A new form of FeF 3 with the pyrochlore structure : Soft chemistry synthesis, crystal structure, thermal transitions and structural correlations with the other forms of FeF 3

Abstract

The topotactic oxidization of the mixed valence fluoride NH 4Fe 2F 6, which exhibits the ordered modified pyrochlore structure leads to a cubic pyrochlore form of FeF 3, hereafter noted Pyr-FeF 3 (S.G. Fd3m, Z = 16 a = 10,325(2) A ̊ ). Fe 3+ and F − ions occupy positions 16 c (0 0 0) and 48 f (x 1 8 1 8 with x = 0,3104 (5)) respectively. The best oxidization reaction occurs in a boiling solution of Br 2 in acetonitrile (T = 81°C). The treatment leads to a zeolitic association whose composition varies from (NH 3) 0.10FeF 3 to (NH 3) 0.25FeF 3 according to the experimental conditions. A similar reaction allows the preparation of HTBFeF 3 (3) from the fluorinated bronze (NH 4) 0.25FeF 3. PyrFeF 3 orders magnetically at 20(2) K ; this low value is related to the highly frustrating character of the Fe 3+ cationic sublattice.