(H2O)[V2III F6] and Pyr-VF3:Hydrothermal synthesis, structure determination, and magnetic characterization of new fluorides with the pyrochlore type

Article abstract of DOI:10.1006/jssc.2001.9286

Small green triangular crystals of a new tridimensional hydrated vanadium(III) fluoride have been hydrothermally prepared in a simple way by heating a mixture of V:HF:H₂O in the molar ratio 1:1:50 at 200°C for 3 days. Its structure was solved by X-ray diffraction in the cubic space group Fd3m (No. 227) with a = 10.4636(2)A. (H₂O)[V₂^IIIF₆] presents the structure of the well-known pyrochlore type. The cavities contain water molecules which are lost by heating (T < 250°C) without any structural change. The thermal stability of this new form of VF₃ (Pyr-VF₃) with the pyrochlore structure is compared to that of the already known Pyr-FeF₃ (1). The magnetic behavior of Pyr-VF₃ is described.

Full text of DOI:10.1006/jssc.2001.9286

Journal of Solid State Chemistry 162, 266}269 (2001)
doi:10.1006/jssc.2001.9286, available online at http://www.idealibrary.com on
III
2
(
H O)[V F ] and Pyr-VF : Hydrothermal Synthesis, Structure
2
6
3
Determination, and Magnetic Characterization
of New Fluorides with the Pyrochlore Type
Karin Barthelet, J eH ro( me Marrot, Didier Riou, and G eH rard F eH reyꢀ
Institut Lavoisier UMR 8637, Universit e& de Versailles St Quentin en Yv., 45 avenue des Etats-Unis, 78035 Versailles Cedex, France
IN HONOR OF PROFESSOR PAUL HAGENMULLER ON THE OCCASION OF HIS 80TH BIRTHDAY
vanadium metal (200 mesh/99.5#%, Chempur), #uorhyd-
Small green triangular crystals of a new tridimensional hy- ric acid (48%, Prolabo), and deionized water in a molar
drated vanadium(III) 6uoride have been hydrothermally pre-
pared in a simple way by heating a mixture of V : HF : H O in the
molar ratio 1 : 1 : 50 at 2003C for 3 days. Its structure was solved
ratio 1:1:50. The optimal synthesis requires heating of the
mixture at 2003C for 5 days in a Te#on-lined stainless-steel
autoclave and cooling to room temperature for 1 day. The
pH of the solution varies from 1 to 2}3 throughout the
synthesis. After "ltration, a homogenous green polycrystal-
line powder is obtained with a yield of 50% based on
vanadium. It is then washed and dried at room temperature.
Examination of the pure sample under polarized light shows
2
by X-ray di4raction in the cubic space group Fd3
ꢀ
m (No. 227)
III
with a ؍ 10.4636(2) A
>
. (H
2
O)[V
2
6
F ] presents the structure of
the well-known pyrochlore type. The cavities contain water
molecules which are lost by heating (T : 2503C) without any
structural change. The thermal stability of this new form of VF
3
(
Pyr-VF
the already known Pyr-FeF
VF
3
) with the pyrochlore structure is compared to that of
(1). The magnetic behavior of Pyr- the presence of small green triangular single crystals.
3
3
is described. ( 2001 Elsevier Science
Chemical Analysis
INTRODUCTION
The thermogravimetric analysis of the product was per-
formed under oxygen #ow on a TA Instrument TGA2050
apparatus with a heating rate of 53C/min from 203C to
In the 1970s and the 1980s, a lot of work was devoted to
the pyrochlore structure (2) which was a good model for
chemical exchanges and physical properties [see for in-
stance Ref. (3) and references therein]. The inorganic frame-
work contained either two types of cations with di!erent
charges, the anions being either F, O, or S, or one type of
cation with two di!erent anions with di!erent charges
mainly O and F). Only two examples, Pyr-FeF (4) and,
further, Pyr-WO (5), obtained by &&chimie douce'' reactions,
contained one cation and one anion. Further attempts to
obtain new pyrochlores of this type failed.
6
003C. Two weight losses are observed. The "rst one (9.1%),
occurring between 50 and 2503C, is in good agreement with
the departure of the water molecules located in the cavities
of the structure (th."7.7%). The second (16.4% between
2
50 and 4003C, th."14.5%) corresponds to the oxidation
of VF into V O as shown by the di!ractogram of the
residue.
ꢁ
ꢂ ꢄ
(
ꢁ
The measured density (2.78(2)), obtained under helium
with a Micromeritics multipycnometer, compares well with
the theoretical value (2.714).
ꢁ
We present here the hydrothermal synthesis, the structure,
and the magnetic properties of two new examples of this kind:
(
H O)[V'''F ] and its isotypic anhydrous form, Pyr-VF .
Structure Determination
ꢂ
ꢂ ꢃ
ꢁ
One of the small triangular crystals was glued on a glass
ber and analyzed by single-crystal X-ray di!raction on
EXPERIMENTAL
"
Synthesis
a Siemens three-circle di!ractometer equipped with a CCD
detector and working with the MoKa monochromatized
The title compound is obtained as a pure phase by hydro-
thermal synthesis under autogenous pressure by mixing
>
wavelength (j"0.71073 A). The SAINT program was used
to reduce the collected data and to correct them for
Lorentz-polarization e!ects. The SADABS program al-
lowed a semi-empirical absorption correction to be applied.
ꢀ
To whom correspondence should be addressed. E-mail: riou@chimie.
uvsq.fr.
2
66
0
(
022-4596/01 $35.00
2001 Elsevier Science
All rights reserved.

STUDY OF (H O)[V'''F ] AND Pyr-VF
267
ꢂ
ꢂ
ꢃ
ꢁ
TABLE 1
Crystallographic and Chemical Data
over, "eld-cooled and zero-"eld-cooled experiments were
performed to get accurate information on the magnetic
ordering temperature.
Chemical formula
Formula weight (g ) mol\ꢀ)
Crystal size
Lattice parameters
System
Space group
(H O)[V'''F ]
ꢂ
ꢂ ꢃ
234
0.05;0.02;0.02 mm
a"10.4636(2) A
Cubic
Fd3m (No. 227)
1145.63(4) Aꢁ, 8
DISCUSSION
>
(H O)[V'''F ] exhibits the well-known tridimensional
ꢂ
ꢂ ꢃ
ꢀ
pyrochlore type structure which exhibits cages delimited
by six-membered-ring windows and containing the water
molecules.
Volume/Z
>
d
d
(g ) cm\ꢁ)
(g ) cm\ꢁ)
2.714
2.78(2)
1849
102
0.1641
0.056, 0.171
#
!ꢀ#ꢁꢀ!ꢂ%$
ꢃ%!ꢄꢁꢅ%$
The structure is built up from the connection of in"nite
tilted chains of octahedra of vanadium oriented in two
perpendicular directions (Fig. 1). These octahedra are very
Collected int.
Ref (I52p(I))
R
*ꢆꢂ
R , wR (Fꢂ)
>
regular with six equivalent distances V}F of 1.974(2) A and
ꢀ
ꢂ
ꢇ
Residual electronic density
>
1.397, !0.883 e.A\ꢁ
F}V}F angles of 88.6(2)3, 91.4(2)3, and 180.03. Each octahed-
ron is linked via its two trans #uorine apices to two oc-
tahedra of the same chain. The four remaining atoms ensure
The structure was solved and re"ned using SHELX-TL in the connection between three chains, one perpendicular to
the cubic space group Fd3m (No. 227). The vanadium was the two others with V}F}V angles values of 139.1(2)3. The
rst located from direct methods. Further Fourier di!erence inorganic framework generates the presence, in the [111]
syntheses provided the positions of #uorine and oxygen direction, of intersecting tunnels. The free aperture of the
atoms. An anisotropic thermal factor was applied to all hexagonal windows is approximately 2.6 A. The trivalent
ꢀ
"
>
nonhydrogen atoms. After the "nal re"nement using (32e) oxidation state of the vanadium (calculated, 2.88) is con-
sites for the water molecules, the reliability factors R1(F ) "rmed by the valence bond calculation using the data of
ꢇ
and wR2(Fꢂ) converged to 0.056 and 0.171 respectively for Brese and O'Kee!e (6).
ꢇ
1
849 re#ections (I52p(I)). These relatively high values for
The originality of this work resides principally in the
the reliability factors could be explained by the very small simplicity of the synthesis compared to that of isostructural
size of the crystal (0.05;0.02;0.02 mm) and consequently compound FeF , which needed more steps. Unfortunately,
ꢁ
by the rather poor quality of the data.
the attempts to synthesize iron(III) #uoride in the same way
All data collection conditions are reported in Table 1. failed: instead of FeF , FeF )4H O is obtained. Moreover,
ꢁ
ꢂ
ꢂ ꢃ
ꢂ
Table 2 provides the atomic coordinates. Anisotropic ther- although FeF , (H O)[V'''F ], and Pyr-VF have the same
ꢁ
ꢂ
ꢁ
mal factors can be obtained by request from the authors.
Thermodi!ractometry experiments were carried out
under vacuum between 50 and 8003C (13C/min) on a Sie-
mens D5000 di!ractometer using a Pt strip heating element
and an HTK 16 Anton Parr furnace. Diagrams were
recorded every 303C up to 2503C, and then, every 103C.
structural type, their thermal behaviors are quite di!erent.
Magnetism
The magnetization (M) of the sample was measured on
anhydrous Pyr-VF as a function of the temperature in the
ꢁ
range 2}500 K with a Quantum Design Squid device. The
resulting magnetic susceptibility was then deduced. More-
TABLE 2
4
Atomic Coordinates (؋10 ) and Equivalent Isotropic
2
3
III
>
2 2 6
Displacement Parameters (A ؋ 10 ) for (H O)[V F ]
Atoms
Wycko!
x
y
z
¹xꢇ##ꢁꢈ ;(eq)
V
F
Ow
16d
48f
32e
0
1250
1491(18)
5000
6250
1491(18)
0
1
1
8(1)
20(2)
29(11)
!659(4)
1491(18) 0.25
FIG. 1. Perspective view of the (H O)[V'''F ] pyrochlore structure
ꢂ
ꢂ ꢃ
(
water molecules are omitted for clarity).

2
68
BARTHELET ET AL.
FIG. 2. Thermodi!ractograms performed under vacuum, of (H O) [V'''F ] from 25 to 8003C.
ꢂ
ꢂ ꢃ
Whereas Pyr-FeF transformed successively into HTB (hex-
ꢁ
2
2
1
1
50
00
50
00
agonal tungsten bronze) at ca. 3003C and into ReO -type
ꢁ
FeF (1) at ca. 3803C, the stability of the homologous
ꢁ
vanadium solid is much lower. Indeed, the thermo-
di!ractogram performed under vacuum (Fig. 2) shows that
the dehydration of (H O)[V'''F ], beginning at 503C, is
ꢂ
ꢂ ꢃ
structurally characterized by an increase of the (111) re#ec-
tion, without any change in the di!raction pattern. That
means that the dehydrated Pyr-VF form keeps the pyro-
ꢁ
chlore structure. Moreover, the decrease of I
above
ꢀ
ꢀꢀ
2
503C indicates that Pyr-VF begins to decompose at
this temperature, before any transformation into a-VF ,
with the simultaneous appearance of the peaks correspond-
5
0
ꢁ
ꢁ
0
ing to V O . This oxide probably comes from the hydrolysis
0
50
100
150
200
250
300
ꢂ ꢁ
of Pyr-VF by the water molecules remaining in the
Temperature (K)
ꢁ
chamber.
Pyr-VF is antiferromagnetic below 5.0(1) K as proved
both by the s\ꢀ(¹) (Fig. 3a) and FC/ZFC curves (Fig. 3b).
The former exhibits an unusual positive asymptotic Curie
0
.30
.25
.20
.15
.10
.05
ꢁ
FC
0
0
0
0
0
temperature (h "20 K) indicating ferromagnetic coup-
ꢈ
lings, which can be explained by the dꢂ electronic structure
of Vꢁ> (7) involved in a 1803 superexchange interaction. The
di!erence between FC and ZFC curves below ¹ is consis-
ZFC
,
tent with a nonaligned magnetic structure. It may be
assumed that, as Pyr-FeF (8), the frustrated magnetic
ꢁ
structure is formed by four magnetic subnetworks with
spins at 1093 one from each other. This could also explain
the low ¹ value of Pyr-VF , which is in the same range as
,
ꢁ
that of Pyr-FeF (19 K).
ꢁ
Finally, the cages which appear in the pyrochlore struc-
ture after removal of the water could be "lled by alkaline
ions via exchange reactions for providing new mixed-
valence V''}V''' #uorides. The study is currently in progress
and will be published elsewhere.
0
5
10
15
20
Temperature (K)
FIG. 3. (a) s\ꢀ"f (¹) curve for Pyr-VF . (b) Zero-"eld-cooled and
eld-cooled (H"100 G) curves for Pyr-VF .
ꢁ
ꢁ
"

STUDY OF (H O)[V'''F ] AND Pyr-VF
269
ꢂ
ꢂ
ꢃ
ꢁ
ACKNOWLEDGMENTS
3. C. N. R. Rao and B. Raveau, &&Transition metal oxides.'' VCH Pub-
lishers, New York, 1995.
4
5
. R. De Pape and G. F eH rey, Mater. Res. Bull. 21, 971 (1986).
. R. Nedjar, M. M. Borel, M. Hervieu, and B. Raveau, Mater. Res.
Bull. 23, 91 (1988).
The authors are indebted to Dr. M. Nogues (LMOV, Versailles) for
magnetic measurements.
6
7
. N. E. Brese and M. O'Kee!e, Acta Crystallogr. B 47, 192 (1991).
. J. B. Goodenough, A. Wold, R. J. Arnot, and N. Menyuk, Phys. Rev.
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2
. G. Fe
H
rey and J. Pannetier, Eur. J. Solid State Inorg. Chem. 31, 697 (1994).
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. D. Babel, Z. Anorg. Allgem. Chem. 387, 3820 (1972).