Mixed-valent iron fluoride hydrates and their thermal-decomposition products

Abstract

The mixed-valent iron fluoride hydrate Fe₂F₅·7H₂O has been confirmed by chemical and thermal analysis. The unit cell has been found by powder and single-crystal diffractometry to be triclinic with a= 6·582, b= 8·988, c= 10·542 × 10^–10 m, and α= 105·81, β= 123·28, and γ= 82·93°. Thermal analysis shows that at 75 °C in flowing nitrogen the heptahydrate loses water to form the dihydrate, Fe₂F₅·2H₂O. Further dehydration in nitrogen produces two phases (A and B) not previously reported and the two components FeF₂ and FeF₃. Phase B occurs in a very narrow temperature range, 190–230 °C, but gives sharp reflections which have been indexed according to the cubic unit cell, a= 10·37 × 10¹⁰ m. Phase A is stable over a wider temperature range (190–400 °C) and gives broadened lines in the X-ray Guinier-powder photograph. These have been indexed on the basis of the hexagonal unit cell, a= 6·40, c= 7·56 × 10^–10 m. The components FeF₂ and FeF₃ first appear at ca. 250 °C and above 500 °C are the only crystalline phases present. The two dehydration steps in the decomposition of the heptahydrate occur at much lower temperatures in flowing HF than in N₂. The first step also yields the dihydrate, but the second step gives an anhydrous product which contains a phase A′ with an X-ray pattern similar to that of A but no reflections of B are detected. No ferromagnetism is exhibited by any of the new phases.