DOE OSTI2020
Tb4Al2O9 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. there are four inequivalent Tb3+ sites. In the first Tb3+ site, Tb3+ is bonded to seven O2- atoms to form distorted TbO7 pentagonal bipyramids that share corners with three AlO4 tetrahedra, edges with two equivalent TbO6 octahedra, and edges with two AlO4 tetrahedra. There are a spread of Tb–O bond distances ranging from 2.27–2.55 Å. In the second Tb3+ site, Tb3+ is bonded to six O2- atoms to form TbO6 octahedra that share corners with five AlO4 tetrahedra and edges with two equivalent TbO7 pentagonal bipyramids. There are a spread of Tb–O bond distances ranging from 2.24–2.39 Å. In the third Tb3+ site, Tb3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Tb–O bond distances ranging from 2.28–2.67 Å. In the fourth Tb3+ site, Tb3+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of Tb–O bond distances ranging from 2.28–2.37 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 three equivalent TbO6 octahedra, a cornercorner with one TbO7 pentagonal bipyramid, a cornercorner with one AlO4 tetrahedra, and an edgeedge with one TbO7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 51–62°. There are a spread of Al–O bond distances ranging from 1.75–1.78 Å. In the second Al3+ site, Al3+ is bonded to four O2- atoms to form AlO4 tetrahedra that share corners with two equivalent TbO6 octahedra, corners with two equivalent TbO7 pentagonal bipyramids, a cornercorner with one AlO4 tetrahedra, and an edgeedge with one TbO7 pentagonal bipyramid. The corner-sharing octahedra tilt angles range from 39–54°. There are a spread of Al–O bond distances ranging from 1.76–1.78 Å. There are nine inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to two Tb3+ and one Al3+ atom. In the second O2- site, O2- is bonded in a 4-coordinate geometry to three Tb3+ and one Al3+ atom. In the third O2- site, O2- is bonded to two Tb3+ and two Al3+ atoms to form a mixture of distorted edge and corner-sharing OTb2Al2 tetrahedra. In the fourth O2- site, O2- is bonded in a 4-coordinate geometry to three Tb3+ and one Al3+ atom. In the fifth O2- site, O2- is bonded in a distorted trigonal planar geometry to two Tb3+ and one Al3+ atom. In the sixth O2- site, O2- is bonded to four Tb3+ atoms to form OTb4 tetrahedra that share corners with five OTb2Al2 tetrahedra and edges with two OTb4 tetrahedra. In the seventh O2- site, O2- is bonded in a 4-coordinate geometry to three Tb3+ and one Al3+ atom. In the eighth O2- site, O2- is bonded to four Tb3+ atoms to form a mixture of edge and corner-sharing OTb4 tetrahedra. In the ninth O2- site, O2- is bonded to three Tb3+ and one Al3+ atom to form a mixture of distorted edge and corner-sharing OTb3Al tetrahedra.