DOE OSTI2020
NaCaAlF6 is zeta iron carbide-derived structured and crystallizes in the trigonal P321 space group. The structure is three-dimensional. Na1+ is bonded to six F1- atoms to form distorted NaF6 octahedra that share corners with two equivalent AlF6 octahedra, corners with six equivalent CaF6 octahedra, and edges with two equivalent AlF6 octahedra. The corner-sharing octahedra tilt angles range from 51–61°. There are a spread of Na–F bond distances ranging from 2.22–2.36 Å. Ca2+ is bonded to six F1- atoms to form CaF6 octahedra that share corners with four equivalent AlF6 octahedra, corners with six equivalent NaF6 octahedra, and an edgeedge with one AlF6 octahedra. The corner-sharing octahedra tilt angles range from 43–61°. There are a spread of Ca–F bond distances ranging from 2.28–2.34 Å. There are two inequivalent Al3+ sites. In the first Al3+ site, Al3+ is bonded to six F1- atoms to form AlF6 octahedra that share corners with six equivalent CaF6 octahedra and edges with three equivalent NaF6 octahedra. The corner-sharing octahedra tilt angles range from 43–44°. All Al–F bond lengths are 1.84 Å. In the second Al3+ site, Al3+ is bonded to six equivalent F1- atoms to form AlF6 octahedra that share corners with six equivalent NaF6 octahedra and edges with three equivalent CaF6 octahedra. The corner-sharing octahedral tilt angles are 51°. All Al–F bond lengths are 1.83 Å. There are three inequivalent F1- sites. In the first F1- site, F1- is bonded in a distorted trigonal planar geometry to one Na1+, one Ca2+, and one Al3+ atom. In the second F1- site, F1- is bonded in a 3-coordinate geometry to one Na1+, one Ca2+, and one Al3+ atom. In the third F1- site, F1- is bonded in a distorted trigonal planar geometry to one Na1+, one Ca2+, and one Al3+ atom.