DOE OSTI2020
Ho4Al2O9 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. there are four inequivalent Ho3+ sites. In the first Ho3+ site, Ho3+ is bonded to six O2- atoms to form HoO6 octahedra that share corners with five AlO4 tetrahedra and edges with two equivalent HoO7 hexagonal pyramids. There are a spread of Ho–O bond distances ranging from 2.20–2.38 Å. In the second Ho3+ site, Ho3+ is bonded to seven O2- atoms to form distorted HoO7 hexagonal pyramids that share corners with three AlO4 tetrahedra, edges with two equivalent HoO6 octahedra, and edges with two AlO4 tetrahedra. There are a spread of Ho–O bond distances ranging from 2.25–2.50 Å. In the third Ho3+ site, Ho3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ho–O bond distances ranging from 2.25–2.66 Å. In the fourth Ho3+ site, Ho3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ho–O bond distances ranging from 2.25–2.79 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 HoO7 hexagonal pyramids, corners with two equivalent HoO6 octahedra, a cornercorner with one AlO4 tetrahedra, and an edgeedge with one HoO7 hexagonal pyramid. The corner-sharing octahedra tilt angles range from 40–53°. There is two shorter (1.76 Å) and two longer (1.77 Å) Al–O bond length. In the second Al3+ site, Al3+ is bonded to four O2- atoms to form AlO4 tetrahedra that share a cornercorner with one HoO7 hexagonal pyramid, corners with three equivalent HoO6 octahedra, a cornercorner with one AlO4 tetrahedra, and an edgeedge with one HoO7 hexagonal pyramid. 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. There are nine inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to three Ho3+ and one Al3+ atom. In the second O2- site, O2- is bonded to four Ho3+ atoms to form OHo4 tetrahedra that share corners with five OHo3Al tetrahedra and edges with two OHo4 tetrahedra. In the third O2- site, O2- is bonded in a 4-coordinate geometry to three Ho3+ and one Al3+ atom. In the fourth O2- site, O2- is bonded to three Ho3+ and one Al3+ atom to form distorted OHo3Al tetrahedra that share corners with six OHo4 tetrahedra and an edgeedge with one OHo2Al2 tetrahedra. In the fifth O2- site, O2- is bonded to two Ho3+ and two Al3+ atoms to form distorted OHo2Al2 tetrahedra that share corners with three OHo4 tetrahedra and edges with two OHo3Al tetrahedra. In the sixth O2- site, O2- is bonded in a distorted trigonal planar geometry to two Ho3+ and one Al3+ atom. In the seventh O2- site, O2- is bonded in a 4-coordinate geometry to three Ho3+ and one Al3+ atom. In the eighth O2- site, O2- is bonded to four Ho3+ atoms to form a mixture of edge and corner-sharing OHo4 tetrahedra. In the ninth O2- site, O2- is bonded in a 4-coordinate geometry to three Ho3+ and one Al3+ atom.