Ba3Hf2O7 crystallizes in the tetragonal I4/mmm space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to four equivalent O(1) and eight equivalent O(2) atoms to form BaO12 cuboctahedra that share corners with four equivalent Ba(1)O12 cuboctahedra, faces with four equivalent Ba(1)O12 cuboctahedra, and faces with eight equivalent Hf(1)O6 octahedra. In the second Ba site, Ba(2) is bonded in a 9-coordinate geometry to four equivalent O(2) and five equivalent O(3) atoms. Hf(1) is bonded to one O(1), one O(3), and four equivalent O(2) atoms to form HfO6 octahedra that share corners with five equivalent Hf(1)O6 octahedra and faces with four equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-1°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted linear geometry to four equivalent Ba(1) and two equivalent Hf(1) atoms. In the second O site, O(2) is bonded in a distorted linear geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Hf(1) atoms. In the third O site, O(3) is bonded to five equivalent Ba(2) and one Hf(1) atom to form a mixture of distorted corner and edge-sharing OBa5Hf octahedra. The corner-sharing octahedral tilt angles are 4°.
Ba3Hf2O7 crystallizes in the tetragonal I4/mmm space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to four equivalent O(1) and eight equivalent O(2) atoms to form BaO12 cuboctahedra that share corners with four equivalent Ba(1)O12 cuboctahedra, faces with four equivalent Ba(1)O12 cuboctahedra, and faces with eight equivalent Hf(1)O6 octahedra. All Ba(1)-O(1) bond lengths are 2.95 Å. All Ba(1)-O(2) bond lengths are 2.95 Å. In the second Ba site, Ba(2) is bonded in a 9-coordinate geometry to four equivalent O(2) and five equivalent O(3) atoms. All Ba(2)-O(2) bond lengths are 2.87 Å. There is one shorter (2.69 Å) and four longer (2.95 Å) Ba(2)-O(3) bond lengths. Hf(1) is bonded to one O(1), one O(3), and four equivalent O(2) atoms to form HfO6 octahedra that share corners with five equivalent Hf(1)O6 octahedra and faces with four equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-1°. The Hf(1)-O(1) bond length is 2.11 Å. The Hf(1)-O(3) bond length is 2.07 Å. All Hf(1)-O(2) bond lengths are 2.08 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted linear geometry to four equivalent Ba(1) and two equivalent Hf(1) atoms. In the second O site, O(2) is bonded in a distorted linear geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Hf(1) atoms. In the third O site, O(3) is bonded to five equivalent Ba(2) and one Hf(1) atom to form a mixture of distorted corner and edge-sharing OBa5Hf octahedra. The corner-sharing octahedral tilt angles are 4°.