Synthesis and Magnetic Properties of Pyrochlore Solid Solutions

Communication / Preliminary communication
has
bhas
solid solutions (Bi1,524–xMxCu0,476)[Sb1,524Cu0,476]O7+ÿ
Synthesis and study of magnetic properties
with M = Ca and Pb
6, bd du Maréchal-Juin, 14050 Caen cedex 04, France
Email address: [email protected] (M. Sellami).
CR Chemistry 9 (2006) 1209–1214
1631-0748/$ - see front matter © 2006 French Academy of Sciences. Published by Elsevier SAS. All rights reserved.
Available online on March 10, 2006
Faculty of Sciences, University of Science and Technology of Oran, BP 1505, 31000 El-Mnaouer, Oran, Algeria
Corresponding author.
Keywords: Bismuth; Copper; Calcium; Lead; Pyrochlor; Solid solution; Oxide; Magnetic susceptibility; Mössbauer spectrometry of 121Sb
Laboratory of Physical Chemistry of Materials – Catalysis and Environment, Department of Chemistry,
doi:10.1016/j.crci.2006.02.001
http://france.elsevier.com/direct/CRAS2C/
Laboratory of Crystallography and Materials Science, ENSICAEN–CNRS UMR 6508,
Keywords: Bismuth; Copper; Calcium; Lead; Pyrochlore; Solid solution; Oxide; Magnetic susceptibility; Mössbauer spectrometry
Received on July 19, 2005; accepted after revision on January 24, 2006
Solid solutions of the pyrochlore type, with the general chemical formula (Bi1,524–xMxCu0,476)[Sb1,524Cu0,476]O7+ÿ (M = Ca2+,
The +5 oxidation state of antimony was demonstrated by Mössbauer spectroscopy of 121Sb. Magnetic susceptibility measurements
showed that the effective moment of copper in these compounds decreases upon substitution of bismuth by calcium.
cubic, with space group Fd3m. The homogeneity domains obtained are 0 ÿ x ÿ 0.30 and 0 ÿ x ÿ 0.26 for calcium and the
© 2006 French Academy of Sciences. Published by Elsevier SAS. All rights reserved.
system, with Fd3m space group. Domains of homogeneity are 0 ÿ x ÿ 0.30 and 0 ÿ x ÿ 0.26 for calcium and lead, respectively. HAS
The solid solutions with pyrochlore-like structure and general chemical formula (Bi1.524–xMxCu0.476)[Sb1.524Cu0.476]O7+ÿ (M =
lead, respectively. A steady decrease in the lattice parameter was observed for the solid calcium solution. The state
Pb2+), were synthesized by high-temperature chemical reaction in the solid state. Both solutions crystallize in the system
To cite this article: M. Sellami et al., CR Chimie 9 (2006).
regular decrease of the cell parameter has been observed in the calcium solid solution. The state oxidation +5 of antimony has
Synthesis and magnetic properties study of solids solutions (Bi1.524–xMxCu0.476)[Sb1.524Cu0.476]O7+ÿ with M = Ca and Pb.
magnetic moment of copper in these compounds decreases when bismuth is substituted for calcium. To cite this article:
M. Sellami et al., CR Chimie 9 (2006).
© 2006 French Academy of Sciences. Published by Elsevier SAS. All rights reserved.
Abstract
Summary
been put in evidence by the Mössbauer spectrometry of 121Sb. Magnetic susceptibility measurements showed that the effective
Noureddine BettaharAhmed Bekka
Mayouf Sellami a,*, Ninh Nguyen ,,
*
b
has
121Sb
Ca2+, Pb2+) have been synthesized by solid-state chemical reaction at high temperature. The two solutions crystallize in the cubic
Machine Translated by Google

Transition metal oxides exhibiting multiple oxidation states
have been the subject of numerous studies. In particular, high-
temperature superconducting copper perovskite phases have
been of great interest, both in basic research and technological
applications. Oxides with pyrochlore-type structures also have
interesting applications [1]. Pyrochlore is a natural mineral with
the formula (Na,Ca,U)2(Nb, Ta)2O6(OH,F) [JCPDS/13-254]. It
was the first compound in a family with the formula A2B2X6Xÿ.
The crystal system is cubic, with space group Fd3m and lattice
parameter a ÿ 10 Å; the unit cell comprises eight formula units
[2].
The polycrystalline compounds were synthesized
by the classical solid-state reaction method of the
oxides SbÿOÿ (99%), BiÿOÿ (99.9%), CuO (99%),
CaO (99%), and PbO (98%) (Aldrich Chemical
Company Ltd). The precursors, weighed according
to the required stoichiometric proportions, were
thoroughly mixed by careful grinding in an agate
mortar until a perfectly homogeneous mixture was
obtained. This mixture was first heated at 700 °C for
24 h before being ground again. The samples then
underwent two heating-grinding cycles at a temperature of
900 °C for 72 h. The samples are finally finely ground
and compacted into 13 mm diameter pellets which
are subjected to sintering at 1000 °C for 24 h.
Electrical measurements were performed using
the four-point method at room temperature. Magnetic
susceptibility measurements were carried out using
a Faraday balance over a temperature range of 80
to 600 K. The oxidation state of antimony in the
compounds was determined by Mössbauer
spectroscopy of 121Sb.
M. Sellami et al. / CR Chimie 9 (2006) 1209–1214
1210
In this work, we undertook to synthesize two new solid solutions
(Bi1,524–xCaxCu0,476)- [Sb1,524Cu0,476]O7+ÿ and (Bi1,524–
xPbxCu0,476)[Sb1,524- Cu0,476]O7+ÿ and to study their magnetic
and electrical properties.
• reducing distortion would decrease the mesh parameter a;
The characterization of the samples by X-ray diffraction was
carried out using an X'pert type automatic powder diffractometer
operating in Bragg-Brentano type focusing geometry (ÿ/2ÿ) and
using the ÿÿ wavelength of copper.
The evolution of the lattice parameter refined as a function of the
lead rate x appears to be irregular, although the ionic radius of lead
(RVIII = 1.29 Å) is greater than that of bismuth (Table 2).
2. Experience
The solid lead solution (Bi1.524–xPbxCu0.476)
3. Results and discussion
In previous work [3], a new copper-antimony-bismuth pyrochlore
compound with the formula (Bi1,524Cu0,476)[Sb1,524Cu0,476]O7+ÿ
was identified, with a lattice parameter a = 10.430(1) Å. In this
compound, bismuth, with its high ionic radius, occupies site A, with
a coordination number VIII, and antimony, with its low ionic radius,
occupies site B, with an octahedral coordination number VI, in
accordance with the stability conditions of the pyrochlore structure.
Copper is distributed between the two sites A and B in equal
proportions, as previously reported by Champarnaud et al. [4]. The
electrical resistivity of the compound at room temperature is on
the order of 105 ÿ cm and the value of the effective moment ÿCu of
copper, equal to 2.27 ÿB, is significantly higher than that usually
observed for Cu2+, namely 1.73 ÿB.
1. Introduction
[Sb1,524Cu0,476]O7+ÿ is formed up to x = 0.26.
The irregular evolution of the lattice parameter has already been
encountered in work where lead, an inert non-bonded electron pair
cation "E*Pb" partially substitutes bismuth, a non-bonded electron
pair cation "E*Bi" [6], which would result in two contradictory effects:
3.1. X-ray Diffraction
The pure phases obtained crystallize in the cubic system with
the space group Fd3m. The refinement of the lattice parameter
reveals a regular decrease in it, in agreement with the difference
between the ionic radii of bismuth (RVIII = 1.17 Å) and of calcium
(RVIII = 1.12 Å) [5] (Fig. 2).
Beyond this fraction, phase mixing occurs. Indeed, the
diffractogram of the composition x = 0.40 (Fig. 1) shows
the presence of additional lines. Table 1 lists the 2ÿ
positions, relative intensity, and Miller indices of the main
lines indexed in a pyrochlore structure of the compound
at x = 0.
The solid solution (Bi1,524–xCaxCu0,476)
[Sb1,524Cu0,476 ]O7+ÿ exists in the region 0 ÿ x ÿ 0.30.
Fig. 1 shows the shape of the diffractogram of the compound x = 0.
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I/Imax
5
511
49.43
Fig. 1. X-ray diffractograms of compounds x = 0 and 0.40 of the solid solution (Bi1,524–xCaxCu0,476)[Sb1,524Cu0,476]O7+ÿ.
0.23
10.4357(9)
13
28.47 29.72 34.35
0 10,430(1)
45.17
9
N
2ÿ (°) 80.06
5.12 1.58
82.71 92.64 99.64
5.98 2.88 2.89
14
0.26
10,435(1)
hkl
41.37 40.25
108
0.15
10.435(9)
100
1211
3
662440
8.85
16
622
2
3.11
Parameter a (Å)
14.64
4
M. Sellami et al. / CR Chimie 9 (2006) 1209–1214
33.43
58.80
7.32
840
37.64
Table 1
311
72.42
222
0.1
10.441(5)
331
1.76
0.20
10.446(6)
1211
Table 2
400 444 844
111
7
4.00
61.60
800
Fraction x
10 22
in the mesh. The values of the isomeric displacement
• The larger size of the Pb ion would increase the
parameter a. highlighted the presence of a single antimony site
singlet form. The analyses of these spectra have
values observed for quadrupole bursting (ÿ)
There is probably a competition between the effect of
= 0.23 and x(Pb) = 0.26 (Fig. 3), are presented under the
are zero, highlighting a spherical distribution of charges
around the Sb nuclei.
(ÿ), ranging between 8.48 and 8.55 mm/s (Table 3) by
size and effect of the unbonded electron pair.
The Mössbauer spectra of 121Sb obtained at room
temperature, for the three compounds x = 0; x(Ca)
+5 of the Sb ion in all samples, while the
relative to InSb, are consistent with the oxidation state
3.2. Mössbauer spectrometry of 121Sb
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M
effective Cu2+ of these phases;
• either the high values of MCu in x = 0 and 0.15
the solid calcium solution. We note that it
cubic (coordinate VIII). This symmetry implies
mixed valence of copper, with ÿ = 0.329. This last value
seems high for a compound
to these Cu2+ ions the eg configuration
synthesized in air like this one. It should also be noted
of these products in solution and of the volatility of bis-
muth, respectively;
reducing atmosphere due to low solubility
t2g5 giving
for Cu3+ and Cu2+ ions (2.83 and 1.73 ÿB respectively)
and that of our unsubstituted compound
(Table 4) also revealed a decrease in this
pyrochlore [7–10]. But, taking into account the values of the
effective moment usually observed
+2.59; we would therefore be in the presence of a phase at
the existence of a hole in the t2g band, this could
iodometric assay or by an ATG in
in the paramagnetic range (300–600 °K)
compounds contain an excess of ÿ oxygen, which
causes partial oxidation of Cu2+ to Cu3+,
there is a decrease in magnetic susceptibility
• either this increase in M is due to the fact that these
Fig . 4 shows the evolution of the ÿ1 curves
value of 2.38 to 1.49 ÿB, when x increases from 0 to 0.30
when the substitution rate x increases. The calculation
are linked to the energy scheme of Cu2+ ions occupying
a symmetry site together with Bi3+ ions
respectively.
as in the case of many type compounds
(T) of
that we were unable to determine this rate ÿ by a
leading to spin-orbit coupling which, consequently, could
increase the value of the moment
of the Cu ion in this compound led to a charge
of the value of the effective moment (MCu) of the copper ion
(x = 0 and MCu = 2.38 ÿB), the estimate on the load
3.3. Magnetic susceptibility
–
Attribution
Fig. 3. Mössbauer spectra of 121Sb at 293 °K of the compounds x =0;
–
substitution of the solid solution (Bi1.524–xCaxCu0.476)-
Fig. 2. Variation of the lattice parameter a as a function of the fraction x
1212
ÿ (mm/s)
1.57 (2)
1.66 (3)
1.74 (3)
–
SbV
Table 3
M. Sellami et al. / CR Chimie 9 (2006) 1209–1214
SbV
xCa = 0.23 and xPb = 0.26.
Compounds
Bi1,52Sb1,52Cu0,95O7
Bi1,29Ca0.23Sb1,52Cu0,95O7
Bi1,26Pb0.26Sb1,52Cu0,95O7
ÿ = isomeric displacement with respect to InSb; ÿ = quadrupole breakup; ÿ = full width at half maximum.
ÿ (mm/s)
SbV
ÿ (mm/s)
+8.479 (7)
+8.552 (9)
+8.505 (9)
SðS þ 1Þ p than the coupling between the spins (M ¼ 2 S =
1/2). Several hypotheses could be put forward:
These results show that, for compounds x = 0
and 0.15, these values are higher than the theoretical value of
the Cu2+ ion (M = 1.73 ÿB), if we do not take into account ,
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
[Sb1,524Cu0,476]O7+ÿ.
4
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vary significantly depending on the temperature
to think that the oxygen level is reduced when a trivalent
ion (Bi3+) is substituted by a divalent ion (Ca2+).
We mentioned this above.
(T) of our compounds (inset in Fig. 4) seems
the four points gives a resistivity on the order of
temperature and it can reach a high value (2,
25 ÿB at 300 °K) [11]. The slight curvature observed
copper by the usual assay methods, such as
to agree with this consideration. However,
This could potentially lead to a reduction in
even though in these phases, 50% of the copper ions
adapted to the size of this ion. With this symmetry,
had this coordination, it couldn't do
Electrical measurements taken on all the
low (1.49 ÿB). However, we were unable to verify these
reductions in oxygen levels and the charge of the
• either in the cubic site, the presence of the Cu2+ ion
at low temperature (< 300 °K) on the curves
samples at room temperature by the method
4. Conclusion
Furthermore, the observation of a significant decrease
the effective magnetic moment of the Cu2+ ion can
the charge of copper, especially for the compound
x = 0.30, whose MCu has a particularly
increase the value of the total effective moment,
observed for both types of sites
being much smaller than the Bi3+ ion leads to a
rearrangement of the copper's neighbors and consequently,
of MCu in the calcium-substituted phases leaves
can lead to a more tetrahedral coordination
copper in the structure, up to 2.38 ÿB.
105 ÿ cm.
Part of the copper would be transferred to site A. The occupation of site B
In the solid solution (Bi1.524–xPbxCu0.476)-
to think that the oxygen level is less affected by lead substitution
than by calcium substitution. This effect is
[12] and Eu2B2O7 (B = Ti, Ru and Pb) [13,14] and is found
give the example of Pb2B2O7–x (x ÿ 1, B = Ru, Re and Ir)
formula A2PbO4 (A = Ca [15], Sr [16] and Cd [17]). By
less pronounced than with the calcium substitution described
above. Fig. 5 shows the shape of the inverse curve
in both sites of the compound according to the formula
of molar magnetic susceptibility as a function of the
temperature for x = 0 and x = 0.26; the value of MCu decreases
from 2.38 to 2.11 ÿB, respectively. This leaves us
probably due to the oxidation of some of the Pb2+ in
also in lead compounds in coordination VI, of
Elsewhere, lead can also occupy site A: one can
[Sb1,524Cu0,476]O7+ÿ, the variation of the effective moment of
[18]. This leads us to believe that the lead would be distributed
lead has already been observed in some compounds
(Bi3+ 1.524–xPb2+x–yCu2+0.476+y)(Sb5+1.524Cu2+0.476–yPb4+y)
of the pyrochlore type, with the formula Ln2Pb2O7 (Ln = La–Gd)
copper with the substitution of bismuth by lead is
O2-
Pb4+ (RPbVI = 0.775 Å) occupying site B, while a
7+ÿ.
Table 4
1213
0.15
2.06
+0.181
Fig. 4. Inverse of molar magnetic susceptibility as a function of
Fraction x 0.30
1.492.38
+0.329
Calculated value of ÿ
M. Sellami et al. / CR Chimie 9 (2006) 1209–1214
-0.1818
0
the temperature of the solid solution (Bi1.524–xCaxCu0.476)-
M
ÿ1
[Sb1,524Cu0,476]O7+ÿ.
Effective moment MCu (ÿÿ)
solid (Bi1,524–xCaxCu0,476)[Sb1,524Cu0,476]O7+ÿ with
In this work, two solid solutions of the type
of the cubic lattice parameter a and the effective moment of
copper as a function of the substitution fraction x.
In contrast, for the solid solution (Bi1.524–xPbxCu0.476)
0 ÿ x ÿ 0.30 is characterized by a regular decrease
[Sb1,524Cu0,476]O7+ÿ with 0 ÿ x ÿ 0.26, the evolution
of the lattice parameter is not regular; it is correlated with
pyrochlore was synthesized. The solution
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