DOE OSTI2020
Ca9Si6(WO7)4 crystallizes in the triclinic P-1 space group. The structure is three-dimensional. there are five inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ca–O bond distances ranging from 2.29–2.60 Å. In the second Ca2+ site, Ca2+ is bonded to six O2- atoms to form distorted CaO6 octahedra that share a cornercorner with one CaO6 octahedra, corners with four SiO4 tetrahedra, and edges with four CaO6 octahedra. The corner-sharing octahedral tilt angles are 70°. There are a spread of Ca–O bond distances ranging from 2.20–2.68 Å. In the third Ca2+ site, Ca2+ is bonded to six O2- atoms to form distorted CaO6 octahedra that share a cornercorner with one CaO6 octahedra, corners with four SiO4 tetrahedra, a cornercorner with one WO5 trigonal bipyramid, and edges with four CaO6 octahedra. The corner-sharing octahedral tilt angles are 67°. There are a spread of Ca–O bond distances ranging from 2.20–2.78 Å. In the fourth Ca2+ site, Ca2+ is bonded to six O2- atoms to form CaO6 octahedra that share corners with two CaO6 octahedra, corners with three SiO4 tetrahedra, a cornercorner with one WO5 trigonal bipyramid, an edgeedge with one CaO6 octahedra, and an edgeedge with one WO5 trigonal bipyramid. The corner-sharing octahedra tilt angles range from 67–70°. There are a spread of Ca–O bond distances ranging from 2.38–2.53 Å. In the fifth Ca2+ site, Ca2+ is bonded in a linear geometry to two equivalent O2- atoms. Both Ca–O bond lengths are 2.16 Å. There are two inequivalent W+3.50+ sites. In the first W+3.50+ site, W+3.50+ is bonded to five O2- atoms to form WO5 trigonal bipyramids that share corners with two CaO6 octahedra, a cornercorner with one SiO4 tetrahedra, an edgeedge with one CaO6 octahedra, and an edgeedge with one WO5 trigonal bipyramid. The corner-sharing octahedra tilt angles range from 19–49°. There are a spread of W–O bond distances ranging from 1.93–2.25 Å. In the second W+3.50+ site, W+3.50+ is bonded in a rectangular see-saw-like geometry to four O2- atoms. There are a spread of W–O bond distances ranging from 1.95–2.22 Å. There are three inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with two CaO6 octahedra, corners with two SiO4 tetrahedra, and a cornercorner with one WO5 trigonal bipyramid. The corner-sharing octahedra tilt angles range from 67–73°. There is two shorter (1.63 Å) and two longer (1.65 Å) Si–O bond length. In the second Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with five CaO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 44–74°. There are a spread of Si–O bond distances ranging from 1.62–1.66 Å. In the third Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four CaO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 54–68°. There are a spread of Si–O bond distances ranging from 1.62–1.68 Å. There are fourteen inequivalent O2- sites. In the first O2- site, O2- is bonded in a 2-coordinate geometry to one Ca2+ and two Si4+ atoms. In the second O2- site, O2- is bonded to three Ca2+ and one Si4+ atom to form distorted OCa3Si tetrahedra that share corners with two equivalent OCa3W tetrahedra, a cornercorner with one OCa3Si trigonal pyramid, and an edgeedge with one OCa3Si tetrahedra. In the third O2- site, O2- is bonded in a distorted T-shaped geometry to one Ca2+ and two equivalent W+3.50+ atoms. In the fourth O2- site, O2- is bonded in a bent 150 degrees geometry to two Si4+ atoms. In the fifth O2- site, O2- is bonded in a 4-coordinate geometry to three Ca2+ and one Si4+ atom. In the sixth O2- site, O2- is bonded to three Ca2+ and one Si4+ atom to form distorted OCa3Si trigonal pyramids that share corners with four OCa3W tetrahedra and an edgeedge with one OCa3Si trigonal pyramid. In the seventh O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ca2+ atoms. In the eighth O2- site, O2- is bonded in a 1-coordinate geometry to two W+3.50+ and one Si4+ atom. In the ninth O2- site, O2- is bonded in a bent 150 degrees geometry to two Si4+ atoms. In the tenth O2- site, O2- is bonded in a distorted see-saw-like geometry to three Ca2+ and one W+3.50+ atom. In the eleventh O2- site, O2- is bonded to three Ca2+ and one W+3.50+ atom to form distorted corner-sharing OCa3W tetrahedra. In the twelfth O2- site, O2- is bonded in a distorted T-shaped geometry to one Ca2+ and two W+3.50+ atoms. In the thirteenth O2- site, O2- is bonded in a 1-coordinate geometry to two Ca2+, one W+3.50+, and one Si4+ atom. In the fourteenth O2- site, O2- is bonded in a 4-coordinate geometry to three Ca2+ and one Si4+ atom.