α-Li₃InF₆, a ternary fluoride with a new structure type

Reaction of anhydrous binary fluorides LiF and InF₃ in sealed platinum tube at the temperature gradient 680/670°C yields α-Li₃InF₆. This low-temperature form crystallizes in the space group P2₁/m with the cell dimensions a = 4.797(3) Å, b = 8.679(1) Å, c = 10.366(1) Å and β = 90.90(1)°. The framework is built from InF₆ and LiF₆ octahedra, beside [Li₂F₆] bitetrahedra. The b-axis seems to be the backbone in the structural features. Running along it, zigzag anionic chains $({\rm LiF}_4 )_n^{3n - }$, resulting from one—edge shared [Li₂F₁₀] groups and double files of single InF₆ octahedra sharing common corners with [Li₂F₆], create an approximately hexagonally packed framework in the plane (1 0 0). The antiferroelectric behaviour evident from dielectric studies is confirmed by the antiparallel displacements of In and Li atoms from octahedral centres.

 This structure is mainly featured by the disorder observed in the lightest atoms. These atoms, which exhibit two kinds of environments, have been refined as two partially occupied sites for the tetrahedral one. Within the Li₃MF₆ compounds (M: trivalent or 3d element), this new structure reveals unexpected structural features, inducing its ferroic properties.