DOE OSTI2020
Ca6Hf19O44 crystallizes in the trigonal R-3c space group. The structure is three-dimensional. there are four inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded in a body-centered cubic geometry to eight O2- atoms. There are a spread of Ca–O bond distances ranging from 2.43–2.60 Å. In the second Ca2+ site, Ca2+ is bonded in a 8-coordinate geometry to six O2- atoms. There are a spread of Ca–O bond distances ranging from 2.23–2.43 Å. In the third Ca2+ site, Ca2+ is bonded in a body-centered cubic geometry to eight O2- atoms. There are a spread of Ca–O bond distances ranging from 2.43–2.60 Å. In the fourth Ca2+ site, Ca2+ is bonded in a body-centered cubic geometry to eight O2- atoms. There are a spread of Ca–O bond distances ranging from 2.43–2.60 Å. There are eleven inequivalent Hf4+ sites. In the first Hf4+ site, Hf4+ is bonded to six O2- atoms to form HfO6 octahedra that share corners with two HfO6 octahedra, corners with four HfO7 pentagonal bipyramids, and edges with two equivalent HfO7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 48°. There are two shorter (2.04 Å) and four longer (2.10 Å) Hf–O bond lengths. In the second Hf4+ site, Hf4+ is bonded to seven O2- atoms to form distorted HfO7 pentagonal bipyramids that share corners with two HfO6 octahedra, a cornercorner with one HfO7 pentagonal bipyramid, edges with two HfO6 octahedra, and edges with three HfO7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 41–46°. There are a spread of Hf–O bond distances ranging from 2.04–2.37 Å. In the third Hf4+ site, Hf4+ is bonded to seven O2- atoms to form distorted HfO7 pentagonal bipyramids that share corners with two HfO6 octahedra, a cornercorner with one HfO7 pentagonal bipyramid, and edges with six HfO7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 40–53°. There are a spread of Hf–O bond distances ranging from 2.06–2.23 Å. In the fourth Hf4+ site, Hf4+ is bonded to seven O2- atoms to form distorted HfO7 pentagonal bipyramids that share corners with two HfO6 octahedra, a cornercorner with one HfO7 pentagonal bipyramid, and edges with six HfO7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 40–53°. There are a spread of Hf–O bond distances ranging from 2.06–2.23 Å. In the fifth Hf4+ site, Hf4+ is bonded to six O2- atoms to form HfO6 octahedra that share corners with two equivalent HfO6 octahedra, corners with four HfO7 pentagonal bipyramids, and edges with two equivalent HfO7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 48°. There are four shorter (2.07 Å) and two longer (2.10 Å) Hf–O bond lengths. In the sixth Hf4+ site, Hf4+ is bonded to seven O2- atoms to form distorted HfO7 pentagonal bipyramids that share corners with two HfO6 octahedra, a cornercorner with one HfO7 pentagonal bipyramid, and edges with six HfO7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 40–53°. There are a spread of Hf–O bond distances ranging from 2.06–2.23 Å. In the seventh Hf4+ site, Hf4+ is bonded to six O2- atoms to form HfO6 octahedra that share corners with two equivalent HfO6 octahedra, corners with four HfO7 pentagonal bipyramids, and edges with two equivalent HfO7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 48°. There are four shorter (2.07 Å) and two longer (2.10 Å) Hf–O bond lengths. In the eighth Hf4+ site, Hf4+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are six shorter (2.21 Å) and two longer (2.54 Å) Hf–O bond lengths. In the ninth Hf4+ site, Hf4+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are six shorter (2.21 Å) and two longer (2.54 Å) Hf–O bond lengths. In the tenth Hf4+ site, Hf4+ is bonded to six O2- atoms to form HfO6 octahedra that share corners with two equivalent HfO6 octahedra, corners with four HfO7 pentagonal bipyramids, and edges with two equivalent HfO7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 48°. There are four shorter (2.07 Å) and two longer (2.10 Å) Hf–O bond lengths. In the eleventh Hf4+ site, Hf4+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are six shorter (2.21 Å) and two longer (2.54 Å) Hf–O bond lengths. There are eight inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to three Hf4+ atoms. In the second O2- site, O2- is bonded in a 4-coordinate geometry to four Hf4+ atoms. In the third O2- site, O2- is bonded to four Hf4+ atoms to form distorted OHf4 tetrahedra that share corners with eight OCa2Hf2 tetrahedra, corners with two equivalent OCaHf3 trigonal pyramids, edges with three OHf4 tetrahedra, and an edgeedge with one OCaHf3 trigonal pyramid. In the fourth O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form distorted OCaHf3 trigonal pyramids that share corners with eight OCa2Hf2 tetrahedra, edges with three OCa2Hf2 tetrahedra, and an edgeedge with one OCaHf3 trigonal pyramid. In the fifth O2- site, O2- is bonded to two Ca2+ and two Hf4+ atoms to form OCa2Hf2 tetrahedra that share corners with eight OCa2Hf2 tetrahedra, corners with two equivalent OCaHf3 trigonal pyramids, edges with three OCa2Hf2 tetrahedra, and an edgeedge with one OCaHf3 trigonal pyramid. In the sixth O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form distorted OCaHf3 tetrahedra that share corners with nine OCa2Hf2 tetrahedra, a cornercorner with one OCaHf3 trigonal pyramid, and edges with three OCa2Hf2 tetrahedra. In the seventh O2- site, O2- is bonded in a 4-coordinate geometry to one Ca2+ and three Hf4+ atoms. In the eighth O2- site, O2- is bonded to two Ca2+ and two Hf4+ atoms to form distorted OCa2Hf2 tetrahedra that share corners with seven OCa2Hf2 tetrahedra, corners with three equivalent OCaHf3 trigonal pyramids, edges with two OCa2Hf2 tetrahedra, and an edgeedge with one OCaHf3 trigonal pyramid.