DOE OSTI2020
Nd4Al2O9 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. there are four inequivalent Nd3+ sites. In the first Nd3+ site, Nd3+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Nd–O bond distances ranging from 2.35–3.01 Å. In the second Nd3+ site, Nd3+ is bonded to seven O2- atoms to form distorted NdO7 hexagonal pyramids that share corners with three AlO4 tetrahedra, edges with two equivalent NdO6 octahedra, and edges with two AlO4 tetrahedra. There are a spread of Nd–O bond distances ranging from 2.33–2.58 Å. In the third Nd3+ site, Nd3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Nd–O bond distances ranging from 2.39–2.76 Å. In the fourth Nd3+ site, Nd3+ is bonded to six O2- atoms to form NdO6 octahedra that share corners with five AlO4 tetrahedra and edges with two equivalent NdO7 hexagonal pyramids. There are a spread of Nd–O bond distances ranging from 2.33–2.47 Å. There are two inequivalent Al3+ sites. In the first Al3+ site, Al3+ is bonded to four O2- atoms to form AlO4 tetrahedra that share corners with two equivalent NdO7 hexagonal pyramids, corners with two equivalent NdO6 octahedra, a cornercorner with one AlO4 tetrahedra, and an edgeedge with one NdO7 hexagonal pyramid. The corner-sharing octahedra tilt angles range from 37–52°. There are a spread of Al–O bond distances ranging from 1.76–1.78 Å. In the second Al3+ site, Al3+ is bonded to four O2- atoms to form AlO4 tetrahedra that share a cornercorner with one NdO7 hexagonal pyramid, corners with three equivalent NdO6 octahedra, a cornercorner with one AlO4 tetrahedra, and an edgeedge with one NdO7 hexagonal pyramid. The corner-sharing octahedra tilt angles range from 47–61°. There is two shorter (1.76 Å) and two longer (1.78 Å) Al–O bond length. There are nine inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to three Nd3+ and one Al3+ atom. In the second O2- site, O2- is bonded in a 4-coordinate geometry to three Nd3+ and one Al3+ atom. In the third O2- site, O2- is bonded in a 3-coordinate geometry to three Nd3+ and one Al3+ atom. In the fourth O2- site, O2- is bonded to three Nd3+ and one Al3+ atom to form distorted corner-sharing ONd3Al tetrahedra. In the fifth O2- site, O2- is bonded to four Nd3+ atoms to form ONd4 tetrahedra that share corners with five ONd3Al tetrahedra and edges with two ONd4 tetrahedra. In the sixth O2- site, O2- is bonded to four Nd3+ atoms to form ONd4 tetrahedra that share corners with four ONd3Al tetrahedra and edges with two ONd4 tetrahedra. In the seventh O2- site, O2- is bonded in a 4-coordinate geometry to three Nd3+ and one Al3+ atom. In the eighth O2- site, O2- is bonded in a 4-coordinate geometry to three Nd3+ and one Al3+ atom. In the ninth O2- site, O2- is bonded in a 4-coordinate geometry to two Nd3+ and two Al3+ atoms.