equivalent O(6)Nd4 tetrahedra, corners with three equivalent O(11)Nd3Nb tetrahedra,  an edgeedge with one O(8)Nd3Nb tetrahedra,  an edgeedge with one O(3)Nd4 tetrahedra, and edges with two equivalent O(5)KNd2Nb tetrahedra. 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  a cornercorner with one O(1)Nd3Nb tetrahedra,  a cornercorner with one O(11)Nd3Nb tetrahedra, corners with two equivalent O(5)KNd2Nb tetrahedra, corners with two equivalent O(4)Nd2MgNb tetrahedra, corners with three equivalent O(12)Nd3Nb tetrahedra, corners with three equivalent O(8)Nd3Nb tetrahedra,  an edgeedge with one O(2)KNd2Nb tetrahedra,  an edgeedge with one O(1)Nd3Nb tetrahedra,  an edgeedge with one O(11)Nd3Nb tetrahedra, edges with two equivalent O(6)Nd4 tetrahedra, and  an edgeedge with one O(7)Nd2MgNb trigonal pyramid. In the fourth O site, O(4) is bonded to one Mg(1), one Nd(1), one Nd(3), and one Nb(1) atom to form distorted ONd2MgNb tetrahedra that share  a cornercorner with one O(1)Nd3Nb tetrahedra,  a cornercorner with one O(11)Nd3Nb tetrahedra,  a cornercorner with one O(8)Nd3Nb tetrahedra, corners with two equivalent O(3)Nd4 tetrahedra, corners with three equivalent O(12)Nd3Nb tetrahedra,  an edgeedge with one O(1)Nd3Nb tetrahedra,  an edgeedge with one O(6)Nd4 tetrahedra, and edges with two equivalent O(7)Nd2MgNb trigonal pyramids. In the fifth O site, O(5) is bonded to one K(1), one Nd(2), one Nd(4), and one Nb(2) atom to form distorted OKNd2Nb tetrahedra that share  a cornercorner with one O(1)Nd3Nb tetrahedra,  a cornercorner with one O(11)Nd3Nb tetrahedra,  a cornercorner with one O(12)Nd3Nb tetrahedra, corners with two equivalent O(3)Nd4 tetrahedra, corners with three equivalent O(8)Nd3Nb tetrahedra,  an edgeedge with one O(11)Nd3Nb tetrahedra,  an edgeedge with one O(6)Nd4 tetrahedra, and edges with two equivalent O(2)KNd2Nb tetrahedra. 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,  a cornercorner with one O(8)Nd3Nb tetrahedra, corners with two equivalent O(2)KNd2Nb tetrahedra, corners with three equivalent O(1)Nd3Nb tetrahedra, corners with three equivalent O(11)Nd3Nb tetrahedra, corners with two equivalent O(7)Nd2MgNb trigonal pyramids,  an edgeedge with one O(5)KNd2Nb tetrahedra,  an edgeedge with one O(4)Nd2MgNb tetrahedra,  an edgeedge with one O(12)Nd3Nb tetrahedra,  an edgeedge with one O(8)Nd3Nb tetrahedra, and edges with two equivalent O(3)Nd4 tetrahedra. In the seventh O site, O(7) is bonded to one Mg(1), one Nd(1), one Nd(3), and one Nb(1) atom to form distorted ONd2MgNb trigonal pyramids that share  a cornercorner with one O(11)Nd3Nb tetrahedra,  a cornercorner with one O(12)Nd3Nb tetrahedra,  a cornercorner with one O(8)Nd3Nb tetrahedra, corners with two equivalent O(6)Nd4 tetrahedra, corners with three equivalent O(1)Nd3Nb tetrahedra,  an edgeedge with one O(12)Nd3Nb tetrahedra,  an edgeedge with one O(3)Nd4 tetrahedra, and edges with two equivalent O(4)Nd2MgNb tetrahedra. In the eighth O site, O(8) is bonded to one Nd(1), one Nd(2), one Nd(4), and one Nb(2) atom to form ONd3Nb tetrahedra that share  a cornercorner with one O(2)KNd2Nb tetrahedra,  a cornercorner with one O(4)Nd2MgNb tetrahedra,  a cornercorner with one O(6)Nd4 tetrahedra, corners with two equivalent O(12)Nd3Nb tetrahedra, corners with three equivalent O(5)KNd2Nb tetrahedra, corners with three equivalent O(3)Nd4 tetrahedra,  a cornercorner with one O(7)Nd2MgNb trigonal pyramid,  an edgeedge with one O(2)KNd2Nb tetrahedra,  an edgeedge with one O(1)Nd3Nb tetrahedra,  an edgeedge with one O(6)Nd4 tetrahedra, and edges with two equivalent O(11)Nd3Nb tetrahedra. In the ninth O site, O(9) is bonded in a distorted linear geometry to one K(2) and one Nb(1) atom. In the tenth O site, O(10) is bonded in a 3-coordinate geometry to one K(1), one K(2), and one Nb(2) atom. In the eleventh O site, O(11) is bonded to one Nd(1), one Nd(2), one Nd(4), and one Nb(2) atom to form distorted ONd3Nb tetrahedra that share  a cornercorner with one O(5)KNd2Nb tetrahedra,  a cornercorner with one O(4)Nd2MgNb tetrahedra,  a cornercorner with one O(3)Nd4 tetrahedra, corners with two equivalent O(1)Nd3Nb tetrahedra, corners with three equivalent O(2)KNd2Nb tetrahedra, corners with three equivalent O(6)Nd4 tetrahedra,  a cornercorner with one O(7)Nd2MgNb trigonal pyramid,  an edgeedge with one O(5)KNd2Nb tetrahedra,  an edgeedge with one O(12)Nd3Nb tetrahedra,  an edgeedge with one O(3)Nd4 tetrahedra, and edges with two equivalent O(8)Nd3Nb tetrahedra. 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 distorted ONd3Nb tetrahedra that share  a cornercorner with one O(2)KNd2Nb tetrahedra,  a cornercorner with one O(5)KNd2Nb tetrahedra,  a cornercorner with one O(6)Nd4 tetrahedra, corners with two equivalent O(8)Nd3Nb tetrahedra, corners with three equivalent O(4)Nd2MgNb tetrahedra, corners with three equivalent O(3)Nd4 tetrahedra,  a cornercorner with one O(7)Nd2MgNb trigonal pyramid,  an edgeedge with one O(11)Nd3Nb tetrahedra,  an edgeedge with one O(6)Nd4 tetrahedra, edges with two equivalent O(1)Nd3Nb tetrahedra, and  an edgeedge with one O(7)Nd2MgNb trigonal pyramid.
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 3-coordinate geometry to one O(10), one O(2), and one O(5) atom. The K(1)-O(10) bond length is 2.72 Å. The K(1)-O(2) bond length is 2.71 Å. The K(1)-O(5) bond length is 2.70 Å. In the second K site, K(2) is bonded in a distorted water-like geometry to one O(10) and one O(9) atom. The K(2)-O(10) bond length is 2.96 Å. The K(2)-O(9) bond length is 2.52 Å. Mg(1) is bonded in an L-shaped geometry to one O(4) and one O(7) atom. The Mg(1)-O(4) bond length is 2.06 Å. The Mg(1)-O(7) bond length is 2.06 Å. There are four inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a body-centered cubic 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 2.49 Å. The Nd(1)-O(11) bond length is 2.56 Å. The Nd(1)-O(12) bond length is 2.47 Å. The Nd(1)-O(3) bond length is 2.39 Å. The Nd(1)-O(4) bond length is 2.54 Å. The Nd(1)-O(6) bond length is 2.38 Å. The Nd(1)-O(7) bond length is 2.55 Å. The Nd(1)-O(8) bond length is 2.54 Å. In the second Nd site, Nd(2) is bonded in a 6-coordinate geometry to one O(11), one O(2), one O(3), one O(5), one O(6), and one O(8) atom. The Nd(2)-O(11) bond length is 2.45 Å. The Nd(2)-O(2) bond length is 2.42 Å. The Nd(2)-O(3) bond length is 2.27 Å. The Nd(2)-O(5) bond length is 2.44 Å. The Nd(2)-O(6) bond length is 2.27 Å. The Nd(2)-O(8) bond length is 2.44 Å. In the third Nd site, Nd(3) is bonded in a 6-coordinate geometry to one O(1), 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 2.44 Å. The Nd(3)-O(12) bond length is 2.45 Å. The Nd(3)-O(3) bond length is 2.29 Å. The Nd(3)-O(4) bond length is 2.47 Å. The Nd(3)-O(6) bond length is 2.30 Å. The Nd(3)-O(7) bond length is 2.48 Å. In the fourth Nd site, Nd(4) is bonded in a body-centered cubic geometry to one O(1), one O(11), 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 2.59 Å. The Nd(4)-O(11) bond length is 2.49 Å. The Nd(4)-O(12) bond length is 2.61 Å. The Nd(4)-O(2) bond length is 2.45 Å. The Nd(4)-O(3) bond length is 2.41 Å. The Nd(4)-O(5) bond length is 2.47 Å. The Nd(4)-O(6) bond length is 2.41 Å. The Nd(4)-O(8) bond length is 2.50 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a distorted trigonal bipyramidal geometry to one O(1), one O(12), one O(4), one O(7), and one O(9) atom. The Nb(1)-O(1) bond length is 2.01 Å. The Nb(1)-O(12) bond length is 2.01 Å. The Nb(1)-O(4) bond length is 2.04 Å. The Nb(1)-O(7) bond length is 2.04 Å. The Nb(1)-O(9) bond length is 1.79 Å. In the second Nb site, Nb(2) is bonded in a distorted trigonal bipyramidal geometry to one O(10), one O(11), one O(2), one O(5), and one O(8) atom. The Nb(2)-O(10) bond length is 1.83 Å. The Nb(2)-O(11) bond length is 2.03 Å. The Nb(2)-O(2) bond length is 1.99 Å. The Nb(2)-O(5) bond length is 1.99 Å. The Nb(2)-O(8) bond length is 2.03 Å. There are twelve inequivalent O sites. In the first O site, O(1) is bonded to one Nd(1), one Nd(3), one Nd(4), and one Nb(1) atom to form distorted ONd3Nb tetrahedra that share  a cornercorner with one O(2)KNd2Nb tetrahedra,  a cornercorner with one O(5)KNd2Nb tetrahedra,  a cornercorner with one O(4)Nd2MgNb tetrahedra,  a cornercorner with one O(3)Nd4 tetrahedra, corners with two equivalent O(11)Nd3Nb tetrahedra, corners with three equivalent O(6)Nd4 tetrahedra, corners with three equivalent O(7)Nd2MgNb trigonal pyramids,  an edgeedge with one O(4)Nd2MgNb tetrahedra,  an edgeedge with one O(8)Nd3Nb tetrahedra,  an edgeedge with one O(3)Nd4 tetrahedra, and edges with two equivalent O(12)Nd3Nb tetrahedra. In the second O site, O(2) is bonded to one K(1), one Nd(2), one Nd(4), and one Nb(2) atom to form distorted OKNd2Nb tetrahedra that share  a cornercorner with one O(1)Nd3Nb tetrahedra,  a cornercorner with one O(12)Nd3Nb tetrahedra,  a cornercorner with one O(8)Nd3Nb tetrahedra, corners with two equivalent O(6)Nd4 tetrahedra, corners with three equivalent O(11)Nd3Nb tetrahedra,  an edgeedge with one O(8)Nd3Nb tetrahedra,  an edgeedge with one O(3)Nd4 tetrahedra, and edges with two equivalent O(5)KNd2Nb tetrahedra. 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  a cornercorner with one O(1)Nd3Nb tetrahedra,  a cornercorner with one O(11)Nd3Nb tetrahedra, corners with two equivalent O(5)KNd2Nb tetrahedra, corners with two equivalent O(4)Nd2MgNb tetrahedra, corners with three equivalent O(12)Nd3Nb tetrahedra, corners with three equivalent O(8)Nd3Nb tetrahedra,  an edgeedge with one O(2)KNd2Nb tetrahedra,  an edgeedge with one O(1)Nd3Nb tetrahedra,  an edgeedge with one O(11)Nd3Nb tetrahedra, edges with two equivalent O(6)Nd4 tetrahedra, and  an edgeedge with one O(7)Nd2MgNb trigonal pyramid. In the fourth O site, O(4) is bonded to one Mg(1), one Nd(1), one Nd(3), and one Nb(1) atom to form distorted ONd2MgNb tetrahedra that share  a cornercorner with one O(1)Nd3Nb tetrahedra,  a cornercorner with one O(11)Nd3Nb tetrahedra,  a cornercorner with one O(8)Nd3Nb tetrahedra, corners with two equivalent O(3)Nd4 tetrahedra, corners with three equivalent O(12)Nd3Nb tetrahedra,  an edgeedge with one O(1)Nd3Nb tetrahedra,  an edgeedge with one O(6)Nd4 tetrahedra, and edges with two equivalent O(7)Nd2MgNb trigonal pyramids. In the fifth O site, O(5) is bonded to one K(1), one Nd(2), one Nd(4), and one Nb(2) atom to form distorted OKNd2Nb tetrahedra that share  a cornercorner with one O(1)Nd3Nb tetrahedra,  a cornercorner with one O(11)Nd3Nb tetrahedra,  a cornercorner with one O(12)Nd3Nb tetrahedra, corners with two equivalent O(3)Nd4 tetrahedra, corners with three equivalent O(8)Nd3Nb tetrahedra,  an edgeedge with one O(11)Nd3Nb tetrahedra,  an edgeedge with one O(6)Nd4 tetrahedra, and edges with two equivalent O(2)KNd2Nb tetrahedra. 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,  a cornercorner with one O(8)Nd3Nb tetrahedra, corners with two equivalent O(2)KNd2Nb tetrahedra, corners with three equivalent O(1)Nd3Nb tetrahedra, corners with three equivalent O(11)Nd3Nb tetrahedra, corners with two equivalent O(7)Nd2MgNb trigonal pyramids,  an edgeedge with one O(5)KNd2Nb tetrahedra,  an edgeedge with one O(4)Nd2MgNb tetrahedra,  an edgeedge with one O(12)Nd3Nb tetrahedra,  an edgeedge with one O(8)Nd3Nb tetrahedra, and edges with two equivalent O(3)Nd4 tetrahedra. In the seventh O site, O(7) is bonded to one Mg(1), one Nd(1), one Nd(3), and one Nb(1) atom to form distorted ONd2MgNb trigonal pyramids that share  a cornercorner with one O(11)Nd3Nb tetrahedra,  a cornercorner with one O(12)Nd3Nb tetrahedra,  a cornercorner with one O(8)Nd3Nb tetrahedra, corners with two equivalent O(6)Nd4 tetrahedra, corners with three equivalent O(1)Nd3Nb tetrahedra,  an edgeedge with one O(12)Nd3Nb tetrahedra,  an edgeedge with one O(3)Nd4 tetrahedra, and edges with two equivalent O(4)Nd2MgNb tetrahedra. In the eighth O site, O(8) is bonded to one Nd(1), one Nd(2), one Nd(4), and one Nb(2) atom to form ONd3Nb tetrahedra that share  a cornercorner with one O(2)KNd2Nb tetrahedra,  a cornercorner with one O(4)Nd2MgNb tetrahedra,  a cornercorner with one O(6)Nd4 tetrahedra, corners with two equivalent O(12)Nd3Nb tetrahedra, corners with three equivalent O(5)KNd2Nb tetrahedra, corners with three equivalent O(3)Nd4 tetrahedra,  a cornercorner with one O(7)Nd2MgNb trigonal pyramid,  an edgeedge with one O(2)KNd2Nb tetrahedra,  an edgeedge with one O(1)Nd3Nb tetrahedra,  an edgeedge with one O(6)Nd4 tetrahedra, and edges with two equivalent O(11)Nd3Nb tetrahedra. In the ninth O site, O(9) is bonded in a distorted linear geometry to one K(2) and one Nb(1) atom. In the tenth O site, O(10) is bonded in a 3-coordinate geometry to one K(1), one K(2), and one Nb(2) atom. In the eleventh O site, O(11) is bonded to one Nd(1), one Nd(2), one Nd(4), and one Nb(2) atom to form distorted ONd3Nb tetrahedra that share  a cornercorner with one O(5)KNd2Nb tetrahedra,  a cornercorner with one O(4)Nd2MgNb tetrahedra,  a cornercorner with one O(3)Nd4 tetrahedra, corners with two equivalent O(1)Nd3Nb tetrahedra, corners with three equivalent O(2)KNd2Nb tetrahedra, corners with three equivalent O(6)Nd4 tetrahedra,  a cornercorner with one O(7)Nd2MgNb trigonal pyramid,  an edgeedge with one O(5)KNd2Nb tetrahedra,  an edgeedge with one O(12)Nd3Nb tetrahedra,  an edgeedge with one O(3)Nd4 tetrahedra, and edges with two equivalent O(8)Nd3Nb tetrahedra. 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 distorted ONd3Nb tetrahedra that share  a cornercorner with one O(2)KNd2Nb tetrahedra,  a cornercorner with one O(5)KNd2Nb tetrahedra,  a cornercorner with one O(6)Nd4 tetrahedra, corners with two equivalent O(8)Nd3Nb tetrahedra, corners with three equivalent O(4)Nd2MgNb tetrahedra, corners with three equivalent O(3)Nd4 tetrahedra,  a cornercorner with one O(7)Nd2MgNb trigonal pyramid,  an edgeedge with one O(11)Nd3Nb tetrahedra,  an edgeedge with one O(6)Nd4 tetrahedra, edges with two equivalent O(1)Nd3Nb tetrahedra, and  an edgeedge with one O(7)Nd2MgNb trigonal pyramid.