The dimorphism of LiMnFeF 6: A new kind of cationic order in the structural type Na 2SiF 6

LiMnFeF ₆ shows a dimorphism; the transition temperature α → β occurs at 560°C. The symmetry of the two phases is trigonal (space group P321, Z = 3) and the unit cell data are very close to each other: a_α = 8.684(2), c_α = 4.657(1), a_β = 8.723(2), c_β = 4.745(1) Å. The absolute structure of α-LiMnFeF ₆ was determined from single-crystal X-ray data ( R = 0.020). This crystalline form exhibits a new kind of cationic order in the structural type Na ₂SiF ₆. The cationic distribution of β-LiMnFeF ₆ was specified from X-ray and neutron powder diffraction data; this compound is isotypical with the LiMnGaF ₆ structure. The different kinds of cationic distribution related to the Na ₂SiF ₆ type are discussed and a classification of the compounds Li(Na) M^IIM^IIIF ₆ is given.