K2CaNd4Nb2O12 crystallizes in the monoclinic Cm 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(7), two equivalent O(2), two equivalent O(4), and two equivalent O(6) atoms. In the second K site, K(2) is bonded in a 10-coordinate geometry to one Ca(1), one O(6), two equivalent O(2), two equivalent O(4), two equivalent O(5), and two equivalent O(7) atoms. Ca(1) is bonded in a 6-coordinate geometry to one K(2), two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms. There are four inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a 8-coordinate geometry to two equivalent O(1), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. In the second Nd site, Nd(2) is bonded in a 7-coordinate geometry to one O(6), two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms. In the third Nd site, Nd(3) is bonded in a 7-coordinate geometry to one O(7), two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms. In the fourth Nd site, Nd(4) is bonded in a distorted body-centered cubic geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a distorted trigonal bipyramidal geometry to one O(6), two equivalent O(1), and two equivalent O(4) atoms. In the second Nb site, Nb(2) is bonded in a distorted trigonal bipyramidal geometry to one O(7), two equivalent O(2), and two equivalent O(5) atoms. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Ca(1), one Nd(1), one Nd(3), one Nd(4), and one Nb(1) atom. In the second O site, O(2) is bonded to one K(1), one K(2), one Nd(2), one Nd(4), and one Nb(2) atom to form distorted OK2Nd2Nb trigonal bipyramids that share corners with two equivalent O(3)CaNd4 trigonal bipyramids,  an edgeedge with one O(3)CaNd4 trigonal bipyramid,  an edgeedge with one O(2)K2Nd2Nb trigonal bipyramid, and  a faceface with one O(2)K2Nd2Nb trigonal bipyramid. In the third O site, O(3) is bonded to one Ca(1), one Nd(1), one Nd(2), one Nd(3), and one Nd(4) atom to form distorted OCaNd4 trigonal bipyramids that share corners with two equivalent O(2)K2Nd2Nb trigonal bipyramids,  an edgeedge with one O(3)CaNd4 trigonal bipyramid,  an edgeedge with one O(2)K2Nd2Nb trigonal bipyramid, and  a faceface with one O(3)CaNd4 trigonal bipyramid. In the fourth O site, O(4) is bonded in a 6-coordinate geometry to one K(1), one K(2), one Ca(1), one Nd(1), one Nd(3), and one Nb(1) atom. In the fifth O site, O(5) is bonded in a 5-coordinate geometry to one K(2), one Nd(1), one Nd(2), one Nd(4), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a 5-coordinate geometry to one K(2), two equivalent K(1), one Nd(2), and one Nb(1) atom. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to one K(1), two equivalent K(2), one Nd(3), and one Nb(2) atom.
K2CaNd4Nb2O12 crystallizes in the monoclinic Cm 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(7), two equivalent O(2), two equivalent O(4), and two equivalent O(6) atoms. The K(1)-O(7) bond length is 2.63 Å. Both K(1)-O(2) bond lengths are 2.80 Å. Both K(1)-O(4) bond lengths are 2.91 Å. Both K(1)-O(6) bond lengths are 3.01 Å. In the second K site, K(2) is bonded in a 10-coordinate geometry to one Ca(1), one O(6), two equivalent O(2), two equivalent O(4), two equivalent O(5), and two equivalent O(7) atoms. The K(2)-Ca(1) bond length is 2.93 Å. The K(2)-O(6) bond length is 2.55 Å. Both K(2)-O(2) bond lengths are 2.72 Å. Both K(2)-O(4) bond lengths are 3.25 Å. Both K(2)-O(5) bond lengths are 2.96 Å. Both K(2)-O(7) bond lengths are 2.99 Å. Ca(1) is bonded in a 6-coordinate geometry to one K(2), two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms. Both Ca(1)-O(1) bond lengths are 2.45 Å. Both Ca(1)-O(3) bond lengths are 2.36 Å. Both Ca(1)-O(4) bond lengths are 2.32 Å. There are four inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a 8-coordinate geometry to two equivalent O(1), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Both Nd(1)-O(1) bond lengths are 2.58 Å. Both Nd(1)-O(3) bond lengths are 2.55 Å. Both Nd(1)-O(4) bond lengths are 2.81 Å. Both Nd(1)-O(5) bond lengths are 2.49 Å. In the second Nd site, Nd(2) is bonded in a 7-coordinate geometry to one O(6), two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms. The Nd(2)-O(6) bond length is 2.92 Å. Both Nd(2)-O(2) bond lengths are 2.44 Å. Both Nd(2)-O(3) bond lengths are 2.37 Å. Both Nd(2)-O(5) bond lengths are 2.48 Å. In the third Nd site, Nd(3) is bonded in a 7-coordinate geometry to one O(7), two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms. The Nd(3)-O(7) bond length is 2.85 Å. Both Nd(3)-O(1) bond lengths are 2.42 Å. Both Nd(3)-O(3) bond lengths are 2.36 Å. Both Nd(3)-O(4) bond lengths are 2.52 Å. In the fourth Nd site, Nd(4) is bonded in a distorted body-centered cubic geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms. Both Nd(4)-O(1) bond lengths are 2.80 Å. Both Nd(4)-O(2) bond lengths are 2.40 Å. Both Nd(4)-O(3) bond lengths are 2.48 Å. Both Nd(4)-O(5) bond lengths are 2.59 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(6), two equivalent O(1), and two equivalent O(4) atoms. The Nb(1)-O(6) bond length is 1.85 Å. Both Nb(1)-O(1) bond lengths are 2.10 Å. Both Nb(1)-O(4) bond lengths are 1.98 Å. In the second Nb site, Nb(2) is bonded in a distorted trigonal bipyramidal geometry to one O(7), two equivalent O(2), and two equivalent O(5) atoms. The Nb(2)-O(7) bond length is 1.89 Å. Both Nb(2)-O(2) bond lengths are 1.99 Å. Both Nb(2)-O(5) bond lengths are 2.06 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Ca(1), one Nd(1), one Nd(3), one Nd(4), and one Nb(1) atom. In the second O site, O(2) is bonded to one K(1), one K(2), one Nd(2), one Nd(4), and one Nb(2) atom to form distorted OK2Nd2Nb trigonal bipyramids that share corners with two equivalent O(3)CaNd4 trigonal bipyramids,  an edgeedge with one O(3)CaNd4 trigonal bipyramid,  an edgeedge with one O(2)K2Nd2Nb trigonal bipyramid, and  a faceface with one O(2)K2Nd2Nb trigonal bipyramid. In the third O site, O(3) is bonded to one Ca(1), one Nd(1), one Nd(2), one Nd(3), and one Nd(4) atom to form distorted OCaNd4 trigonal bipyramids that share corners with two equivalent O(2)K2Nd2Nb trigonal bipyramids,  an edgeedge with one O(3)CaNd4 trigonal bipyramid,  an edgeedge with one O(2)K2Nd2Nb trigonal bipyramid, and  a faceface with one O(3)CaNd4 trigonal bipyramid. In the fourth O site, O(4) is bonded in a 6-coordinate geometry to one K(1), one K(2), one Ca(1), one Nd(1), one Nd(3), and one Nb(1) atom. In the fifth O site, O(5) is bonded in a 5-coordinate geometry to one K(2), one Nd(1), one Nd(2), one Nd(4), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a 5-coordinate geometry to one K(2), two equivalent K(1), one Nd(2), and one Nb(1) atom. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to one K(1), two equivalent K(2), one Nd(3), and one Nb(2) atom.