Materials Research Bulletin 40 (2005) 1569–1576
www.elsevier.com/locate/matresbu
New lead vanadium phosphate with langbeinite-type
structure: Pb1.5V2(PO4)3
R.V. Shpanchenko a,*, O.A. Lapshina a, E.V. Antipov a,
J. Hadermann b, E.E. Kaul c, C. Geibel c
a
b
Department of Chemistry, Moscow State University, 119992 Moscow, Russia
EMAT University of Antwerp (RUCA), Groenenborgerlaan 171, 2020 Antwerp, Belgium
c
Max-Planck Institute CPfS Nöthnitzer Str. 40, 01187 Dresden, Germany
Received 25 November 2004; received in revised form 3 February 2005; accepted 13 April 2005
Abstract
The new lead vanadium phosphate Pb1.5V2(PO4)3 was synthesized by solid state reaction and characterized by
X-ray powder diffraction, electron microscopy, and magnetic susceptibility measurements. The crystal structure of
Pb1.5V2(PO4)3 (a = 9.78182(8) Å, S.G. P213, Z = 4) was determined from X-ray powder diffraction data and
belongs to the langbeinite-type structures. It is formed by corner-linked V3+O6 octahedra and tetrahedral phosphate
groups resulting in a three-dimensional framework. The lead atoms are situated in the structure interstices and only
partially occupy their positions. An electron microscopy study confirmed the structure solution. Magnetic
susceptibility measurements revealed Curie–Weiss (CW) behavior for Pb1.5V2(PO4)3 at high temperature whereas
at around 14 K an abrupt increase on the susceptibility was observed.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides; D. Crystal structure; D. Magnetic properties
1. Introduction
Vanadium compounds with a low (<5+) oxidation state of the V-atoms attract the attention of
investigators since they often exhibit unusual magnetic behavior. Depending on the oxidation state of the
vanadium atoms and their coordination these compounds demonstrate a great variety of possible
* Corresponding author. Tel.: +7 95 9393490; fax: +7 95 9394788.
E-mail address: [email protected] (R.V. Shpanchenko).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.04.037
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structures forming three-dimensional frameworks, layered structures or even infinite chains of vanadium
polyhedra [1,2]. The V3+ cation usually has a regular octahedral coordination and these octahedra may be
connected by different manners in these structures.
The PO4 phosphate group is a convenient structural unit for the synthesis of new low-valent vanadium
compounds since it is stable under redox processes. Only a few compounds in the Pb–V–P–O system
were reported where the vanadium oxidation state is lower than 5+. PbV2P2O10 [3] contains vanadium 4+
in octahedra and square pyramids which together with the phosphate tetrahedra form a three-dimensional
framework. PbV2(P2O7)2 [4] is isostructural with other MV2(P2O7)2 pyrophosphates (where M = Sr, Ba,
Cd) [5–7]. Their structures are three-dimensional networks of corner-linked VO6 octahedra and P2O7
groups. The crystal structure of Pb2V2VO(PO4)4 is built up by V3+O6 octahedra and V4+O5 pyramids
connected via PO4 groups and may be described in terms of a [V2P4O16]1 framework [8]. The structure
of Pb(VO)3(P2O7)2 [9] contains isolated parallel infinite chains of corner-linked VO6 octahedra with
vanadyl bonds directed along the chains. P2O7 groups are connected to the neighboring octahedra by
common corners forming a network with infinite hexagonal channels occupied by lead ions.
Pb(VO)(P2O7) [10] has a closely related structural motif to Pb(VO)3(P2O7)2.
Thus, only one pyrophosphate, namely PbV2(P2O7)2, contains vanadium in oxidation state 3+. In other
compounds the vanadium atoms are tetravalent or have a mixed valence (3+ and 4+) (Pb2(VO)(V2P4O16)).
Therefore, the aim of our research was the preparation of new lead vanadium (3+) phosphates, their crystal
structure determination, electron microscopy study and magnetic properties measurements.
2. Experimental
A powder sample of Pb1.5V2(PO4)3 was obtained by the reaction of V2O3, Pb2P2O7 and NH4H2PO4 in a
molar ratio of 1:1:2, respectively. A pressed pellet of the initial mixture was placed in a corundum crucible
and annealed in a tube furnace in purified argon flow in two stages. At the beginning the mixture was slowly
heated up to 850 8C for 3 h to eliminate NH3 and H2O released during the decomposition of NH4H2PO4. The
resulting product was annealed at 1200 8C for 40 h. The X-ray powder diffraction study of the brown powder
revealed a presence of V2O3 as admixture whereas the main peaks belong to Pb1.5V2(PO4)3. We failed to
prepare a single phase sample probably due to the high volatility of lead at high temperatures. Pb2P2O7 was
synthesized by heating the equimolar mixture of PbO and (NH4)H2PO4 at 700 8C in air for 3 days with
intermediate regrinding. V2O3 was prepared by reducing V2O5 at 600 and 800 8C in hydrogen flow.
X-ray powder diffraction (XRPD) data were collected on a STOE STADI/P powder diffractometer
(transmission mode, Cu Ka1-radiation, Ge-monochromator, linear-PSD). The structure refinement was
carried out with the GSAS program [11]. The transmission electron microscopy (EM) study was
performed with a JEOL 4000EX microscope. HREM image simulation was done using the MacTempas
software. The magnetic susceptibility measurements were carried out with a commercial Quantum
Design MPMS SQUID magnetometer 0.01, 0.1 and 1 T magnetic fields between 2 and 400 K.
3. Results and discussion
The XRD pattern of the Pb1.5V2(PO4)3 phosphate was indexed in a primitive cubic cell with
the lattice parameter a = 9.78182(8) Å. The peak positions and their intensities for the new
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Table 1
Experimental and crystallographic parameters for Pb3V(PO4)3 and Pb1.5V2(PO4)3
Chemical formula
Pb1.5V2(PO4)3
Molecular weight
Crystal system
Space group (no.)
Cell constants (Å)
Volume (Å3)
Z
Dcalc (g cm3)
Radiation and wavelength (Å)
u or 2u range (8)
m (mm1)
Colour
Diffractometer
Reliability factors
Goodness of fit, x 2
Package for structure refinement
Admixture (wt.%)
697.6
Cubic
P213 [198]
a = 9.7881(2)
937.77(5)
4
4.912
Cu Ka1, 1.5406
6–106
10.93
Brown
STADI/P
RwP = 0.026, RP = 0.020
1.44
GSAS
V2O3, 8.0
Pb1.5V2(PO4)3 were close to those for KTi2(PO4)3 [12] and BaKCr2(PO4)3 [13] having the langbeinite-like structure.
The structure refinement was carried out for a two phase mixture containing this phase and 8 wt.% of
V2O3. The atomic coordinates for KBaCr2(PO4)3 were taken as the starting ones. At the first step only the
positions of the heavy atoms were refined followed by the determination of the occupancies for the two
lead positions at a fixed displacement parameter of U = 0.025 Å2. Actually, the refined occupancy values
0.88(2) and 0.62(2) do not depend on a variation of the thermal parameter (0.01–0.025 Å2) and they are
in a good agreement with the expected amount of Pb atoms per formula unit (1.5). The displacement
parameters for the oxygen atoms were constrained and refined together. The experimental and
crystallographic parameters for Pb1.5V2(PO4)3 are summarized in Table 1. The atomic positions,
displacement parameters and main interatomic distances in the Pb1.5V2(PO4)3 structure are listed in
Table 2
Atomic positions and displacement parameters in the Pb1.5V2(PO4)3 structure
Atom
Position
x/a
y/b
z/c
100 Uiso (Å2)
Pb(1) a
Pb(2) b
V(1)
V(2)
P
O(1)
O(2)
O(3)
O(4)
Occupancy.
a
0.88(1) Pb.
b
0.62(1) Pb.
4a
4a
4a
4a
12b
12b
12b
12b
12b
0.07110(7)
0.3045(1)
0.5864(3)
0.8557(2)
0.6251(4)
0.6480(9)
0.7720(9)
0.5669(11)
0.5360(8)
x
x
x
x
0.4602(4)
0.5077(7)
0.4828(7)
0.3212(9)
0.5729(8)
x
x
x
x
0.2693(4)
0.4129(11)
0.2131(8)
0.2681(7)
0.1984(7)
4.4(1)
5.2(2)
2.8(1)
2.4(2)
1.5(1)
5.0(2)
5.0
5.0
5.0
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Fig. 1. The observed, calculated and difference X-ray patterns for Pb1.5V2(PO4)3. Low set of peaks belongs to V2O3.
Tables 2 and 3, respectively. The observed, calculated and difference X-ray patterns for Pb3V(PO4)3 are
shown in Fig. 1.
Pb1.5V2(PO4)3 has a three-dimensional structure belonging to the langbeinite type. The structure
contains V3+O6 octahedra and PO4 tetrahedra (Fig. 2). Every octahedron is linked with the neighboring
ones via tetrahedra by all six vertexes forming a 3D-framework. The vanadium atoms are situated in
slightly distorted octahedra. BVS calculations give values of 3.4 and 3.1 for V(1) and V(2), respectively.
All P–O separations in the phosphate tetrahedron are in the range of 1.48–1.57 Å. The coordination
polyhedra for the lead atoms are shown in Fig. 3. The Pb(1) and Pb(2) atoms randomly occupy two
positions with occupancy 88 and 62%, respectively. Pb(1) coordinates nine oxygen atoms: six of them
(dPb(1)–O(2) = 2.75(1) Å and dPb(1)–O(1) = 2.823(8) Å) form a nearly hexagonal base plane and the
remaining three (dPb(1)–O(4) = 3.010(7) Å) close to the polyhedra by a trigonal top plane. The Pb(2)
atoms are located in strongly distorted octahedra. Additionally three O(2) oxygen atoms are situated at
the 3.24 Å distance but the latter may be considered as nonbonding. One may suggest that the lone pair of
the Pb(2) atom is directed toward the plane formed by the O(4) atoms.
Table 3
The main interatomic distances (Å) in the Pb1.5V2(PO4)3 structure
Pb(1)–O(1)
O(2)
O(4)
Pb(2)–O(3)
O(4)
V(1)–O(2)
O(1)
V(2)–O(4)
O(3)
P(1)–O(1)
O(2)
O(3)
O(4)
3 2.823(8)
3 2.750(10)
3 3.010(7)
3 2.598(10)
3 2.753(8)
3 1.979(8)
3 1.960(9)
3 1.970(8)
3 2.075(10)
1.498(10)
1.556(10)
1.475(8)
1.568(9)
R.V. Shpanchenko et al. / Materials Research Bulletin 40 (2005) 1569–1576
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Fig. 2. Crystal structure of Pb1.5V2(PO4)3. Lead atoms are represented by circles.
The Pb1.5V2(PO4)3 sample was also studied using transmission electron microscopy. The electron
diffraction patterns could be indexed using the cell parameters stated in the X-ray diffraction results. The
only reflection condition is h00:h = 2n, which is in agreement with the space group P213 found from
Fig. 3. Coordination polyhedra of lead atoms in the Pb1.5V2(PO4)3 structure.
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XRD. A high resolution image along the [0 0 1] direction and the corresponding electron diffraction
pattern are shown in Fig. 4. On this diffraction pattern the appearance of the 1 0 0 and 0 1 0 reflections,
which are forbidden by the symmetry P213, is due to double diffraction. The unit cell is indicated
on the high resolution image by a small white square. A calculated image was made using the
model presented in this paper, and is set in the experimental image, indicated by a white outline.
The calculated image was made at a defocus value of +5 nm and a thickness of 5 nm, and is in good
agreement with the experimental image, thus supporting the proposed model. On this image the dark
areas correspond to the positions in between the atom columns, while the bright dots are projections
along the columns of Pb, V and P atoms, which are very near along all projections and thus are imaged as
a single bright dot.
x(T) and 1/x(T) dependence for Pb1.5V2(PO4)3 is shown in Fig. 5. Above 15 K the magnetic
susceptibility follows closely a Curie–Weiss (CW) behavior. Fitting the susceptibility with the CW
model we obtained an effective magnetic moment value of 2.8 mB and a Curie–Weiss temperature of
4 K. The experimental value for the effective moment is in an excellent agreement with the expected
one (2.83
mB) forffi a compound containing one S = 1, V3+ ion per formula unit (according to
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
meff = g sðs þ 1Þn for g = 2). The negative value of the CW temperature indicates an existence
of either weak antiferromagnetic interactions among the moments or a presence of interactions with
different sign and similar magnitude. At around 12–14 K the curve strongly deviates upwards from the
paramagnetic behavior for weak applied fields (100 G) while for larger fields (already at 1000 G)
this behavior is strongly reduced. This may indicate two possible scenarios. In the first one, this
ferromagnetic-like upturn is due to a low amount of a foreign phase which undergoes a ferromagnetic
transition at this temperature while our compound remains paramagnetic. At the second, this material
undergoes a phase transition towards a canted antiferromagnetic state in which a net (but small)
ferromagnetic moment is generated. In order to distinguish between these two possibilities specific
Fig. 4. High resolution electron microscopy image of Pb1.5V2(PO4)3 viewed along the [0 0 1] direction. The corresponding
electron diffraction pattern and calculated image are shown as inserts. The small white square gives the outline of the unit cell.
R.V. Shpanchenko et al. / Materials Research Bulletin 40 (2005) 1569–1576
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Fig. 5. The x(T) and 1/x(T) (insert) dependences for Pb1.5V2(PO4)3.
heat as well as single-crystal magnetic susceptibility measurements are needed. Since the structure is
quite complicated and the possible superexchange paths involve intermediate phosphate groups then it
is hard to estimate from the standard rules the magnitude and sign of possible interactions between the
V3+ magnetic centers.
This result is not something unusual. In the langbeinite-type phosphates KBaFe2(PO4)3 [14] and
KBaCr2(PO4)3 [13] the magnetic measurements revealed a CW behavior at high temperature and some
deviations indicating a magnetic ordering at low temperature. It was found that KBaFe2(PO4)3 is an Ltype ferrimagnet with TN 4 K whereas KBaCr2(PO4)3 undergoes an antiferromagnetic transition at
12 K. The latter result was attributed to a superexchange Cr–Cr interaction.
Pb1.5V2(PO4)3 investigated in the frame of this work is the next lead vanadium phosphate, after
PbV2(P2O7)2, containing only V3+ in its structure. It belongs to the langbeinite-type structures
A2xB2(XO4)3 where the A-cation is an alkali- and/or alkali-earth element, the B-cation is a tri- or
tetravalent transition metal in phosphates and B = Mg, Zn in sulfates. Depending on the valence state
of the transition metal the amount of the A-cation may be varied from 1 to 2 as it was observed in the
vanadium and titanium containing compounds: KTi24+(PO4)3, K2Ti23.5+(PO4)3 [12,15,16] or
K2Ti4+V3+(PO4)3, BaV23.5+(PO4)3 and Ba1.5V23+(PO4)3 [17]. If the number of A-cations is less than
2 per formula unit then their positions (one or both) are only partially occupied like it was observed
for Pb1.5V2(PO4)3, where Pb is the A-cation. When this occurs then the occupancies of the two
positions are non-equivalent. The largest interstices have a higher occupancy than the smaller ones.
So, for KTi2(PO4)3 and K1.75Ti2(PO4)3 they are 0.8, 0.2 and 1.0, 0.75, respectively. The average K–O
distances for these two positions are 2.974 and 2.918 Å, respectively, in both structures. In the case of
Pb1.5V2(PO4)3 additional changes in the coordination arrangement of the Pb atoms takes place in
comparison with that for the K ones. Symmetric potassium cations occupy almost equal nine-top
polyhedra while a sterically active lone pair of the Pb2+ cation demands an asymmetric coordination.
Recently Drob and Glaum reported the crystal structure of Ba1.5V2(PO4)3 [18]. The two barium
positions in this structure have close occupancies (0.772(2) and 0.727(2)). However, the difference
between the average Ba–O distances in the two polyhedra (3.019 and 2.981 Å) is also less than that for
the Pb1.5V2(PO4)3 structure (2.861 and 2.676 Å, respectively).
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Acknowledgements
Authors are grateful to RFBR (grant 04-03-32787) and ICDD (Grant-in-Aid APS91-05) for financial
support. Part of this work has been performed within the framework of the IAP 5-1 of the Belgian
government.
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