BaCaEuNbO6 crystallizes in the orthorhombic P222 space group. There are four inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to four equivalent O(1), four equivalent O(2), and four equivalent O(3) atoms to form distorted BaO12 cuboctahedra that share faces with four equivalent Eu(1)O6 octahedra and faces with four equivalent Nb(1)O6 octahedra. In the second Ba site, Ba(2) is bonded to four equivalent O(4), four equivalent O(5), and four equivalent O(6) atoms to form BaO12 cuboctahedra that share faces with four equivalent Eu(1)O6 octahedra and faces with four equivalent Nb(1)O6 octahedra. In the third Ba site, Ba(3) is bonded in a 8-coordinate geometry to four equivalent O(2) and four equivalent O(6) atoms. In the fourth Ba site, Ba(4) is bonded in a distorted body-centered cubic geometry to four equivalent O(1) and four equivalent O(6) atoms. There are four inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a square co-planar geometry to four equivalent O(5) atoms. In the second Ca site, Ca(2) is bonded in a distorted square co-planar geometry to four equivalent O(4) atoms. In the third Ca site, Ca(3) is bonded in a rectangular see-saw-like geometry to four equivalent O(2) atoms. In the fourth Ca site, Ca(4) is bonded in a square co-planar geometry to four equivalent O(3) atoms. Eu(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form EuO6 octahedra that share corners with six equivalent Nb(1)O6 octahedra,  a faceface with one Ba(1)O12 cuboctahedra, and  a faceface with one Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 21-34°. Nb(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form NbO6 octahedra that share corners with six equivalent Eu(1)O6 octahedra,  a faceface with one Ba(1)O12 cuboctahedra, and  a faceface with one Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 21-34°. There are six inequivalent O sites. In the first O site, O(5) is bonded in a 3-coordinate geometry to one Ba(2), one Ca(1), one Eu(1), and one Nb(1) atom. In the second O site, O(6) is bonded in a 5-coordinate geometry to one Ba(2), one Ba(3), one Ba(4), one Eu(1), and one Nb(1) atom. In the third O site, O(1) is bonded in a 4-coordinate geometry to one Ba(1), one Ba(4), one Eu(1), and one Nb(1) atom. In the fourth O site, O(2) is bonded in a 3-coordinate geometry to one Ba(1), one Ba(3), one Ca(3), one Eu(1), and one Nb(1) atom. In the fifth O site, O(3) is bonded in a 3-coordinate geometry to one Ba(1), one Ca(4), one Eu(1), and one Nb(1) atom. In the sixth O site, O(4) is bonded in a 3-coordinate geometry to one Ba(2), one Ca(2), one Eu(1), and one Nb(1) atom.
BaCaEuNbO6 crystallizes in the orthorhombic P222 space group. There are four inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to four equivalent O(1), four equivalent O(2), and four equivalent O(3) atoms to form distorted BaO12 cuboctahedra that share faces with four equivalent Eu(1)O6 octahedra and faces with four equivalent Nb(1)O6 octahedra. All Ba(1)-O(1) bond lengths are 2.87 Å. All Ba(1)-O(2) bond lengths are 3.02 Å. All Ba(1)-O(3) bond lengths are 3.13 Å. In the second Ba site, Ba(2) is bonded to four equivalent O(4), four equivalent O(5), and four equivalent O(6) atoms to form BaO12 cuboctahedra that share faces with four equivalent Eu(1)O6 octahedra and faces with four equivalent Nb(1)O6 octahedra. All Ba(2)-O(4) bond lengths are 3.00 Å. All Ba(2)-O(5) bond lengths are 3.02 Å. All Ba(2)-O(6) bond lengths are 3.00 Å. In the third Ba site, Ba(3) is bonded in a 8-coordinate geometry to four equivalent O(2) and four equivalent O(6) atoms. All Ba(3)-O(2) bond lengths are 3.08 Å. All Ba(3)-O(6) bond lengths are 2.64 Å. In the fourth Ba site, Ba(4) is bonded in a distorted body-centered cubic geometry to four equivalent O(1) and four equivalent O(6) atoms. All Ba(4)-O(1) bond lengths are 2.64 Å. All Ba(4)-O(6) bond lengths are 3.03 Å. There are four inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a square co-planar geometry to four equivalent O(5) atoms. All Ca(1)-O(5) bond lengths are 2.40 Å. In the second Ca site, Ca(2) is bonded in a distorted square co-planar geometry to four equivalent O(4) atoms. All Ca(2)-O(4) bond lengths are 2.40 Å. In the third Ca site, Ca(3) is bonded in a rectangular see-saw-like geometry to four equivalent O(2) atoms. All Ca(3)-O(2) bond lengths are 2.42 Å. In the fourth Ca site, Ca(4) is bonded in a square co-planar geometry to four equivalent O(3) atoms. All Ca(4)-O(3) bond lengths are 2.38 Å. Eu(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form EuO6 octahedra that share corners with six equivalent Nb(1)O6 octahedra,  a faceface with one Ba(1)O12 cuboctahedra, and  a faceface with one Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 21-34°. The Eu(1)-O(1) bond length is 2.31 Å. The Eu(1)-O(2) bond length is 2.39 Å. The Eu(1)-O(3) bond length is 2.29 Å. The Eu(1)-O(4) bond length is 2.32 Å. The Eu(1)-O(5) bond length is 2.31 Å. The Eu(1)-O(6) bond length is 2.36 Å. Nb(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form NbO6 octahedra that share corners with six equivalent Eu(1)O6 octahedra,  a faceface with one Ba(1)O12 cuboctahedra, and  a faceface with one Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 21-34°. The Nb(1)-O(1) bond length is 2.00 Å. The Nb(1)-O(2) bond length is 2.08 Å. The Nb(1)-O(3) bond length is 2.02 Å. The Nb(1)-O(4) bond length is 2.04 Å. The Nb(1)-O(5) bond length is 2.02 Å. The Nb(1)-O(6) bond length is 2.03 Å. There are six inequivalent O sites. In the first O site, O(5) is bonded in a 3-coordinate geometry to one Ba(2), one Ca(1), one Eu(1), and one Nb(1) atom. In the second O site, O(6) is bonded in a 5-coordinate geometry to one Ba(2), one Ba(3), one Ba(4), one Eu(1), and one Nb(1) atom. In the third O site, O(1) is bonded in a 4-coordinate geometry to one Ba(1), one Ba(4), one Eu(1), and one Nb(1) atom. In the fourth O site, O(2) is bonded in a 3-coordinate geometry to one Ba(1), one Ba(3), one Ca(3), one Eu(1), and one Nb(1) atom. In the fifth O site, O(3) is bonded in a 3-coordinate geometry to one Ba(1), one Ca(4), one Eu(1), and one Nb(1) atom. In the sixth O site, O(4) is bonded in a 3-coordinate geometry to one Ba(2), one Ca(2), one Eu(1), and one Nb(1) atom.