Magnetic structure and properties of Pb8FeIIFe2IIIF24: A 1-D ferrimagnetic chain compound exhibiting a spin-flop transition

Article abstract of DOI:10.1016/S0925-8388(99)00296-0

The magnetic structure of Pb₈Fe^IIFe₂^IIIF₂₄ was solved starting from its neutron diffraction powder pattern obtained at 1.4 K and its magnetic properties were characterized using the data recorded on a SQUID magnetometer and on a magneto-susceptometer. Pb₈Fe^IIFe₂^IIIF₂₄ is triclinic, space group P1; its crystal structure was previously solved on a single crystal. The magnetic structure of this fluoride is built up from ferrimagnetic chains antiferromagnetically coupled approximately along the c-axis of the cell, making up layers parallel to the (b,c) plane. Two successive layers exhibit different orientations of the magnetic moments. The net magnetic moment of Pb₈Fe^IIFe₂^IIIF₂₄ is equal to zero in agreement with an antiferromagnetic structure. This magnetic structure does not evolve in the temperature range: 1.4-20 K. Thus, only one magnetic phase occurs below TN≈21 K. Under a sufficiently strong magnetic field (H>5000 Oe), Pb₈Fe^IIFe₂^IIIF₂₄ exhibits a spin-flop transition.

Full text of DOI:10.1016/S0925-8388(99)00296-0

Journal of Alloys and Compounds 291 (1999) 44–51
L
Magnetic structure and properties of Pb8FeIIFe2IIIF24: a 1-D ferrimagnetic
chain compound exhibiting a spin-flop transition
A. Pierrarda, P. Gredina , , N. Dupont , A. de Kozak , F. Bouree-Vigneron , G. Andre ,
a
a
b
b
*
I. Rosenmanc
a
´
Laboratoire de Cristallochimie du Solide, Universite Pierre-et-Marie-Curie, 4, place Jussieu, 75252 Paris Cedex 05, France
b
´
Laboratoire Leon Brillouin, C.E.A.-Saclay, 91191, Gif-sur-Yvette Cedex, France
cGroupe de Physique des Solides, U.M.R. 7588, 2, place Jussieu, 75251 Paris Cedex 05, France
Received 24 May 1999; accepted 21 June 1999
Abstract
The magnetic structure of Pb8FeIIFe2IIIF24 was solved starting from its neutron diffraction powder pattern obtained at 1.4 K and its
magnetic properties were characterized using the data recorded on a SQUID magnetometer and on a magneto-susceptometer.
Pb8FeIIFe2IIIF24 is triclinic, space group P1; its crystal structure was previously solved on a single crystal. The magnetic structure of this
¯
fluoride is built up from ferrimagnetic chains antiferromagnetically coupled approximately along the c-axis of the cell, making up layers
parallel to the (b,c) plane. Two successive layers exhibit different orientations of the magnetic moments. The net magnetic moment of
Pb8FeIIFe2IIIF24 is equal to zero in agreement with an antiferromagnetic structure. This magnetic structure does not evolve in the
temperature range: 1.4–20 K. Thus, only one magnetic phase occurs below T , TN ¯ 21 K. Under a sufficiently strong magnetic field
(H.5000 Oe), Pb8FeIIFe2IIIF24 exhibits a spin-flop transition.
1999 Published by Elsevier Science S.A. All rights reserved.
Keywords: Iron lead fluoride; Magnetic properties; Neutron diffraction; Magnetic structure; Spin flop transition
1. Introduction
2. Experimental
The magnetic properties of one-dimensional compounds
were abundantly studied these last years. In particular,
theoretical works have focused attention on linear chains in
order to estimate various thermodynamic quantities, such
as correlation functions, magnetic susceptibility or specific
heat. Some authors have investigated the magnetic be-
haviour of linear ferrimagnetic 1-D systems in which two
magnetic cations occupy two or three crystallographic sites
[1–6]. Bimetallic chains of connected lozenges were also
studied from a theoretical point of view [7]. This kind of
ferrimagnetic chain was evidenced in Ba2CaMnFe2F14 [8]
and also in Pb8FeIIFe2IIIF24 [9,10]. The aim of this work is
to solve the magnetic structure of this latter fluoride and to
specify its magnetic properties in relation with its crystal
and magnetic structures.
The binary fluorides used in this work were prepared in
the laboratory.
To obtain iron III fluoride, first FeF3,3H2O is prepared
in solution by reaction in a platinum crucible between
Fe(NO3)3,9H2O (R.P. Normapur) and an excess of hydro-
fluoric acid at 40% (R.P. Normapur); the liquid is evapo-
rated on a sand-bath at 608C and the solid is then slowly
dehydrated in 6 h into FeF3 under an anhydrous hydrogen
fluoride stream at 7508C.
Iron II fluoride is prepared starting from FeCl2,4H2O
(R.P. Normapur). The chloride is first dehydrated into
FeCl2 under an anhydrous hydrogen chloride stream at
2508C in 3 h and then immediately fluorinated in the same
reactor into FeF2 by anhydrous hydrogen fluoride at 6508C
in 4 h.
Lead fluoride PbF2 is synthesized in solution by reaction
between its carbonate (R.P. Rectapur) and an excess of
hydrofluoric acid at 40% (R.P. Normapur); the liquid is
evaporated at 808C and the solid is then dried in 4 h at
5008C under argon.
*Corresponding author.
E-mail address: gredin@ccr.jussieu.fr (P. Gredin)
Pb8FeIIFe2IIIF24 is prepared by reaction in the solid state
0925-8388/99/$ – see front matter
PII: S0925-8388(99)00296-0
1999 Published by Elsevier Science S.A. All rights reserved.

A. Pierrard et al. / Journal of Alloys and Compounds 291 (1999) 44–51
45
˚
˚
of a stoichiometric mixture of the three fluorides PbF2,
FeF2 and FeF3 placed in a platinum crucible under dried
argon, at 5008C during 14 h.
5.597(1) A, c59.440(2) A, a589.75(2)8, b5105.79(2)8,
g589.38(2)8, Z52. The crystal structure was previously
¯
solved in the unconventional space group C1 from X-ray
single crystal data [10]. The cationic subnetwork of
Pb8FeIIFe2IIIF24 is quite the same that the one observed for
the fluorite-type structure adopted by PbF2 at high tem-
perature. The compound is obtained by a strict ordered
substitution of 3/11th of the lead cations of PbF2 by iron
cations, which adopt, as usual in fluorides, an octahedral
coordination. The iron III octahedron is quite regular, with
The neutron diffraction experiments were performed at
´
the Laboratoire Leon Brillouin (C. E. A. Saclay) using the
3T2 and G4.1 diffractometers. The high resolution powder
˚
diffractometer 3T2 (l51.2253 A, 68,2u ,1268) was
used for the refinement of the nuclear structure at 300 K
˚
and the multi-detector (800 cells) G4.1 (l52.4266 A) was
used for the resolution of the magnetic structure and the
thermal evolution study of the low temperature diagrams.
Therefore, twelve diffraction patterns were recorded in the
range 2u57–878, at different temperatures between 43 and
1.4 K. The powder sample was put in a cylindrical
vanadium can and held in a liquid helium cryostat. Nuclear
and magnetic structures were refined using the program
FULLPROF [11]. The nuclear scattering lengths and magnetic
forms were those included in this program.
The magnetic susceptibility was determined between
300 and 2 K using a SQUID magnetometer. The data were
corrected from the diamagnetism of the sample, using the
usual values for Fe21, Fe31, Pb21 and F2 ions [12]. The
field dependence of the magnetization at 5 K was obtained
between 0 and 15,000 Oe using a DSM8 MANICS
magneto-susceptometer.
˚
a mean Fe(III)–F distance of 1.924 A. The iron II
octahedron is distorted by a compressive Jahn–Teller
effect, in agreement with a d6 HS electronic configuration.
The iron octahedra share corners and form infinite
[FeIIFe2IIIF14]n6n2 double-chains (Fig. 1) running along the
b-axis, inserted in a fluorite-like matrix (Fig. 2). The
crystal structure of Pb8FeFe2F24 and related compounds,
must be compared to that of the mineral called ‘usovite’:
Ba2CaMgAl2F14 [8], which exhibits the same
[MIIMI2IIF14]n6n2 double-chains and has therefore about the
same b cell parameter.
4. Nuclear and magnetic structure determinations
The neutron powder diffraction pattern obtained on the
3T2 diffractometer at 300 K was refined starting from the
atomic positions determined from the X-ray single crystal
data [10]. After refinement of the cell parameters, zero-
point, atomic positions and isotropic thermal motions, the
reliability factors converge to rather good values, taking
into account the number of refined parameters and the
complexity of the powder pattern. The characteristics of
the refinement are given in Table 1 and the observed and
calculated diffraction diagrams are shown in Fig. 3.
3. Structural background
Pb8FeIIFeI2IIF24 is triclinic with the cell parameters
determined from X-ray diffraction: a520.118(3) A, b5
˚
The atomic positions determined from the neutron
powder pattern at 300 K are given in Table 2. These are
very close to those obtained from the X-ray diffraction
data [10]. Considering the accuracy of the measurements
and the complexity of the neutron diffraction pattern, the
two techniques lead to the same close atomic positions.
The values of the isotropic thermal factors are high, but in
the range of those obtained for other compounds [13–15].
Moreover, these values exhibit the same unusual order as
observed from X-ray diffraction, i.e., Beq. (Pb) greater than
Beq. (Fe). The h factor exhibits a high value which
indicates a line broadening. This effect can be attributed to
the relatively poor crystallinity of the powder. Indeed, in
this case, the line broadening results from the finite extent
of the coherently diffracting domains within the grains.
To solve the magnetic structure of Pb8FeIIFe2IIIF24,
twelve neutron diffraction patterns were recorded on the
G4.1 diffractometer between 43 and 1.4 K. These patterns
do not allow to refine the atomic positions. Indeed, the
ratio between the number of effective reflections and the
number of intensity dependent parameters is too low.
6n2
n
Fig. 1. (1 0 0) projection of the infinite [FeIIFe2IIIF14
running along the b-axis.
]
double-chains,

46
A. Pierrard et al. / Journal of Alloys and Compounds 291 (1999) 44–51
Fig. 2. (0 1 0) projection of the structure of Pb8FeIIFe2IIIF24 showing a sort of layered structure separating fluorite-type zones from perovskite-type ones.
As the nuclear structure belongs to the triclinic unconven-
tional space group C1, the magnetic structure will be
solved in the lower symmetry space group P1, using the
neutron diffraction pattern obtained at 1.4 K. It can be
noted that large diffusion peaks are observed above the
Therefore the atomic positions found at 300 K were used
in the refinement of the patterns and only the cell parame-
ters, the values and the directions of the magnetic moments
were refined. Above 21 K, no magnetic lines are observed.
Below 21 K, the order temperature, new lines, the
strongest ones of which lie in the 2u range: 6–558, appear
and are attributed to the magnetic structure (Fig. 4). They
can be indexed in a double cell, with the parameters:
¯
´
Neel temperature around 2u588 and 2u5288 (see neutron
diffraction pattern obtained at 24 K in Fig. 4). This
indicates that a short range ordering of the magnetic
´
moments occurs. The diffusion still subsits below the Neel
a
magnetic 5anuclear
,
b
magnetic 5bnuclear
,
c
magnetic 52cnuclear
,
.
temperature, even at 1.4 K, particularly around 2u588,
amagnetic 5anuclear, bmagnetic 5bnuclear and gmagnetic 5gnuclear
Table 1
Crystallographic data and refinement characteristics (nuclear cell)
¯ ¯
Triclinic C1 (P1)
System and space group (nuclear structure)
Refined cell parameters:
a520.133(4) A, b55.6006(1) A, c59.4344(2) A,
˚
˚
˚
a589.625(2)8, b5105.583(2)8, g589.509(2)8, Z52
Wavelength
˚
1.2253 A
2u data range and step
6–126830.058
Number of independent reflections
3278
Effective number of reflections taken into account
for the resolution
342
54
Number of intensity dependent parameters
Halfwidth parameters (H25U.tan2 u 1V.tan u 1W)
U:
V:
W:
0.3225
20.4975
0.2475
h (pseudo-Voigt profile)
Rietveld reliability factors (%)
Rp:
0.30(1)
8.65
9.12
6.31
4.26
Rwp
RB:
RF:
:

A. Pierrard et al. / Journal of Alloys and Compounds 291 (1999) 44–51
47
under the 0 0 1 magnetic diffraction line. Therefore, a
weak long range magnetic disorder exits up to 1.4 K. This
observation is corroborated by the greater value of the
profile shape parameter h for the magnetic phase than for
the nuclear one (see Table 3).
Fig. 5 shows the positions of the iron II and III in the
magnetic cell. A careful analysis of the zero intensity
magnetic lines hkl, with l 5 2n, implies that magnetic
Table 3
Refinement characteristics for the magnetic structure determination
System and space group (magnetic structure)
Refined magnetic cell parameters at 1.4 K:
a520.046(2) A, b55.621(1) A, c518.781(1) A
a5g5908, b5105.43(1)8, Z54
Wavelength
Triclinic P1
Fig. 3. Observed (dotted line) and calculated (solid line) profiles of the
neutron powder pattern of Pb8FeIIFe2IIIF24 obtained on the 3T2 diffrac-
tometer at 300 K. The short vertical lines below the patterns mark the
positions of all Bragg reflections. The bottom curve shows the difference
between observed and calculated profiles.
˚
˚
˚
˚
2.4266 A
2u data range and step
7–55830.18
Number of independent reflections
284
Effective number of reflections taken into account
for the resolution
Table 2
56
4
Atomic coordinates and parameters of isotropic thermal motion (with
e.s.d. in parentheses)
Number of intensity dependent parameters
Halfwidth parameters (H25U.tan2 u 1V.tan u 1W)
2
˚
Atom
x
y
z
Beq. (A )
U:
0.821
20.237
0.078
0.50(2)
0.71(4)
V:
W:
Pb(1)
Pb(2)
Pb(3)
Pb(4)
Fe(2)
Fe(3)
F(1)
F(2)
F(3)
F(4)
F(5)
F(6)
F(7)
F(8)
F(9)
F(10)
F(11)
F(12)
0.7236(14)
0.6317(13)
0.8232(13)
0.0852(13)
0
0.0063(23)
0.0085(21)
20.0029(22)
0.0034(22)
0
0.6387(17)
0.1701(17)
0.1023(17)
0.4383(16)
0
1.10(13)
1.10(13)
1.10(13)
1.10(13)
0.32(8)
h (pseudo-Voigt profile) for the nuclear phase
h (pseudo-Voigt profile) for the magnetic phase
Rietveld reliability factors (%)
Rp:
Rwp
RB:
11.1
11.4
3.67
6.63
0.5460(13)
0.7241(16)
0.1787(16)
0.6835(16)
0.6415(15)
0.9562(15)
0.8227(15)
0.4469(15)
0.9786(5)
0.4128(15)
0.5475(15)
0.9097(15)
0.2283(16)
0.0038(21)
0.2550(31)
0.2511(29)
0.2466(33)
0.0618(27)
0.2120(28)
0.0600(27)
0.2772(27)
0.3131(27)
0.1840(27)
0.0493(27)
0.0406(28)
0.2615(31)
0.7491(16)
0.1233(21)
0.3960(21)
0.3870(21)
0.8597(20)
0.3625(20)
0.3564(20)
0.1139(20)
0.0979(20)
0.3686(20)
0.3390(19)
0.8573(20)
0.1342(21)
0.32(8)
:
1.53(13)
1.53(13)
1.53(13)
1.53(13)
1.53(13)
1.53(13)
1.53(13)
1.53(13)
1.53(13)
1.53(13)
1.53(13)
1.53(13)
RMagnetic
:
Fig. 5. : (0 1 0) projection of the structure of Pb8FeIIFe2IIIF24 showing the
orientations of the projections of the magnetic moments. The nuclear and
the magnetic cells are drawn with hatched lines and solid lines respective-
ly.
Fig. 4. Thermal evolution of the neutron powder patterns for
Pb8FeIIFe2IIIF24 between 24 and 1.4 K showing the increase of the
magnetic lines in the 2u range: 7–558.

48
A. Pierrard et al. / Journal of Alloys and Compounds 291 (1999) 44–51
¢
moments linked by the c translation are opposite to each
other i.e.:
¢
¢
¢
¢
S1 5 2 S3; S2 5 2 S4
¢
¢
¢
¢
S5 5 2 S7; S6 5 2 S8
¢
¢
¢
¢
S9 5 2 S11; S10 5 2 S12
Assuming that the magnetic moments of iron III in a
[FeIIFeI2IIF14]n6n2 chain have the same directions and
¢
¢
¢
¢
¢
¢
¢
¢
values (i.e.: S5 5 S9, S6 5 S10, S7 5 S11 and S8 5 S12), all
the relations between the magnetic moments allow to find
a starting model for the refinement of the magnetic
structure. Moreover, the curve of the magnetic suscep-
tibility versus the temperature is characteristic of ferrimag-
netic chains and so, we assume that the magnetic moments
of iron III exhibit in a same chain an antiferromagnetic
coupling with the magnetic moments of iron II. The
measurements of the magnetization versus the field show
that the net magnetic moment is approximately null for
H50, which leads to an antiferromagnetic structure at field
H50 and at T ,21 K (Fig. 12).
Fig. 6. : Definition of the u and f angles of the spherical coordinates in
relation with the crystallographic axes.
Table 4
Positions of the magnetic atoms, values and directions of the magnetic
momentsa
Atom Label
x
y
z
M
f
u
(mB)
(8) (8)
Fe21 S1
Fe21 S2
Fe21 S3
Fe21 S4
Fe31 S5
Fe31 S6
Fe31 S7
Fe31 S8
Fe31 S9
Fe31 S10
Fe31 S11
Fe31 S12
0
0.5
0
0.5
0.5460
0.0460
0.5460
0.0460
0
0.5
0
0.5
0.0038
0.5038
0.0038
0.5038
0
0
0.5
0.5
3.70(6) 19 165(3)
3.70(6) 19 215(3)
3.70(6) 19 345(3)
3.70(6) 19
35(3)
In fact, different possible solutions can be checked. For
a first solution (model I), the magnetic moments of iron III
exhibit an antiferromagnetic coupling with the magnetic
moments of iron II located in the same chain and they are
both collinear. For a second one (model II), the magnetic
moments of iron III exhibit in a same chain an anti-
ferromagnetic coupling with the magnetic moments of iron
II and they can be both non-collinear, i.e. giving an angle
slightly different from 1808. The model II compared to the
model I, leads to the same reliability factors and at the final
stage of the refinements, the u angles and the magnetic
moments appear to be the same, taking into account the
standard deviations. Therefore, only model I was retained.
The refinement of the nuclear and magnetic cell parame-
ters shows that the angles a and g become very close to
908 (respectively 89.95(3) and 89.94(5)) below 20 K.
Therefore, the angles a and g were fixed at 908 for the
magnetic cell and a system of spherical coordinates defined
in Fig. 6, is used to solve the magnetic structure of
Pb8FeIIFe2IIIF24. This choice gives better results than the
use of cartesian coordinates, due to the reduction of the
number of refinable parameters. The characteristics of the
refinement are given in Table 3. The f angle can vary
from 0 to 268, without sensible modifications on the
calculated neutron diffraction pattern. Nevertheless, the
best reliability factors were obtained for f5198 and so, f
is fixed at this value for all the refinements. The values of
the magnetic moments and their directions are given in
spherical coordinates in Table 4. The difference curve
between calculated and experimental powder diffraction
patterns is given in Fig. 7.
0.3746 4.38(5) 19 215(3)
0.3746 4.38(5) 19 165(3)
0.8746 4.38(5) 19
35(3)
0.8746 4.38(5) 19 345(3)
20.5460 20.0038 20.3746 4.38(5) 19 215(3)
20.0460 20.5038 20.3746 4.38(5) 19 165(3)
20.5460 20.0038 20.8746 4.38(5) 19
35(3)
20.0460 20.5038 20.8746 4.38(5) 19 345(3)
a The coordinates of the iron ions are given in the magnetic cell:
nuclear 3bnuclear 32cnuclear
a
.
agreement with a d6 HS electronic configuration. Indeed,
the orbital component of the magnetic moment is close to
zero and a ‘spin only’ magnetic moment is assumed. So,
Fig. 7. Observed (dotted line) and calculated (solid line) profiles of the
neutron powder pattern of Pb8FeIIFe2IIIF24 obtained on the G4.1 diffrac-
tometer at 1.4 K. The short vertical lines below the patterns mark the
positions of all Bragg reflections (nuclear and magnetic lines). The bottom
curve shows the difference between observed and calculated profiles.
As described in the structural part, the [FeIIF6]42
octahedron exhibits a compressive Jahn–Teller effect, in

A. Pierrard et al. / Journal of Alloys and Compounds 291 (1999) 44–51
49
law. They are satured below 9 K. Nevertheless, as indi-
cated above, the values of the Fe21 and Fe31 magnetic
moments are lower than the expected ones for the free
ions. This fact can be explained by the presence of the
diffusion which still subsits even at 1.4 K.
The magnetic structure of Pb8FeIIFe2IIIF24 can be com-
pared to that of the usovite-type compound
Ba2CaMnFe2F14 in which the same type of ferrimagnetic
chains has been evidenced [8]. Nevertheless, on the
contrary of Pb8FeIIFe2IIIF24, Ba2CaMnFe2F14 is built up
from ferromagnetic [CaMnFe2F14]4nn2 layers, which are
perpendicular to the a-axis. In the a direction, the layers
are coupled antiferromagnetically and all the magnetic
moments are compensated, leading to an antiferromagnetic
compound.
Fig. 8. Orientations of the magnetic moments on the (a/cos(198), b)
plane.
5. Magnetic properties of Pb8FeIIFe2IIIF24
the magnetic moments of iron II and III (Table 4)
correspond approximately to 90% of their theoretical
values, 4 and 5 mB respectively.
Fig. 10 shows the plot of the reciprocal molar suscep-
tibility xM versus the temperature, obtained under 1000
21
and 15,000 Oe magnetic fields. A Curie–Weiss law is well
obeyed above 200 K, with CM 511.84 cm3.K.mol21 and
up 5285 K. The observed value for the Curie constant CM
is in agreement with the calculated one for two iron III and
one iron II exhibiting ‘spin only’ moments (11.74
The magnetic structure of Pb8FeIIFe2IIIF24 can be de-
scribed as built up from successive antiferromagnetic
[FeI2IFeI4IIF28]n12n2 layers parallel to the (b,c) plane. Indeed,
these layers are composed with ferrimagnetic
[FeIIFe2IIIF14]n6n2 chains running along the b-axis of the
magnetic cell. Two successive ferrimagnetic chains exhibit
an antiferromagnetic coupling along the c-axis of the
magnetic cell. Then, all the net magnetic moments are
compensated. Two successive layers are separated along
the a-axis by fluorite-type [Pb16F20]1n2n1 blocks and ex-
hibit different orientations of their iron II and III magnetic
moments (Figs. 5,8).
cm3.K.mol21). Below 200 K, xM versus temperature,
21
under a 1000 Oe magnetic field, shows a non-linear
´
decrease until 21 K, the Neel temperature. The plot of xT
versus temperature displayed in Fig. 11, exhibits a mini-
mum of xT around 120 K. This magnetic behaviour,
characteristic of ferrimagnetic chains, is also observed for
Ba2CaMnFe2F14 [8]. This kind of ferrimagnetic chain was
theoretically studied by Xu et al. [7] considering a classical
Heisenberg spin Hamiltonian.
The field dependence of the magnetization at 5 K was
measured between 0 and 15,000 Oe (Fig. 12). The curve is
characteristic of a spin-flop transition which occurs for a
Fig. 9 gives the evolution of the iron II and III magnetic
moments versus the temperature refined with the same
´
magnetic model, giving a Neel temperature around 21 K.
The values of the magnetic moments follow a Brillouin
Fig. 10. Plot of the reciprocal molar susceptibility of Pb8FeIIFe2IIIF24
versus the temperature. The insert in the figure shows the plot of the
reciprocal molar susceptibility versus the temperature for magnetic fields:
H51000 Oe and H515,000 Oe.
Fig. 9. Evolution of the magnetic moments of iron II and III versus the
temperature.

50
A. Pierrard et al. / Journal of Alloys and Compounds 291 (1999) 44–51
calculated one using the magnetic moments issued from
the resolution of the magnetic structure, due probably to a
residual angle between the magnetic moments. The exist-
ence of a spin-flop transition explains the different be-
haviour observed at low temperature for the plots of the
reciprocal molar susceptibility xM versus the tempera-
21
ture, obtained under 15,000 and 1000 Oe magnetic fields.
Under a 15,000 Oe magnetic field, the compound is
ferrimagnetic and the plot xM versus the temperature is
21
characteristic of a 1-D ferrimagnetic lozenges chain
[FeIIFe2IIIF14]n6n2 [7,8].
Because of all the measurements were carried on
powdered samples, it is difficult to specify if the magnetic
moments gradually rotate when the magnetic field in-
creases (spin-flop transition), or if the magnetic moments
go over directly from an antiferromagnetic alignment to a
satured paramagnetic alignment (metamagnetic transition).
To solve this problem, the magnetization curve versus the
magnetic field must be studied on a single crystal, as a
function of the crystallographic directions.
Fig. 11. Plot of xT versus temperature for Pb8FeIIFe2IIIF24
.
critical magnetic field around Hs55000 Oe. Indeed, the
magnetization exhibits a weak linear increase from 0 to
5000 Oe. Above 5000 Oe, the magnetization increase
becomes non linear and reaches a value of 4.2 mB for
15,000 Oe. The magnetization cycle versus the magnetic
field shows a weak hysteresis as expected for a spin-flop
transition.
6. Conclusion
To explain this behaviour, in connection with the
magnetic structure of Pb8FeIIFe2IIIF24, we propose a transi-
tion mechanism. Indeed, the coupling between the mag-
netic moments of iron II and iron III in a same chain is
strong. On the contrary, the coupling between adjacent
chains and all the more between the layers, is sufficiently
weak for that the magnetic moments of iron II and
correlatively iron III, fall into line when the magnetic field
increases and becomes sufficiently strong. Pb8FeIIFe2IIIF24
falls over from an antiferromagnetic state to a satured
paramagnetic state, sometimes called ferromagnetic state.
Therefore, the value of the net magnetic moment of the
compound must reach the one of a [FeIIFe2IIIF14]n6n2 chain,
i.e. approximately 5 mB. The measured value at 5 K and
under a 15,000 Oe magnetic field is weakly lower than the
The magnetic structure of Pb8FeIIFe2IIIF24 is solved and
the results of the refinements appear to be consistent. The
magnetic properties are in agreement with the magnetic
structure. A spin-flop transition is evidenced for a mag-
netic field greater than 5000 Oe at 5 K. Nevertheless, to
characterize more precisely this transition, it is necessary
to study the properties of this fluoride (magnetization
versus magnetic field, heat capacity) on a single crystal,
for different temperatures below 21 K.
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