Crystal structure refinement and magnetic properties of Fe4(P2O7)3 studied by neutron diffraction and moessbauer techniques

Article abstract of DOI:10.1006/jssc.2001.9415

Fe₄(P₂O₇)₃ was prepared from Fe(PO₃)₃ and FePO₄ at 940°C under oxygen. The unit cell is monoclinic, space group P2₁/n, with a = 7.389(2) A, b = 21.337(1) A, c = 9.517(2) A, β = 90(1)°, and Z = 4. The crystallographic structure has been determined from a single crystal through direct methods and difference Fourier synthesis and refined to R = 0.10 (R_w = 0.09). The three-dimensional framework is built up from Fe₂O₉ clusters of two face-sharing octahedra, linked by bent diphosphates P₂O₇ (P-O-P ～ 156°). Fe₄(P₂O₇)₃ is antiferromagnetic below T_N = 50 K. The magnetic structure has been determinated by means of powder neutron diffraction. There are four antiferromagnetic iron sublattices corresponding to the four crystallographically distinct iron atoms. The magnetic moments are antiferromagnetically coupled inside the Fe₂O₉ dimers, in agreement with the Goodenough rules. They are parallel to the c axis and have 4.55(5) μ_B value at 1.7 K. The magnetic interactions are discussed. Moessbauer spectra are fitted with four doublets and sextuplets in the paramagnetic and antiferromagnetic states, respectively. Their rather high isomer shifts are explained by the inductive effect.

Full text of DOI:10.1006/jssc.2001.9415

Journal of Solid State Chemistry 163, 412}420 (2002)
doi:10.1006/jssc.2001.9415, available online at http://www.idealibrary.com on
Crystal Structure Refinement and Magnetic Properties of Fe4(P2O7)3
Studied by Neutron Diffraction and Mossbauer Techniques
L. K. Elbouaanani,* B. Malaman,* R. Gerardin,*ꢁꢀ and M. Ijjaali-
*Laboratoire de Chimie du Solide Mine&ral, Universite& Henri Poincare&-Nancy 1, associe& au CNRS (UMR 7555), B.P. 239, 54506 Vandoeuvre-les-Nancy Cedex,
France; and - Laboratoire de Chimie Mine&rale, Faculte& des Sciences et Techniques Fes Saiss, B.P. 2202,Fes, Morocco
Received June 22, 2001; in revised form September 26, 2001; accepted September 28, 2001
These Fe O units are constituted of a trigonal prism
ꢅ ꢀꢃ
ꢃ>
centered on Fe which shares its two triangular faces with
Fe4(P2O7)3 was prepared from Fe(PO3)3 and FePO4 at 9403C
ꢅ>
under oxygen.The unit cell is monoclinic, space group P21"n, two FeO octahedra occupied by Fe . In order to describe
ꢆ
with a ؍ 7.389(2) A> , b ؍ 21.337(1) A> , c ؍ 9.517(2) A> , ꢀ ؍ 90(1)3,
the Fe (P O ) structure from that of Fe (P O ), B. Mala-
ꢄ
ꢃ ꢄ ꢂ
ꢃ ꢃ ꢄ
and Z ؍ 4.The crystallographic structure has been determined
from a single crystal through direct methods and di4erence
Fourier synthesis and re5ned to R ؍ 0.10 (Rw ؍ 0.09). The three-
dimensional framework is built up from Fe2O9 clusters of two
face-sharing octahedra, linked by bent diphosphates P2O7
(P+O+P&1563).Fe 4(P2O7)3 is antiferromagnetic below
TN ؍ 50 K.The magnetic structure has been determinated by
means of powder neutron di4raction.There are four antifer-
romagnetic iron sublattices corresponding to the four crystallog-
raphically distinct iron atoms.The magnetic moments are
antiferromagnetically coupled inside the Fe2O9 dimers, in agree-
ment with the Goodenough rules.They are parallel to the c axis
and have 4.55(5) ꢁB value at 1.7 K. The magnetic interactions are
discussed.Mo ssbauer spectra are 5tted with four doublets and
sextuplets in the paramagnetic and antiferromagnetic states,
respectively.Their rather high isomer shifts are explained by the
inductive e4ect. ꢀ 2002 Elsevier Science (USA)
man et al. (4) suggest a mechanism based on the creation of
vacancies on the iron lattice (namely a substitution
ꢃ>
ꢅ>
Fe PFe ). This mechanism leads to the series:
ꢃ>
ꢃ>
ꢅ>
Fe (P O ) (8Fe )PFe ᮀ (P O ) (5Fe ,2Fe
)
ꢇ
ꢃ ꢄ ꢂ
ꢄ ꢀ ꢃ ꢄ ꢂ
ꢃ>
ꢅ>
PFe ᮀ (P O ) (2Fe ,4Fe ).
ꢆ ꢃ ꢃ ꢄ ꢂ
Moreover, the magnetic properties of these three com-
pounds have been determined (4) yielding a better know-
ledge of the exchange interactions in such compounds.
According to these results, it appeared very interesting to
fully characterize the iron(III) term of the series: Fe (P O )
ꢂ
ꢃ ꢄ ꢅ
(Fe
ꢈꢉꢅꢅꢅ ꢃ ꢄ ꢂ
The iron(III) diphosphate Fe (P O ) was discovered in
(P O ) ).
ꢂ
ꢃ ꢄ ꢅ
1962by D 'Yvoire (3) by heating progressively a mixture of
Fe(PO ) and Fe(PO ) to 9503C. This author only gives an
unindexed X-ray powder di!raction pattern: JCPDS card
no. 36-318 which actually is the only crystallographic refer-
ence for this compound.
ꢂ
ꢅ ꢅ
Key Words: iron phosphate; single crystal; crystal structure;
magnetic structure; 57Fe Mossbauer spectrometry.
In 1964, Remy and Boule (5) indicated the existence of
Cr (P O ) which, according to these authors, would be
INTRODUCTION
ꢂ
ꢃ ꢄ ꢅ
isotypic with Fe (P O ) prepared by D'Yvoire (3).
ꢂ
ꢃ ꢄ ꢅ
Iron phosphates have been widely studied in our laborat-
ory on account of their passivating properties and their
importance in catalysis. Modaressi (1) and Ijjaali (2) have
showed that Fe (P O ) is the ferric term of a four-diphos-
Later, in 1983, Schlesinger et al. (6) succeeded in the
synthesis of single crystals of V (P O ) and Cr (P O ) .
ꢂ
ꢃ ꢄ ꢅ
ꢂ ꢃ ꢄ ꢅ
Both compounds crystallize in the orthorhombic system
(Pbn2 or Pbnm) with the following lattice parameters:
ꢂ
ꢃ
ꢃ ꢄ ꢅ
ꢀ
phate series, Fe (P O ), Fe (P O ) , Fe (P O ) , and Fe
ꢃ ꢄ
ꢄ
ꢃ ꢄ ꢂ
ꢅ
ꢃ ꢄ ꢃ
ꢂ
(P O ) , within which the "rst three terms have closely
ꢃ ꢄ ꢅ
a (A
>
)
b (A
>
)
c (A
>
)
S.G.
related structures.
In the iron(II) diphosphate Fe (P O ) the P O groups
ꢃ
ꢃ ꢄ
ꢃ ꢄ
V (P O )
ꢃ ꢄ ꢅ
Cr (P O )
ꢃ ꢄ ꢅ
9.601(2)
9.38(1)
21.425(5) 7.470(4)
21.00(4) 7.26(2)
Pbnm
Pbnm
connect FeO octahedra which share edges to form thus
ꢂ
ꢂ
ꢆ
a planar hexagonal lattice of octahedra. These sheets are
parallely stacked. In the mixed-valent diphosphate
Fe (P O ) the P O groups connect Fe O
trimers.
Finally, the structure of V (P O ) was determined by
ꢃ ꢄ ꢅ
Palkina et al. (7) in the space group Pmcn with the cell
ꢅ
ꢃ ꢄ ꢃ ꢃ ꢄ ꢅ ꢀꢃ
ꢂ
ꢀ To whom correspondence should be addressed. E-mail: Rene.
Gerardin@lcsm.uhp-nancy.fr.
parameters a"7.443 A, b"9.560 A, c"21.347 A
>
>
> .
412
0022-4596/02 $35.00
ꢀ 2002 Elsevier Science (USA)
All rights reserved.

STRUCTURE AND MAGNETIC PROPERTIES OF Fe (P O )
ꢆ ꢁ
413
ꢅ
ꢂ
In this structure, vanadium atoms occupy two 8(d) sites. minimization techniques (11) to evaluate the hyper"ne spec-
Each vanadium sublattice (V(1) or V(2)) is characterized by tral parameters.
the occurrence of V O units made of two face-sharing VO
octahedra. These dimers are interconnected via the diphos-
phate groups.
ꢂ ꢃ
ꢄ
CRYSTAL STRUCTURE
(a) Structure Analysis
In the "rst step, a single crystal (ꢀ&70 ꢀm) was studied
At the same time, Modaressi (1) synthesized a single
crystal of Fe (P O ) . According to this author, Fe (P O )
ꢅ
ꢂ ꢆ ꢁ
ꢅ ꢂ ꢆ ꢁ
crystallizes in an orthorhombic unit cell (Pmnm) with by conventional X-ray techniques (oscillating crystal and
a"7.389 A, b"21.337 A, c"9.517 A. Nevertheless, all Weissenberg camera, Co Kꢂ). An orthorhombic unit cell of
(similar to those
>
>
>
attempts to determine the structure have failed and the a"7.389 A> , b"21.337 A> , c"9.517 A>
best "t to their data was obtained in the space group given in Ref. (1)) was deduced. The condition limiting pos-
Pmn2 (with a structural arrangement close to that observed sible re#ections [(h0l) with h#l"2n] leads to three pos-
ꢀ
in V (P O ) ) but yielding the poor reliability factor sible space groups: Pmnm, Pmn2 , and P2 nm.
ꢅ
ꢂ ꢆ ꢁ
ꢀ
ꢀ
R"20%.
In the second step, the crystal was mounted on an auto-
The present study clearly shows that the crystal structure matic di!ractometer. Analysis of the intensities shows, be-
of Fe (P O ) has a framework similar to that of V (P O ) , side the (h0l) with h#l"2n condition previously observed,
ꢅ
ꢂ ꢆ ꢁ
ꢅ ꢂ ꢆ ꢁ
but in a less symmetric monoclinic unit cell. In this paper, the additional extinction (0k0) with k"2n#1 which has
we also report on the magnetic properties of Fe (P O )
ꢂ ꢆ ꢁ
studied by bulk magnetization, Mossbauer spectrometry, senberg patterns.
been unambiguously con"rmed by very long duration Weis-
ꢅ
and neutron di!raction experiments.
This last condition implies the occurrence of a 2 screw
axis parallel to b, which is incompatible with the orthor-
ꢀ
EXPERIMENTAL
hombic groups Pmnm, Pmn2 , and P2 nm. Only the P1
ꢀ
ꢀ
2 /n1 space group ful"lls all these constraints.
ꢀ
Fe (P O ) was prepared by "ring, in an alumina cru-
Thus, the structure of Fe (P O ) has been studied in the
ꢅ
ꢂ ꢆ ꢁ
ꢅ ꢂ ꢆ ꢁ
cible, the mixture Fe(PO ) #3FePO , under dried oxygen, nonconventional monoclinic space group P2 /n. The condi-
ꢁ ꢁ
for 72h at 940 3C. In air, the resulting phosphate sets in tions for the data collection and the re"nement of the
ꢅ
ꢀ
a highly solid block which is di$cult to grind (8). structure are listed in Table 1. Absorption was neglected (ꢄr
ꢁ>
ꢇ
\
The starting compounds were prepared by mixing aque- ꢀ1). Atomic scattering factors for Fe , P , and O were
ous solutions of Fe(NO ) ) 9H O and H PO with the ratio taken from Ref. (12). All computer programs used were
ꢁ ꢁ
ꢂ
ꢁ
ꢅ
ꢀ
ꢀ
Fe/P " and respectively. The solutions were evaporated
ꢁ
ꢀ
to dryness and the residues were subsequently heated to
4003C under nitrogen, and then at 9003C under oxygen for
2 4 h.
TABLE 1
Summary of Data Collection and Structure Re5nement
Single crystals were obtained by long annealing (15 days),
in sealed silica tubes, within a gold crucible, under vacuum,
Molar mass (g.mol\ꢀ)
Crystal size (ꢀm)
Symmetry
745.38
ꢁ+70
Monoclinic
7.389(2)
21.337(1)
9.517(2)
90(1)
1500.4
4
3.3
P2 /n
MoKꢂ
Graphite
ꢃ}2ꢃ
2
ꢃ(21
2
at 9003C, with traces of FeCl as mineralizer agent.
ꢂ
A single crystal (ꢀ&70 ꢀm) was selected for the structure
a (A
>
)
)
)
determination. The data were collected on a Nonius CAD
4 automatic di!ractometer at the Service Commun de Dif-
fractometrie de l'Universite Henri Poincare-Nancy I.
Magnetic measurements were performed (between 4.2
and 300 K) on a MANICS magnetosusceptometer in mag-
netic "eld up to 1.6 T.
b (A
>
c (A
>
ꢀ (3)
< (A
>
ꢁ)
Z
ꢁ
(g.cm\ꢁ)
ꢀꢁꢂ
Space group
ꢀ
Neutron di!raction experiments were carried out at the
Institut Laue Langevin (ILL), Grenoble. Several di!raction
patterns were recorded with the one-dimensional curved
Radiation
Monochromator
Scanning
Take-o! (3)
multidetector D1b using a neutron wavelength of 2.520 A> ,
Record limits (3)
Linear absorption coe$cient ꢄ(cm\ꢀ)
Recorded intensities
Recorded intensities with ꢅ(I)/I(0.33
F(000)
in the temperature range 120}1.7 K. Analysis of the patterns
was performed by Rietveld pro"le re"nements (9) using the
software Fullprof (10).
The Mossbauer data were collected with a constant accel-
eration spectrometer with 1024 channels. Isomer shifts are
reported with respect to ꢂ-iron at room temperature. The
Mossbauer e!ect data were analyzed by using least-squares
4
1560
1147
1483
146
10%
9%
Number of parameters
Final R value
Final R value
ꢃ

414
ELBOUAANANI ET AL.
TABLE 2
(b) Description
Atomic Coordinates for Fe4(P2O7)3
The mean feature of the structure of Fe (P O ) is the
ꢅ
ꢂ ꢆ ꢁ
occurrence of Fe O dimers made of face-sharing FeO
Atom
x
y
z
ꢂ ꢃ
ꢄ
octahedra. Iron atoms are slightly o!-centered in each oc-
tahedron in order to minimize the repulsive interactions
between the common faces (Table 3). One observes two
types of dimers: [Fe(1), Fe(2)] and [Fe(3), Fe(4)] with an
Fe(1)
Fe(2)
Fe(3)
Fe(4)
P(1)
P(2)
P(3)
P(4)
P(5)
P(6)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
0.0521(5)
0.4644(8)
0.4595(8)
0.0582(8)
0.2539(7)
0.2514(7)
0.2522(7)
0.2440(8)
0.2579(8)
0.7572(8)
0.264(2)
0.259(2)
0.248(3)
0.226(2)
0.245(2)
0.259(1)
0.939(3)
0.274(2)
0.572(2)
0.592(3)
0.594(2)
0.913(2)
0.588(2)
0.923(3)
0.923(2)
0.211(2)
0.952(2)
0.948(2)
0.322(3)
0.101(3)
0.381(2)
0.2554(4)
0.2593(3)
0.0009(3)
0.9964(4)
0.2011(3)
0.0627(3)
0.0455(3)
0.8591(4)
0.1855(2)
0.1112(6)
0.923(2)
0.215(1)
0.022(2)
0.131(1)
0.032(2)
0.236(1)
0.073(2)
0.329(1)
0.947(2)
0.074(2)
0.305(1)
0.953(2)
0.182(1)
0.174(1)
0.302(1)
0.874(1)
0.959(2)
0.290(3)
0.109(2)
0.214(2)
0.014(2)
0.7845(7)
0.7901(10)
0.2398(10)
0.2427(10)
0.0553(9)
0.9744(10)
0.5323(9)
0.1543(11)
0.5089(10)
0.1706(9)
0.232(2)
0.656(1)
0.103(2)
0.029(2)
0.377(1)
0.910(2)
0.199(2)
0.750(1)
0.110(2)
0.193(1)
0.923(2)
0.110(2)
0.817(2)
0.803(2)
0.931(2)
0.003(1)
0.392(1)
0.621(1)
0.537(1)
0.129(3)
0.621(1)
Fe}Fe distance of 3.05 and 2.97 A
>
, respectively. The dimers
ꢁ\
are connected by (PO )
entities yielding in"nite (010)
ꢅ
layers made of [Fe(1), Fe(2)] or [Fe(3), Fe(4)] iron planes.
These layers are alternatively stacked along the screw axis
ꢅ\
through (P O ) units (Fig. 1, see also Fig. 7). Hence, this
ꢂ ꢆ
structure is very similar to that of V (P O ) (7).
In the diphosphate units, some P}O distances remain
ꢅ
ꢂ ꢆ ꢁ
short (Table 3), but the average values are comparable to the
1.536 A> value given by Corbridge (17). All the conforma-
tions are of staggered form. Furthermore, the P}O}P links
O(8)
O(9)
O(10)
O(11)
O(12)
O(13)
O(14)
O(15)
O(16)
O(17)
O(18)
O(19)
O(20)
O(21)
TABLE 3
Main Interatomic Distances (A
>
) and Angles (3)
[FeO ] Octahedra
ꢄ
Fe(1)}O(18)
Fe(1)}O(15)
Fe(1)}O(6)
Fe(1)}O(14)
Fe(1)}O(2)
Fe(1)}O(8)
1.89(3)
1.96(2)
1.98(1)
1.99(2)
2.14(2)
2.29(2)
Fe(3)}O(21)
Fe(3)}O(9)
Fe(3)}O(10)
Fe(3)}O(3)
Fe(3)}O(5)
Fe(3)}O(1)
1.80(2)
1.88(3)
1.89(4)
2.08(2)
2.16(2)
2.20(3)
Fe(2)}O(11)
Fe(2)}O(13)
Fe(2)}O(20)
Fe(2)}O(6)
Fe(2)}O(8)
Fe(2)}O(2)
1.86(2)
1.90(2)
1.92(3)
1.96(2)
2.08(2)
2.20(2)
Fe(4)}O(17)
Fe(4)}O(12)
Fe(4)}O(7)
Fe(4)}O(3)
Fe(4)}O(5)
Fe(4)}O(1)
1.81(2)
1.90(3)
1.90(4)
2.01(2)
2.03(2)
2.18(3)
taken from Refs. (13, 14). The cell parameters were deter-
mined by least-squares re"nements of the 20 values of 25
independent re#ection}antire#ections, measured for 2ꢃ
'173:
Fe(1)}Fe(2)
3.048(7)
Fe(3)}Fe(4)
2.967(8)
[PO ] Tetrahedra
ꢅ
P(1)}O(20)
P(1)}O(4)
P(1)}O(6)
P(1)}O(18)
1P(1)}O2
1.36(3)
1.53(2)
1.57(2)
1.58(2)
1.51
P(4)}O(14)
P(4)}O(16)
P(4)}O(13)
P(4)}O(1)
1P(4)}O2
1.48(2)
1.49(2)
1.54(2)
1.56(4)
1.52
a"7.389(2) A> , b"21.337(1) A> , c"9.517(2) A> , and ꢀ"90(1)3.
The iron and phosphorus atomic positions have been
obtained using the direct methods procedure (13). The oxy-
gen atomic positions were determined, step by step, by
di!erence Fourier synthesis. Due to the lack of data (very
small crystal), we only re"ned a general temperature factor
ꢂ
P(2)}O(12)
P(2)}O(3)
P(2)}O(9)
P(2)}O(4)
1P(2)}O2
1.49(2)
1.50(3)
1.55(2)
1.56(2)
1.52
P(5)}O(15)
P(5)}O(11)
P(5)}O(2)
P(5)}O(19)
1P(5)}O2
1.45(2)
1.48(2)
1.53(2)
1.72(4)
1.54
(B "0.58 A
>
)
yielding the "nal residual R"0.10
%
(R "0.09). The atomic parameters are listed in Table 2 P(3)}O(21)
1.44(2)
1.45(4)
1.51(2)
1.67(2)
1.52
P(6)}O(10)
P(6)}O(8)
P(6)}O(7)
P(6)}O(16)
1P(6)}O2
1.47(3)
1.49(2)
1.59(3)
1.70(1)
1.56
ꢃ
P(3)}O(19)
P(3)}O(5)
P(3)}O(17)
1P(3)}O2
and the main interatomic distances and angles are given in
Table 3. It is noteworthy that some P}O distances are very
short. Despite many tests, we did not manage to correct
these anomalies. However, it should be noted that similar
short P}O distances were also observed in some phosphates
and diphosphates (1.43 A) (15, 16).
P(1)}O(4)}P(2)
P(4)}O(16)}P(6)
162(1)
163(1)
P(3)}O(19)}P(5)
142(2)
>

STRUCTURE AND MAGNETIC PROPERTIES OF Fe (P O )
ꢆ ꢁ
415
ꢅ
ꢂ
generates an empty prismatic site with triangular bases,
P6
P4
P4
ꢂ>
similar to that occupied by the Fe ion in Fe (P O ) (see
Fig. 2). This observation suggests a structural relationship
between these two compounds.
P5
P3
P1
P2
ꢁ
ꢂ ꢆ ꢂ
P2
P3
P2
P5
P1
P55
P6
Fe4
Fe4
Fe1
Fe4
Fe4
Fe2
Fe1
Fe3
P6
P4
P4
P6
One can imagine the following mechanism. On the basis
of two dimers &&Fe1}Fe2 }empty prism}Fe1}Fe2 ,'' the
P5
P1
P3
P2
P2
P3
P3
P2
P1
ꢄꢀꢅꢁ
ꢄꢀꢅꢁ
c
ꢂ>
occupation of the prismatic site by Fe ion and the depar-
Fe2
Fe3
ꢁ>
ture of the Fe ions of the two octahedra of the ends lead
to the formation of the trimer Fe O characteristic of
Fe (P O ) . However, in Fe (P O ) , the vertical edges
joining the triangular bases of the prism are generated by
P6
P4
P4
P6
P5
P1
P3
ꢁ ꢀꢂ
ꢅ ꢂ ꢆ ꢁ
P3
P1
P5
b
ꢁ
ꢂ ꢆ ꢂ
edges of PO tetrahedra of the diphosphates groups. That
ꢅ
supposes also a rearrangement of a part of the P O subnet-
work. This remark seriously complicates the considered
process since it imposes a signi"cant three-dimensional re-
organization of the anions. The assumption is nevertheless
tempting and it may appear signi"cant to announce it.
Fe3
ꢂ ꢆ
Fe4
Fe4
Fe1
Fe2
Fe2
Fe1
P5
P4
P6
P5
P3
P2
Fe3
Fe4
Fe3
Fe4
Fe4
Fe3
a
P1
P4
P5
P2
P6
P3
MAGNETIC STUDY
Fe2
Fe1
b
(a) Susceptibility Measurements
The thermal variation of the reciprocal susceptibility (1/ꢆ)
of Fe (P O ) in an applied "eld value of 4930 G (Fig. 3a)
FIG.1. Projection of the framework of Fe (P O ) . (Top) Parallel to
ꢆ ꢁ
ꢅ
ꢂ
[100]. (Bottom) Parallel to [001].
ꢅ
ꢂ ꢆ ꢁ
shows antiferromagnetic behavior below ¹ "50(5) K.
,
are bent and the angles values are of 142, 162, and 1633 for
Above 50 K, the compound is paramagnetic and the
P(3)}O}P(5), P(2)}O}P(1), and P(6)}O}P(4), respectively temperature dependence of 1/ꢆ obeys the Curie}Weiss law
\ꢀ
(Fig. 1).
with the apparent Curie constant of 18.9 emu K per for-
mula unit. This value corresponds to an apparent e!ective
magnetic moment/iron atom of ꢄ "6.14 ꢄ which is
ꢆꢇꢇ
(c) Relationships among Iron Diphosphates
ꢁ>
ꢈ
(d ) in a
slightly higher than the value expected for Fe
The analysis of the structures of Fe (P O ) (18) and H.S. ground state. The Curie temperature ꢃ "!73 K is
ꢁ
ꢂ ꢆ ꢂ
.
Fe (P O ) shows certain similarities. Thus, in the consistent with dominant antiferromagnetic interactions. These
ꢂ ꢆ ꢁ
latter compound, the sequence of dimers along the a axis results agree with the previous data of Rei! and Torardi (19).
ꢅ
Fe3(P2O7)2
Fe4(P2O7)3
c
b
shared prism
(see text)
b
a
FIG.2. Trimer in Fe (P O ) (18) and hypothetical one in Fe (P O ) .
ꢆ ꢂ ꢆ ꢁ
ꢁ
ꢂ
ꢅ ꢂ

416
ELBOUAANANI ET AL.
patterns, in agreement with the magnetic measurements.
20
15
10
5
The neutron di!raction pattern collected and calculated at
1.7 K is shown in Fig. 4b. All these new re#ections can be
indexed on the basis of the nuclear unit cell and obey the
rule (h0l) with h#l"2n#1 (i.e., forbidden by the n glide
plane). Moreover, it must be noted that some nuclear
re#ections have magnetic contributions to their intensities
(Fig. 4c).
Let us recall that there are four independent iron sites
(Table 2) and for each iron sublattice, there are four equiva-
lent positions per unit cell. Iron atoms can be separated in
H = 4930 G
two sets of positions labeled (Fe ; Fe ) and (Fe ; Fe
)
ꢀꢉH
ꢂꢉH
ꢁꢉH
ꢅꢉH
0
50
100
150
200
250
300
corresponding to the two types of Fe O dimers. For the
ꢂ ꢃ
notation Fe , i"1, 4 corresponds to the four crystallo-
GꢉH
T (K)
(a)
graphic iron sites and j"1, 4 corresponds to the four equiv-
0.35
0.30
alent positions, as indicated in Fig. 5.
All possible collinear models, in agreement with the loss
of the glide plane, were tested.
Considering the small number of magnetic data, the mag-
netic moment components were constrained to be equal for
the four independent iron sites. The best re"nement corres-
ponds to the following model:
0.25
0.20
0.15
T = 5 K
*each iron sublattice (i) is antiferromagnetic,
*in each dimer [Fe(1), Fe(2)] and [Fe(3), Fe(4)] the iron
atoms are antiferromagnetically coupled.
0.10
0.05
0.00
*the spin direction lies along the c axis.
0
5
10
H (kG)
15
(b)
FIG.3. (a) Temperature dependence of the inverse magnetic suscepti-
bility of Fe (P O ) . (b) Fe (P O ) magnetization versus applied "eld
ꢅ
ꢂ
ꢆ ꢁ
ꢅ ꢂ ꢆ ꢁ
at 5 K.
At 5 K, the isotherm curve (Fig. 3b) evidences neither
spontaneous magnetization nor
behavior.
a metamagnetic-like
(b) Neutron Di+raction Study
The neutron di!raction pattern at 120 K is characteristic
of only nuclear scattering. The re"ned cell parameters
(R "0.11) are a"7.386(2) A
>
,
b"21.298(5) A
>
,
c"
9.492(2) A
> , and ꢀ"90.03(6)3.
Due to the lack of experimental information as there are
93 variable parameters to determine from 57 independent
neutron data, a full re"nement was not possible. Therefore,
using the set of nuclear parameters de"ned during the
single-crystal X-ray study (Table 2), only the scale factor, the
lattice, and half-width parameters were re"ned at 120 K.
The observed and the calculated di!raction pro"les at
120 K are given in Fig. 4a.
FIG.4. Observed and calculated neutron di!raction patterns of
Fe (P O ) : (a) at 120 K, (b) at 1.7 K, (c) di!erence between the observed
spectra collected at 120 and 1.7 K.
Below 50 K, additional lines characteristic of an antifer-
romagnetic ordering appear in the neutron di!raction
ꢅ
ꢂ ꢆ ꢁ

STRUCTURE AND MAGNETIC PROPERTIES OF Fe (P O )
ꢆ ꢁ
417
ꢅ
ꢂ
Fe43
Fe33
100
98
Fe34
Fe44
T = 295 K
Fe24
Fe14
Fe43
Fe33
Fe13
Fe23
Fe21
Fe11
96
Fe31
Fe41
Fe12
Fe22
Fe42
Fe32
Fe31
Fe41
94
92
90
c
a
b
-1
0
1
2
Velocity (mm/s)
FIG.5. Collinear arrangement of spin for antiferromagnetic ordering
of Fe (P O ) . The gray and white circles indicate opposite spin direction
ꢅ
ꢂ
ꢆ ꢁ
parallel to the c axis. Only Fe atoms (Fe O dimers) are shown and
100
ꢂ
ꢃ
referenced as i, j: i"1!4; j corresponds to equivalent positions x, y, z;
!x#ꢀ, y#ꢀ,!z#ꢀ; !x, !y, !z; and x#ꢀ, -y#ꢀ, z#ꢀ, respec-
99
98
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
tively. The black bonds represent the 2.967 and 3.048 A> intradimer Fe}Fe
distances.
97
96
95
94
T = 4.2 K
The magnetic structure is represented in Fig. 5. At 1.7 K,
-12 -10 -8 -6 -4 -2
0
2
4
6
8
10 12
the best re"nement (R "10.4% and R
"7.2%)
,3!
+ꢊ%
Velocity mm/s
yields ꢄ "4.55(5) ꢄ (Fig. 4b).
$ꢆ
The thermal dependence of the magnetic moment ampli-
tude is shown in Fig. 6. The magnetic moment obeys the
usual Brillouin-type variation versus temperature, leading
FIG.7. Mossbauer spectra of Fe (P O ) obtained at 295 and 4.2 K.
ꢆ ꢁ
ꢅ
ꢂ
to ¹ "57(2) K, in fair agreement with the previous bulk
,
magnetization measurements.
doublets (i.e., orthorhombic cell) leads to too wide peaks
(ꢉ"0.41 mm/s). This result consolidates the monoclinic
structural model proposed in the present crystallographic
study.
(c) Mossbauer Study
Paramagnetic state (¹'55 K). The room temperature
The isomer shifts (ꢇ) are clearly indicative of high-spin
ꢁ>
spectrum (Fig. 7) was "tted with four Lorentz-type symmet- Fe ions in an octahedral oxygen crystal "eld. Let us recall
ric quadrupole doublets, in agreement with the existence of that the usual values measured in oxides are, according to
ꢁ>
the four crystallographic Fe sites.
Menil (20), in the range 0.29}0.50 mm/s for Fe
in VI
The intensity of each doublet was constrained to be equal coordination. The high average value, 0.415 mm/s, observed
as well as the width of the peaks (I, ꢉ). The re"ned values of here is not unexpected, as it has been well established by
the hyper"ne parameters are given in Table 4. An attempt to Gleitzer (8) that, thanks to the inductive e!ect, the Fe}O
analyze the spectrum with two symmetric quadrupole bond in phosphate is rather highly ionic.
The increasing quadrupole splitting values ꢈ"0.40, 0.55,
ꢁ>
0.77, and 0.90 mm/s are clearly indicative of an Fe ion in
a highly distorted octahedral con"guration (see Table 3).
5
Examination of the Fe}O distances shows that the
4
TABLE 4
Mossbauer Spectral Parameters for Fe4(P2O7)3 at 295 K
3
?
ꢇ
ꢈ
ꢉ
I
(%)
2
1
0
Site
(mm/s)
(mm/s)
(mm/s)
d1
d2
d3
d4
0.418
0.414
0.412
0.418
0.406
0.549
0.768
0.904
0.251
0.251
0.251
0.251
25
25
25
25
0
10
20
30
40
50
60
T (K)
?
Relative to room temperature ꢂ-iron foil. ꢇ$0.002, ꢈ$0.002,
FIG.6. Thermal variation of the Feꢁ> magnetic moment in Fe (P O ) .
ꢉ$0.002.
ꢅ
ꢂ ꢆ ꢁ

418
ELBOUAANANI ET AL.
Fe2
P4
P6
P1
P1
Fe1
P4 P5
Fe2
Fe2
Fe1
P1
P5
Fe2
Fe1
P4
P6
P1
P1
P1
Fe1
Fe2
Fe1
P1
P5
P4 P5
a
Fe2
Fe1
Fe2
P4
P6
P1
Fe1
Fe2
Fe1
P1
P4
P5
P5
c
FIG.8. Interdimer Fe}O}P}O}Fe superexchange pathway in a (010) layer.
coordination (4#2) for Fe(1) and Fe(3) and (5#1) for Fe(2)
To discuss the magnetic exchange interactions we have to
and Fe(4) can be related to ꢈ"0.40, 0.55 and ꢈ"0.77, consider two types of couplings: intra- and interdimers.
0.90 mm/s, respectively.
Interactions within the Fe O groups. (1) The cation}
cation direct interactions are important since the cation
separation, through the common face, is relatively small
ꢂ ꢃ
Ordered state (¹"4.2 K). The Mossbauer spectrum of
Fe (P O ) measured at 4.2K is shown in Fig. 7. It has been
ꢅ
ꢂ ꢆ ꢁ
"tted with four magnetic sextets of equal relative area and
width of peak. The calculated hyper"ne spectral parameters
are given in Table 5. The hyper"ne "elds are rather similar,
(mean distance Fe}Fe+3 A
> ). These interactions, occurring
between two half-"lled orbitals, are obviously antiferromag-
netic, as the transferred electron must be antiparallel to the
net spin at both ions (21).
ꢁ>
56.0 and 54.4 T; these values are typical of an Fe high-
spin ion close to the saturation state.
(2) The cation}anion}cation interactions are character-
ized by Fe}O}Fe angles close to 903 (Table 6). Accordingly
they are moderately antiferromagnetic, thanks to delocaliza-
tion superexchange, and partly to correlation superexchange.
The values of the di!erent isomer shifts remain close
(0.567 to 0.580 mm/s) and their temperature dependence is
in agreement with the second-order Doppler shift.
These results nicely explain the data obtained by Rei!
and Torardi (19) on a topotactically prepared Fe (P O )
TABLE 5
ꢅ
ꢂ ꢆ ꢁ
sample.
Mossbauer Spectral Parameters for Fe4(P2O7)3 at 4.2 K
?
ꢇ
2ꢊ
(mm/s)
H
(T)
ꢉ
I
(%)
Site
(mm/s)
(mm/s)
(d) Discussion
The magnetic structure of Fe (P O ) is characterized by s1
0.567
!0.041
!0.411
!0.149
!0.639
55.9
56.0
54.3
54.3
0.270
0.270
0.270
0.270
25
25
25
25
ꢅ
ꢂ ꢆ ꢁ
s20.570
s3
the occurrence of antiferromagnetic Fe O units building
ꢂ ꢃ
0.570
0.580
up in"nite (010) layers via &&O}P}O'' connections (Fig. 8).
s4
These layers are stacked along the b axis and are connected,
one to the others, by &&O}P}O'' and &&O}P}O}P}O''
links.
?
Relative to room temperature ꢂ-iron foil. ꢇ$0.002, ꢈ$0.002,
ꢉ$0.002.

STRUCTURE AND MAGNETIC PROPERTIES OF Fe (P O )
ꢆ ꢁ
419
ꢅ
ꢂ
TABLE 6
Magnetic Interactions in Fe4(P2O7)3 between Trivalent Iron
Angle
Fe}Fe
Interaction
Nature
Sign
n
Path
Fe}O}Fe
Distance
Fe(1)}Fe(2)
Fe(3)}Fe(4)
Direct
''
Superexchange
AF
AF
AF
AF
AF
AF
AF
AF
AF
F
AF
F
AF
AF
F
AF
F
AF
AF
F
AF
F
AF
AF
F
1
1
1
1
1
1
1
1
3.05 A
>
2.97 A
>
Fe(3)}O(3)}Fe(4)
Fe(3)}O(5)}Fe(4)
Fe(3)}O(1)}Fe(4)
Fe(1)}O(2)}Fe(2)
Fe(1)}O(8)}Fe(2)
Fe(1)}O(6)}Fe(2)
Fe(1)}O(18)}P(1)}O(20)}Fe(2)
Fe(1)}O(18)}P(1)}O(6) }Fe(2)
933
903
853
893
883
1013
''
''
''
''
''
Super
-superexchange
Via P(1)
2
2
3.81 A
3.85 A
>
>
>
>
, 12 23
4.34 A
4.76 A
6.03 A
4.86 A
6.04 A
4.42A
4.60 A
5.81 A
4.70 A
5.97 A
4.42A
4.99 A
6.08 A
4.97 A
6.24 A
4.34 A
5.44 A
6.65 A
5.59 A
6.50 A
4.34 A
4.86 A
6.03 A
4.76 A
6.04 A
4.42A
5.26 A
6.35 A
5.28 A
6.49 A
>
, 1063
, 1063
, 1123
, 1123
>
>
>
>
>
>
ꢇ
Fe(1)}O(18)}P(1)}O(6) }Fe(1)
23.87 A
23.90 A
2
2
23.96 A
23.89 A
23.91 A
23.98 A
23.61 A
23.96 A
23.83 A
23.84 A
23.97 A
2
1
1
ꢇ
Fe(2)}O(20)}P(1)}O(6) }Fe(1)
ꢇ
Fe(1)}O(20)}P(1)}O(6) }Fe(2)
3.88 A
>
ꢇ
Fe(3)}O(9)}P(2)}O(12)}Fe(4)
Fe(3)}O(9)}P(2)}O(3) }Fe(3)
Fe(3)}O(9)}P(2)}O(3) }Fe(4)
Via P(2)
Via P(3)
Via P(4)
Via P(5)
Via P(6)
3.78 A
>
, 123
>
>
>
>
>
>
>
>
>
, 1113
, 1113
, 1073
, 1073
, 1083
, 12 13
, 12 13
, 1123
, 1123
, 1113
, 1123
, 1123
, 1103
, 1103
, 1123
, 1133
, 1133
, 1173
, 1173
, 1143
, 1173
, 1173
, 1063
, 1063
ꢇ
>
ꢇ
Fe(4)}O(12)}P(2)}O(3) }Fe(4)
>
ꢇ
Fe(4)}O(12)}P(2)}O(3) }Fe(3)
ꢇ
>
Fe(3)}O(21)}P(3)}O(17)}Fe(4)
>
>
>
>
Fe(3)}O(21)}P(3)}O(5) }Fe(3)
ꢇ
Fe(3)}O(21)}P(3)}O(5) }Fe(4)
ꢇ
Fe(4)}O(17)}P(3)}O(5) }Fe(4)
ꢇ
Fe(4)}O(17)}P(3)}O(5) }Fe(3)
Fe(1)}O(14)}P(4)}O(13)}Fe(2)
Fe(1)}O(14)}P(4)}O(1) }Fe(4)
>
ꢇ
3.89 A
4.17 A
4.19 A
4.10 A
4.08 A
3.82 A
4.16 A
>
>
4.06 A
>
>
>
>
>
>
>
>
>
ꢇ
Fe(1)}O(14)}P(4)}O(1) }Fe(3)
AF
F
AF
AF
F
AF
F
AF
AF
F
>
ꢇ
Fe(2)}O(13)}P(4)}O(1) }Fe(3)
ꢇ
1
1
2
2
>
>
>
>
>
>
>
>
Fe(2)}O(13)}P(4)}O(1) }Fe(4)
ꢇ
Fe(1)}O(15)}P(5)}O(11)}Fe(2)
Fe(1)}O(15)}P(5)}O(2) }Fe(2)
Fe(1)}O(15)}P(5)}O(2) }Fe(1)
ꢇ
24.10 A
24.00 A
2
ꢇ
Fe(2)}O(11)}P(5)}O(2) }Fe(1)
ꢇ
Fe(2)}O(11)}P5}O(2) }Fe(2)
ꢇ
>
>
Fe(3)}O(10)}P(6)}O(7)}Fe(4)
23.79 A
Fe(3)}O(10)}P(6)}O(8) }Fe(1)
1
1
1
1
4.18 A
3.97 A
3.98 A
4.19 A
>
>
>
ꢇ
Fe(3)}O(10)}P(6)}O(8) }Fe(2)
ꢇ
AF
F
AF
>
>
>
Fe(4)}O(7)}P(6)}O(8) }Fe(2)
>
>
ꢇ
Fe(4)}O(7)}P(6)}O(8) }Fe(1)
ꢇ
Note. AF, antiferromagnetic; F, ferromagnetic; n, number of interactions per dimer; path, distance"Fe}O#O}Fe; angle"angle O}P}O; O ,
D
oxygen atom of shared faces.
CONCLUSION
Interactions between the Fe O groups. As shown in
ꢂ ꢃ
Fig. 8, these interactions are assured by di!erent super-
ꢈꢆ
X-ray single-crystal and Fe Mossbauer spectroscopy
superexchange &&Fe}O}P}O}Fe'' bridges. In each (010)
layer, when only the terminal oxygen atoms of the diphos- experiments allow us to conclude that Fe (P O ) crystal-
ꢅ
ꢂ ꢆ ꢁ
phate groups are concerned (Table 6), namely in each [100] lizes in the monoclinic space group P2 /n. Its structure is
ꢀ
row, they are antiferromagnetic whereas a frustrated situ- characterized by the occurrence of face-sharing Fe O units.
ꢂ ꢃ
ation occurs from row to row via oxygen atoms of the The three-dimensional framework is built up from these
shared faces.
dimers linked by bent diphosphates P O (P}O}P&1563).
ꢂ ꢆ
Between the (010) layers, the exchange interactions are In fair agreement with Goodenough's rules, each dimer is
through Fe}O}P}O}Fe (Table 6) and Fe}O}P}O}P}O}Fe antiferromagnetic whereas the &&Fe}O}P}O}Fe'' interdimer
pathways (Fig. 1), which are the weakest as they call upon exchange interactions are essentially antiferromagnetic as
super-superexchange actions.
classically observed in other iron diphosphates.

420
ELBOUAANANI ET AL.
9. H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969).
ACKNOWLEDGMENTS
10. J. Rodriguez-Carvajal, Fullprof Program, Laboratoire Leon Brillouin,
1998.
11. G. Le Caer, Private communication.
12. J. A. Ibers and W. C. Hamilton, &&International Tables for X-Ray
Crystallography,'' Vol. 4. Kynoch, Birmingham, 1976.
13. G. Sheldrick, Shelx 76, Program for Crystal Structure Determination,
Univ. Cambridge, 1976.
We are grateful to the Institut Laue Langevin (Grenoble) for the provis-
ion of neutron facilities.
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