G. Ferey

A new form of FeF3 with the pyrochlore structure : Soft chemistry synthesis, crystal structure, thermal transitions and structural correlations with the other forms of FeF3

The topotactic oxidization of the mixed valence fluoride NH4Fe2F6, which exhibits the ordered modified pyrochlore structure leads to a cubic pyrochlore form of FeF3, hereafter noted Pyr-FeF3 (S.G. Fd3m, Z = 16 a = 10,325(2) A). Fe3+ and F− ions occupy positions 16 c (0 0 0) and 48 f (x 1818 with x = 0,3104 (5)) respectively. The best oxidization reaction occurs in a boiling solution of Br2 in acetonitrile (T = 81°C). The treatment leads to a zeolitic association whose composition varies from (NH3)0.10FeF3 to (NH3)0.25FeF3 according to the experimental conditions. A similar reaction allows the preparation of HTBFeF3 (3) from the fluorinated bronze (NH4)0.25FeF3. PyrFeF3 orders magnetically at 20(2) K ; this low value is related to the highly frustrating character of the Fe3+ cationic sublattice.

Croissance cristalline des composés fluorés de type RbCoF3 perovskite et RbCoCrF6 pyrochlore

Abstract Studies of crystal growth of fluorinated perovskites AMe II F 3 and pyrochlores AMe II Me III F 6 (A = K, Rb, Cs; Me II = Fe, Co, Ni, Zn; Me III = Al, V, Cr, Fe) are reported. In order to avoid high temperature hydrolysis of these compounds thus giving MeO or Me 2 O 3 , two techniques of growth have been selected: 1) flux growth from a mixture containing melted chlorides and 2) the Bridgman-Stockbarger method in sealed platinum crucible. Phase diagrams are used in order to obtain maximum yield of crystals and avoid side crystallization.

Crystal structure of the ordered pyrochlore NH4FeIIFeIIIF6 structural correlations with Fe2F5 · 2H2O and its dehydration product Fe2F5 · H2O

Abstract Crystal structure of NH4FeIIFeIIIF6 is studied in order to explain further its peculiar antiferromagnetic behavior compared to the spin glass one of the pyrochlore family. NH4FeIIFeIIIF6 is orthorhombic, space group Pnma with a = 7.045 (4) A , b = 7.454 (4) A , c = 10.116 (6) A , Z = 4. Diffraction data on single crystals obtained by hydrothermal synthesis, collected on an automatic four circle diffractometer, have been refined by full matrix least-squares calculations to a weighted value of 0.029 (unweighted R = 0.024) for 798 observed reflections. This structure is derived from the pyrochlore structure, with a cationic order between Fe2+ and Fe3+ ions. (FeIIF6)4− octahedra form infinite trans chains along [100] by sharing corners although similar chains of (FeIIIF6)3−) octahedra lie along [010]. This type of FeIIFeIII order is related to a similar one existing in Fe2F5 · 2H2O, the dehydration of which leads to the pyrochlore Fe2F5 · H2O. A mechanism is proposed to explain the formation of this compound.

Caracterisation d'une variete amorphie de FeF3 : Etude thermique, magnetique et Mossbauer

Abstract Amorphous FeF3 has been prepared by fast vaporisation of the bulk material and condensation : it has been characterized by X-ray diffraction and differential thermal analysis. Strong antiferromagnetic interactions, but low spin-freezing temperature are observed. The magnetic arrangment is speromagnetic. The spin glass behaviour is questionable; the Mossbauer study reveals an electric field gradient distribution which, in average, has no definite sign.

Crystal structure and magnetic properties of Ba2Ni3F10

Ba2Ni3F10 is monoclinic (space group C2m), a = 18.542(7) A, b = 5.958(2) A, c = 7.821(3) A, β = 111°92(10). Ba2Co3F10 and Ba2Zn3F10 are isostructural. The structure has been refined from 995 reflections by full-matrix least-squares refinement to a weighted R value of 0.048 (unweighted R, 0.047). The three-dimensional network can be described either by complex chains connected to each other by octahedra sharing corners or with an 18L dense-packing sequence. The basic unit (Ni3F10)4− is discussed and compared to the different unit existing in Cs4Mg3F10. Antiferromagnetic properties of Ba2Ni3F10 (TN = 50 K are described.

Cristallisation par voie hydrothermale des fluorures FeF3; FeF3, H2O; FeF3, 3H2O; et NH4FeF4

Abstract Hydrothermal treatment (1 kbar, 380°C) of ferric fluoride has been studied. In water, FeF 3 , H 2 O is obtained; in aqueous solutions of NH 4 HF 2 , 5M/1, NH 4 FeF 4 crystallizes; crystals of FeF 3 ,3H 2 O are formed in HF 10% solution; FeF 3 crystals appear in HF 49%. Crystallographic data have been determined: the space group of NH 4 FeF 4 is P2 1 2 1 2 with a = b = 7.58 ± 0.01 A , c = 6.36 ± 0.01 A . FeF 3 ,H 2 O is hexagonal (P6 3 22) with a = 44.34 ± 0.04 A and c = 7.55 ± 0.01 A , and exhibits structural relations with hexagonal tungsten bronzes.

Composes ferrimagnetiques fluores de structure Na2SiF6

Abstract The magnetic properties of NaMnCrF 6 and of its antitype LiMnCrF 6 , both related to trigonal Na 2 SiF 6 structure, have been investigated. They are ferrimagnetic with respective Curie temperatures T C of 21 and 23° K. The values of their asymptotic Curie temperatures θ P involve ferromagnetic interactions. The observed difference between their respective saturation magnetization (6 and 4 μB/molecule) is discussed and related to the different types of cationic order, using an approached magnetic model with spins oriented along [001] axis. Magnetic properties of some substituted phases are also described.

Etude Mössbauer de la reduction du pyrochlore FeF3 par l'hydrazine en chimie douce

Using a topotactic oxidation of the orthorhombic modified pyrochlore NH4Fe2F6, a new form of FeF3, with the cubic pyrochlore structure, was previously synthesized. In order to study the reactivity of the fluorides at low temperature, the reduction of this new form with hydrazine was undertaken. Non-stoechiometric cubic pyrochlore phases are obtained. The Fe2+ fraction x in the sample and a cationic Fe2+Fe3+ order in the phase near x=0.5 are evidenced by Mossbauer spectroscopy.