Eu5Nb4O15 crystallizes in the monoclinic Cm space group. There are five inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to two equivalent O(4); two O(6,6); four equivalent O(3); and four O(5,5) atoms to form EuO12 cuboctahedra that share corners with six equivalent Eu(1)O12 cuboctahedra; faces with six Eu(3,3)O12 cuboctahedra; faces with two Nb(2,2)O6 octahedra; and faces with six Nb(1,1)O6 octahedra. In the second Eu site, Eu(2) is bonded in a 12-coordinate geometry to one O(6); two equivalent O(5); three O(2,2); and six O(1,1) atoms. In the third Eu site, Eu(2) is bonded in a 12-coordinate geometry to one O(6); two equivalent O(5); three O(2,2); and six O(1,1) atoms. In the fourth Eu site, Eu(3) is bonded to one O(2), one O(4), two equivalent O(1), two equivalent O(3), two equivalent O(6), and four equivalent O(5) atoms to form distorted EuO12 cuboctahedra that share corners with nine Eu(3,3)O12 cuboctahedra; faces with three equivalent Eu(1)O12 cuboctahedra; faces with three equivalent Nb(2)O6 octahedra; and faces with four Nb(1,1)O6 octahedra. In the fifth Eu site, Eu(3) is bonded to one O(2), one O(4), two equivalent O(1), two equivalent O(3), two equivalent O(6), and four equivalent O(5) atoms to form distorted EuO12 cuboctahedra that share corners with nine Eu(3,3)O12 cuboctahedra; faces with three equivalent Eu(1)O12 cuboctahedra; faces with three equivalent Nb(2)O6 octahedra; and faces with four Nb(1,1)O6 octahedra. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(6), two equivalent O(3), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra; corners with three equivalent Nb(2)O6 octahedra; faces with three equivalent Eu(1)O12 cuboctahedra; and faces with four Eu(3,3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-10°. In the second Nb site, Nb(1) is bonded to one O(4), one O(6), two equivalent O(3), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra; corners with three equivalent Nb(2)O6 octahedra; faces with three equivalent Eu(1)O12 cuboctahedra; and faces with four Eu(3,3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-9°. In the third Nb site, Nb(2) is bonded to one O(2), one O(6), two equivalent O(1), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra,  a faceface with one Eu(1)O12 cuboctahedra, and faces with three equivalent Eu(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 9-10°. In the fourth Nb site, Nb(2) is bonded to one O(2), one O(6), two equivalent O(1), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra,  a faceface with one Eu(1)O12 cuboctahedra, and faces with three equivalent Eu(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 9°. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Eu(3); three Eu(2,2); and one Nb(2) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Eu(3); three Eu(2,2); and one Nb(2) atom. In the third O site, O(2) is bonded in a 5-coordinate geometry to one Eu(3); three Eu(2,2); and one Nb(2) atom. In the fourth O site, O(1) is bonded in a 5-coordinate geometry to one Eu(3); three Eu(2,2); and one Nb(2) atom. In the fifth O site, O(3) is bonded in a distorted linear geometry to two equivalent Eu(1); two Eu(3,3); and two Nb(1,1) atoms. In the sixth O site, O(4) is bonded in a distorted linear geometry to two equivalent Eu(1); two Eu(3,3); and two Nb(1,1) atoms. In the seventh O site, O(5) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), two equivalent Eu(3), one Nb(1), and one Nb(2) atom. In the eighth O site, O(6) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), two equivalent Eu(3), one Nb(1), and one Nb(2) atom. In the ninth O site, O(6) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), two equivalent Eu(3), one Nb(1), and one Nb(2) atom. In the tenth O site, O(5) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), two equivalent Eu(3), one Nb(1), and one Nb(2) atom.
Eu5Nb4O15 crystallizes in the monoclinic Cm space group. There are five inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to two equivalent O(4); two O(6,6); four equivalent O(3); and four O(5,5) atoms to form EuO12 cuboctahedra that share corners with six equivalent Eu(1)O12 cuboctahedra; faces with six Eu(3,3)O12 cuboctahedra; faces with two Nb(2,2)O6 octahedra; and faces with six Nb(1,1)O6 octahedra. Both Eu(1)-O(4) bond lengths are 2.84 Å. Both Eu(1)-O(6,6) bond lengths are 2.77 Å. All Eu(1)-O(3) bond lengths are 2.84 Å. All Eu(1)-O(5,5) bond lengths are 2.78 Å. In the second Eu site, Eu(2) is bonded in a 12-coordinate geometry to one O(6); two equivalent O(5); three O(2,2); and six O(1,1) atoms. The Eu(2)-O(6) bond length is 3.25 Å. Both Eu(2)-O(5) bond lengths are 3.23 Å. There is one shorter (2.48 Å) and two longer (2.88 Å) Eu(2)-O(2,2) bond lengths. There are a spread of Eu(2)-O(1,1) bond distances ranging from 2.47-2.89 Å. In the third Eu site, Eu(2) is bonded in a 12-coordinate geometry to one O(6); two equivalent O(5); three O(2,2); and six O(1,1) atoms. The Eu(2)-O(6) bond length is 3.24 Å. Both Eu(2)-O(5) bond lengths are 3.24 Å. There is one shorter (2.47 Å) and two longer (2.88 Å) Eu(2)-O(2,2) bond lengths. There are two shorter (2.47 Å) and four longer (2.88 Å) Eu(2)-O(1,1) bond lengths. In the fourth Eu site, Eu(3) is bonded to one O(2), one O(4), two equivalent O(1), two equivalent O(3), two equivalent O(6), and four equivalent O(5) atoms to form distorted EuO12 cuboctahedra that share corners with nine Eu(3,3)O12 cuboctahedra; faces with three equivalent Eu(1)O12 cuboctahedra; faces with three equivalent Nb(2)O6 octahedra; and faces with four Nb(1,1)O6 octahedra. The Eu(3)-O(2) bond length is 2.52 Å. The Eu(3)-O(4) bond length is 3.06 Å. Both Eu(3)-O(1) bond lengths are 2.52 Å. Both Eu(3)-O(3) bond lengths are 3.06 Å. Both Eu(3)-O(6) bond lengths are 2.86 Å. There are two shorter (2.85 Å) and two longer (2.86 Å) Eu(3)-O(5) bond lengths. In the fifth Eu site, Eu(3) is bonded to one O(2), one O(4), two equivalent O(1), two equivalent O(3), two equivalent O(6), and four equivalent O(5) atoms to form distorted EuO12 cuboctahedra that share corners with nine Eu(3,3)O12 cuboctahedra; faces with three equivalent Eu(1)O12 cuboctahedra; faces with three equivalent Nb(2)O6 octahedra; and faces with four Nb(1,1)O6 octahedra. The Eu(3)-O(2) bond length is 2.52 Å. The Eu(3)-O(4) bond length is 3.06 Å. Both Eu(3)-O(1) bond lengths are 2.52 Å. Both Eu(3)-O(3) bond lengths are 3.06 Å. Both Eu(3)-O(6) bond lengths are 2.86 Å. There are two shorter (2.85 Å) and two longer (2.86 Å) Eu(3)-O(5) bond lengths. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(6), two equivalent O(3), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra; corners with three equivalent Nb(2)O6 octahedra; faces with three equivalent Eu(1)O12 cuboctahedra; and faces with four Eu(3,3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-10°. The Nb(1)-O(4) bond length is 2.03 Å. The Nb(1)-O(6) bond length is 1.98 Å. Both Nb(1)-O(3) bond lengths are 2.03 Å. Both Nb(1)-O(5) bond lengths are 1.97 Å. In the second Nb site, Nb(1) is bonded to one O(4), one O(6), two equivalent O(3), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra; corners with three equivalent Nb(2)O6 octahedra; faces with three equivalent Eu(1)O12 cuboctahedra; and faces with four Eu(3,3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-9°. The Nb(1)-O(4) bond length is 2.03 Å. The Nb(1)-O(6) bond length is 1.97 Å. Both Nb(1)-O(3) bond lengths are 2.03 Å. Both Nb(1)-O(5) bond lengths are 1.97 Å. In the third Nb site, Nb(2) is bonded to one O(2), one O(6), two equivalent O(1), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra,  a faceface with one Eu(1)O12 cuboctahedra, and faces with three equivalent Eu(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 9-10°. The Nb(2)-O(2) bond length is 1.90 Å. The Nb(2)-O(6) bond length is 2.15 Å. Both Nb(2)-O(1) bond lengths are 1.90 Å. Both Nb(2)-O(5) bond lengths are 2.15 Å. In the fourth Nb site, Nb(2) is bonded to one O(2), one O(6), two equivalent O(1), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra,  a faceface with one Eu(1)O12 cuboctahedra, and faces with three equivalent Eu(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 9°. The Nb(2)-O(2) bond length is 1.90 Å. The Nb(2)-O(6) bond length is 2.15 Å. Both Nb(2)-O(1) bond lengths are 1.90 Å. Both Nb(2)-O(5) bond lengths are 2.15 Å. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Eu(3); three Eu(2,2); and one Nb(2) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Eu(3); three Eu(2,2); and one Nb(2) atom. In the third O site, O(2) is bonded in a 5-coordinate geometry to one Eu(3); three Eu(2,2); and one Nb(2) atom. In the fourth O site, O(1) is bonded in a 5-coordinate geometry to one Eu(3); three Eu(2,2); and one Nb(2) atom. In the fifth O site, O(3) is bonded in a distorted linear geometry to two equivalent Eu(1); two Eu(3,3); and two Nb(1,1) atoms. In the sixth O site, O(4) is bonded in a distorted linear geometry to two equivalent Eu(1); two Eu(3,3); and two Nb(1,1) atoms. In the seventh O site, O(5) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), two equivalent Eu(3), one Nb(1), and one Nb(2) atom. In the eighth O site, O(6) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), two equivalent Eu(3), one Nb(1), and one Nb(2) atom. In the ninth O site, O(6) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), two equivalent Eu(3), one Nb(1), and one Nb(2) atom. In the tenth O site, O(5) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), two equivalent Eu(3), one Nb(1), and one Nb(2) atom.