DOE OSTI2020
Tm4Al2O9 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. there are four inequivalent Tm3+ sites. In the first Tm3+ site, Tm3+ is bonded to six O2- atoms to form TmO6 octahedra that share corners with three equivalent AlO4 tetrahedra, corners with two equivalent AlO4 trigonal pyramids, and edges with two equivalent TmO7 hexagonal pyramids. There are a spread of Tm–O bond distances ranging from 2.16–2.34 Å. In the second Tm3+ site, Tm3+ is bonded to seven O2- atoms to form distorted TmO7 hexagonal pyramids that share a cornercorner with one AlO4 tetrahedra, corners with two equivalent AlO4 trigonal pyramids, edges with two equivalent TmO6 octahedra, an edgeedge with one AlO4 tetrahedra, and an edgeedge with one AlO4 trigonal pyramid. There are a spread of Tm–O bond distances ranging from 2.22–2.47 Å. In the third Tm3+ site, Tm3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Tm–O bond distances ranging from 2.21–2.63 Å. In the fourth Tm3+ site, Tm3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Tm–O bond distances ranging from 2.21–2.78 Å. There are two inequivalent Al3+ sites. In the first Al3+ site, Al3+ is bonded to four O2- atoms to form AlO4 trigonal pyramids that share corners with two equivalent TmO7 hexagonal pyramids, corners with two equivalent TmO6 octahedra, a cornercorner with one AlO4 tetrahedra, and an edgeedge with one TmO7 hexagonal pyramid. The corner-sharing octahedra tilt angles range from 40–53°. There are a spread of Al–O bond distances ranging from 1.75–1.77 Å. In the second Al3+ site, Al3+ is bonded to four O2- atoms to form AlO4 tetrahedra that share a cornercorner with one TmO7 hexagonal pyramid, corners with three equivalent TmO6 octahedra, a cornercorner with one AlO4 trigonal pyramid, and an edgeedge with one TmO7 hexagonal pyramid. The corner-sharing octahedra tilt angles range from 52–61°. There are a spread of Al–O bond distances ranging from 1.74–1.77 Å. There are nine inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to three Tm3+ and one Al3+ atom. In the second O2- site, O2- is bonded to four Tm3+ atoms to form OTm4 tetrahedra that share corners with five OTm3Al tetrahedra and edges with two OTm4 tetrahedra. In the third O2- site, O2- is bonded in a 4-coordinate geometry to three Tm3+ and one Al3+ atom. In the fourth O2- site, O2- is bonded to three Tm3+ and one Al3+ atom to form distorted OTm3Al tetrahedra that share corners with six OTm4 tetrahedra and an edgeedge with one OTm2Al2 tetrahedra. In the fifth O2- site, O2- is bonded to two Tm3+ and two Al3+ atoms to form distorted OTm2Al2 tetrahedra that share corners with three OTm4 tetrahedra and edges with two OTm3Al tetrahedra. In the sixth O2- site, O2- is bonded in a distorted trigonal planar geometry to two Tm3+ and one Al3+ atom. In the seventh O2- site, O2- is bonded in a 4-coordinate geometry to three Tm3+ and one Al3+ atom. In the eighth O2- site, O2- is bonded to four Tm3+ atoms to form a mixture of edge and corner-sharing OTm4 tetrahedra. In the ninth O2- site, O2- is bonded in a 4-coordinate geometry to three Tm3+ and one Al3+ atom.