DOI: 10.17188/1270195
title: Materials Data on CaHf4O9 by Materials Project

description: CaHf4O9 crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. there are three inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are a spread of Ca–O bond distances ranging from 2.34–2.69 Å. In the second 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 2.29–2.54 Å. In the third Ca2+ site, Ca2+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are a spread of Ca–O bond distances ranging from 2.32–2.62 Å. There are nine inequivalent Hf4+ sites. In the first Hf4+ site, Hf4+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of Hf–O bond distances ranging from 2.06–2.18 Å. 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, and edges with five HfO7 pentagonal bipyramids. The corner-sharing octahedra tilt angles range from 40–46°. There are a spread of Hf–O bond distances ranging from 2.11–2.28 Å. In the third 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 49°. There are a spread of Hf–O bond distances ranging from 2.04–2.10 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 four HfO7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 44°. There are a spread of Hf–O bond distances ranging from 2.11–2.25 Å. In the fifth Hf4+ site, Hf4+ is bonded to six O2- atoms to form HfO6 octahedra that share a cornercorner with one HfO6 octahedra, corners with five HfO7 pentagonal bipyramids, and an edgeedge with one HfO7 pentagonal bipyramid. The corner-sharing octahedral tilt angles are 49°. There are a spread of Hf–O bond distances ranging from 2.01–2.15 Å. In the sixth Hf4+ site, Hf4+ is bonded to seven O2- atoms to form distorted HfO7 pentagonal bipyramids that share a cornercorner with one HfO6 octahedra, corners with two HfO7 pentagonal bipyramids, an edgeedge with one HfO6 octahedra, and edges with three HfO7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 47°. There are a spread of Hf–O bond distances ranging from 2.07–2.34 Å. In the seventh Hf4+ site, Hf4+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are a spread of Hf–O bond distances ranging from 2.07–2.57 Å. In the eighth Hf4+ site, Hf4+ is bonded to seven O2- atoms to form distorted HfO7 pentagonal bipyramids that share a cornercorner with one HfO6 octahedra, corners with two HfO7 pentagonal bipyramids, an edgeedge with one HfO6 octahedra, and edges with four HfO7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 49°. There are a spread of Hf–O bond distances ranging from 2.03–2.23 Å. In the ninth Hf4+ site, Hf4+ is bonded to seven O2- atoms to form distorted HfO7 pentagonal bipyramids that share a cornercorner with one HfO6 octahedra, corners with two HfO7 pentagonal bipyramids, and edges with four HfO7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 53°. There are a spread of Hf–O bond distances ranging from 2.08–2.29 Å. There are eighteen inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to four Hf4+ atoms. In the second O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent Ca2+ and two Hf4+ atoms. In the third O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three Hf4+ atoms. In the fourth O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form a mixture of distorted edge and corner-sharing OCaHf3 tetrahedra. In the fifth O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form a mixture of distorted edge and corner-sharing OCaHf3 tetrahedra. In the sixth O2- site, O2- is bonded in a 4-coordinate geometry to one Ca2+ and three Hf4+ atoms. In the seventh O2- site, O2- is bonded to two Ca2+ and two Hf4+ atoms to form distorted OCa2Hf2 tetrahedra that share corners with five OHf4 tetrahedra and edges with three OCaHf3 tetrahedra. In the eighth O2- site, O2- is bonded in a 4-coordinate geometry to two Ca2+ and two Hf4+ atoms. In the ninth O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three Hf4+ atoms. In the tenth O2- site, O2- is bonded in a 4-coordinate geometry to one Ca2+ and three Hf4+ atoms. In the eleventh O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form distorted OCaHf3 tetrahedra that share corners with eight OHf4 tetrahedra and edges with four OCaHf3 tetrahedra. In the twelfth O2- site, O2- is bonded to four Hf4+ atoms to form a mixture of distorted edge and corner-sharing OHf4 tetrahedra. In the thirteenth O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form a mixture of distorted edge and corner-sharing OCaHf3 tetrahedra. In the fourteenth O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form distorted OCaHf3 tetrahedra that share corners with eight OHf4 tetrahedra and an edgeedge with one OCa2Hf2 tetrahedra. In the fifteenth O2- site, O2- is bonded in a 4-coordinate geometry to one Ca2+ and three Hf4+ atoms. In the sixteenth O2- site, O2- is bonded to four Hf4+ atoms to form distorted OHf4 tetrahedra that share corners with eight OHf4 tetrahedra and edges with two OCaHf3 tetrahedra. In the seventeenth O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form a mixture of edge and corner-sharing OCaHf3 tetrahedra. In the eighteenth O2- site, O2- is bonded to one Ca2+ and three Hf4+ atoms to form OCaHf3 tetrahedra that share corners with eight OHf4 tetrahedra and edges with three OCaHf3 tetrahedra.