Distorted chiolite crystal structures of α-Na5M3F14 (M=Cr,Fe,Ga) studied by X-ray powder diffraction

The crystal structures of the chiolite-related room temperature phases α-Na 5 M 3 F 14 (M III =Cr,Fe,Ga) are determined. For all of them, the space group is P 2 1 / n , Z =2; a =10.5096(3) Å, b =7.2253(2) Å, c =7.2713(2) Å, β=90.6753(7)° (M=Cr); a =10.4342(7) Å, b =7.3418(6) Å, c =7.4023(6) Å, β=90.799(5)° (M=Fe), and a =10.4052(1) Å, b =7.2251(1) Å, c =7.2689(1), β=90.6640(4)° (M=Ga). Rietveld refinements produce final R F factors 0.036, 0.033, and 0.035, and R WP factors, 0.125, 0.116, and 0.096, for M III =Cr, Fe, and Ga, respectively. The MF 6 polyhedra in the defective isolated perovskite-like layers deviate very few from perfect octahedra. Subtle octahedra tiltings lead to the symmetry decrease from the P 4/ mnc space group adopted by the Na 5 Al 3 F 14 chiolite aristotype to the P 2 1 / n space group adopted by the title series. Facile twinning precluded till now the precise characterization of these compounds.