Ferric Fluorides: crystalline, amorphous and nanostructured states J. M. Greneche Laboratoire de Physique de l`Etat Condensé, UMR CNRS 6087, Université du Maine, Le Mans, Cedex 9, France Ferric Fluorides: crystalline, amorphous and nanostructured states J. M. Greneche Laboratoire de Physique de l`Etat Condensé, UMR CNRS 6087, Université du Maine, Le Mans, Cedex 9, France

The fundamental interest of ferric fluorides FeF 3 results from the occurrence of three polymorphic crystalline phases (see next slide) which display original non-collinear magnetic arrangements resulting from the presence of triangular cationic platelets and antiferromagnetic interactions (see Figure 1 and 2): the more stable crystalline rhombohedral r–FeF 3 phase is antiferromagnetically ordered below T N = 363 K while the magnetic structures of hexagonal tungsten bronze (HTB) and the pyrochlore (pyr) phases consist of 3 and 4 antiferromagnetic sublattices below ~ 100 K and ~ 20 K, respectively. The hyperfine parameters estimated from Mössbauer spectra are consistent with the presence of HS Fe 3+ ions located in octahedral units while in-field Mössbauer spectra give evidence for collinear and non collinear magnetic structures at low temperature.

r-FeF 3 (ReO 3 ) HTB-FeF 3 pyr-FeF 3 Figure 1

T N = 363K T N  20K T N  100K Figure 2

Amorphous ferric fluorides can be prepared by vapour quenching techniques or fluorination route. They behave as speromagnets below T F ~ 30–40 K, as clearly revealed by in-field Mössbauer spectrometry (see Figure 3). Such a structure does consist of a topologically frustrated Fe network composed of 3, 4, 5 and 6 membered cationic rings. In addition, the quadrupolar spectra consist of symmetrical broadened line doublets: the isomer shift values which are close to those of the three crystalline phases, and the shape of the quadrupolar splitting values of the amorphous varieties suggest that Fe 3+ ions are located in corner-sharing octahedral sites. Consequently, it is concluded that the structure does result from a dense random packing of corner sharing FeF 6 octahedral units, as illustrated in Figure 4.

Figure 3

Dense Random Packing of Octahedral Units (without dangling bonds) Amorphous Phase Speromagnet T f  35K Figure 4

Mechanically milled powders of FeF 3 were prepared from the rhombohedral crystalline phase of FeF 3 using a commercial Fritsch Pulverisette 7 planetary ball-mill with zircon vial and balls under an argon atmosphere, to avoid reduction and oxidation. As illustrated in Figure 5 (note the square root scale to enlarge the low part of peaks) X-ray pattern can be well described using a model composed of two components: a nanocrystalline phase and a pseudo-amorphous phase where the long-range order is lost, attributed to the grain boundaries.

Figure 5

Mössbauer spectra have been recorded at several temperatures on powders ground with different milling times and milling energies. A unique fitting model was successfully achieved to describe all Mössbauer spectra: the components are attributed to crystalline grains, an amorphous structure due to grain boundaries and to an interfacial layer comprised between these two zones. In addition an in-field Mössbauer spectrum recorded on ground powders allows to confirm previous model with the presence of a narrow line sextet assigned to antiferromagnetic crystalline grains and a speromagnetic component due to grain boundaries (Figure 6). Their proportions have been found in quantitative agreement with those estimated from X-ray diffraction and also from 69 Ga and 71 Ga NMR experiments.

SP AF mm/s Figure 6

The set of hyperfine data allow to propose a structural model which consists of a cubic packing and a random packing of corner sharing octahedral FeF 6 units attributed to nanocrystalline grains and grain boundaries, respectively. The structural nature of nanostructured powders can be thus illustrated by the 2D schematic representation (where a square corresponds to an octadedron), in Figure 7.

Figure 7

The temperature dependence of the hyperfine field of the nanocrystalline grains and the grain boundaries are then compared to those of bulk r-FeF 3 (unmilled phase) and the amorphous phase in Figure 8. The low temperature agreement supports the static magnetic behaviour (interacting AF single domain grains through SP grain boundaries). At high temperature, the lowering of the hyperfine field of the crystalline phase is due superparamagnetic fluctuations resulting: when their thickness exceed the magnetic correlation length (about 1nm in ionics) the paramagnetic grain boundaries prevent the crystalline grains to magnetically interact. Within the intermediate temperature range (50<T<200K), the crystalline grains magnetically polarize the grain boundaries, inducing thus small hyperfine fields.

Figure 8

The different forms of ferric fluorides are an excellent example to illustrate how the magnetic frustration originates non colinear magnetic structures. In addition, these results unambiguously demonstrate the high efficiency of 57 Fe Mössbauer spectrometry as a powerful tool: indeed, its local probe behaviour and its sensitivity to dynamics provides relevant information relative to structural, microstructural, static and dynamic magnetic properties, in crystalline, amorphous and nanocrystalline solid systems.

Some references: Ferey G., Varret F. and Coey J.M.D J. Phys. C: Solid State Phys. 12 L531 Coey J.M.D. and Murphy P.J.K. J. Non-Cryst. Solids 50 (1982) 125 Greneche J.M., Teillet J. and Coey J.M.D. J. Non-Cryst. Solids 83 (1986) Greneche J.M., Teillet J. and Coey J.M.D. J. Physique, 48 (1987) Greneche J.M., Le Bail A., Leblanc M., Mosset A., Varret F., Galy J. and Ferey G J. Phys. C: Solid State Phys Ferey F., Leblanc M., De Pape R. and Pannetier J Inorganic Solid Fluorides ed Hagenmuller (New York: Academic) p 395 Greneche.J.M. and Varret F Mössbauer Spectroscopy Applied to Magnetism and Materials Science ed G. Long and F. Grandjean (New York: Plenum) p 161 Bureau B., Guérault H., Silly G., Buzaré J Y. and Greneche J.M J. Phys.: Condens. Matter 11 L423 Guérault H. and Greneche J.M J. Phys.: Condens. Matter H Guérault, M Tamine and J M Grenèche J. Physics : Condensed Matter 12 (2000) 9497