THE NAF-CAF2-AIF3 SYSTEM - STRUCTURES OF BETA-NACAALF6 AND NA4CA4AL7F33

The crystal structure of β-NaCaAlF6 has been determined from X-ray powder diffraction data (RI = 0.054, RP = 0.102). The cell is trigonal (Na2SiF6-type; SG P 321) with a = 8.9295(9)Å, c = 5.0642(2)Å, and Z = 3. This compound is isotypical with β-LiMnFeF6. The α-NaCaAlF6 phase is indexed in a monoclinic cell (a = 8.738(3)Å, b = 5.190(3)Å, c = 20.342(9)Å, β = 91.451(6)°). Na4Ca4Al7F33 is cubic (SG I m3m) with a = 10.781(3)Å and Z = 2. The crystal structure is refined from 182 reflections to R = 0.039 (RW = 0.031). Aluminum and calcium octahedra build up a three-dimensional network [Ca4Al7F33]4- in which are inserted the Na+ ions. Finally, a comparison of the Na4Ca4Al7F33 structure with the Na5Al3F14 and Na3Sr4Al5F26 structures is given. © 1990.