Ferric Fluorides:
crystalline, amorphous and nanostructured
states
J. M. Greneche
Laboratoire de Physique de l`Etat Condensé, UMR CNRS 6087,
Université du Maine, 72085 Le Mans, Cedex 9, France
jean-marc.greneche@univ-lemans.fr
The fundamental interest of ferric fluorides FeF3 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–FeF3 phase is
antiferromagnetically ordered below TN= 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 Fe3+ ions located in octahedral units while in-field
Mössbauer spectra give evidence for collinear and non collinear
magnetic structures at low temperature.
r-FeF3
(ReO3)
FFe 3+
c
b
c
a
a
b
Al+3
F
pyr-FeF3
Ow
F
Fe 3+
HTB-FeF3
Figure 1
TN = 363K
TN  20K
TN  100K
Figure 2
Amorphous ferric fluorides can be prepared by vapour
quenching techniques or fluorination route. They behave as
speromagnets below TF ~ 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 Fe3+ 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 FeF6 octahedral units, as illustrated in
Figure 4.
V [mm/s]
-12
-6
0
6
12
6
12
relative transmission
1,00
0,98
0,96
0,94
a-FeF3 4.2K 7T
0,92
1,00
0,96
a-FeF3 4.2K
0,92
1,00
0,95
0,90
r-FeF3 4.2K
0,85
-12
-6
0
V [mm/s]
Figure 3
Amorphous Phase
Speromagnet Tf  35K
Dense Random Packing of Octahedral Units (without
dangling bonds)
Figure 4
Mechanically milled powders of FeF3 were prepared
from the rhombohedral crystalline phase of FeF3 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.
75
Sqrt(I)
50
25
0
20
40
60
80
100
2
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 69Ga and 71Ga NMR experiments.
SP
AF
-14
-12
-12
-10
-8
-6
-6
-4
-2
0
0
2
4
mm/s
6
6
8
10
12
12
14
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 FeF6 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-FeF3 (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.
60
Bulk r-FeF3
40
Bhf (T)
<Bhf>GB
<Bhf>Grains
20
<Bhf>am
Tf
0
0
100
TN
200
T (K)
300
400
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 57Fe
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:
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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) 27-34
Greneche J.M., Teillet J. and Coey J.M.D. J. Physique, 48 (1987) 1709-1714
Greneche J.M., Le Bail A., Leblanc M., Mosset A., Varret F., Galy J. and Ferey G. 1988
J. Phys. C: Solid State Phys.21 1351
Ferey F., Leblanc M., De Pape R. and Pannetier J. 1985 Inorganic Solid Fluorides ed
Hagenmuller (New York: Academic) p 395
Greneche .J.M. and Varret F. 1993 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. 1999 J. Phys.:
Condens. Matter 11 L423
Guérault H. and Greneche J.M. 2000 J. Phys.: Condens. Matter 12 4791
H Guérault, M Tamine and J M Grenèche J. Physics : Condensed Matter 12 (2000)
9497