Journal of Alloys and Compounds 486 (2009) 299–303
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Ac electrical properties and dielectric relaxation of the new mixed crystal
(Na
0.8
Ag
0.2
)
2
PbP
2
O
7
B. Louati
∗
, F. Hlel, K. Guidara
Laboratoire de l’état solide, Faculté des Sciences de Sfax, B. P. 802, 3018 Sfax, Tunisia
article info
Article history:
Received 23 December 2008
Received in revised form 18 June 2009
Accepted 19 June 2009
Available online 2 July 2009
Keywords:
(Na0.8Ag0.2)2PbP2O7
X-ray diffraction
ac conductivity
Dielectric measurements
abstract
The new diphosphate compound containing sodium, silver and lead, (Na
0.8
Ag
0.2
)
2
PbP
2
O
7
, has been syn-
thesized and characterized by X-ray diffraction (XRD) and electrical technique. (Na
0.8
Ag
0.2
)
2
PbP
2
O
7,
is of
triclinic symmetry with the space group P
¯
1
. The unit cell parameters are: a = 5.511(3) Å, b = 6.910(4) Å,
c = 9.457(6) Å, ˛ = 105.72(4)
◦
, ˇ = 96.81(5)
◦
, = 108.40(4)
◦
and V = 320.6(4) Å
3
. The ac conductivity and
dielectric properties of (Na
0.8
Ag
0.2
)
2
PbP
2
O
7
compound have been studied at temperatures in the range
573–692 K and frequencies (300 Hz to 5 MHz). The ac conductivity data reveal that for high frequency
ac
(ω) is proportional to ω
n
. The values of the frequency exponent, n, were found to increase by increasing
temperature.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Inorganic phosphates cover a large class of diverse materials
whose applications include: catalysts, solid electrolytes for batter-
ies [1–3], linear and non-linear optical components [4,5] and laser
materials [6,7]. However, the study of phosphates has become more
popular particularly, after the development of NASICON groups of
fast ionic conductors [8–10].
Double phosphates of A
2
BP
2
O
7
(A = alkaline ion and B = divalent
cation) form a large family of materials [11–13]. The structures
of these phosphates are diverse, and it is difficult to classify
these materials as a function of classical fundamental criteria
such as cation size, coordination number or chemical bonding.
The Na
2
PbP
2
O
7
crystal is one of the inorganic phosphates. The
double phosphates Na
2
PbP
2
O
7
have been prepared by melting
using phosphate flux. Single crystals obtained are transparent
and of parallelepipedic form. Na
2
PbP
2
O
7
is of triclinic symmetry
with the space group P
¯
1
(Z = 2) and the unit cell parame-
ters are a = 5.533(4)
´
˚
A, b = 6.942(4)
´
˚
A, c = 9.440(4)
´
˚
A, ˛ = 105.58(2)
◦
,
ˇ = 97.03(4)
◦
, = 108.32(4)
◦
and V = 323.4(3)Å
3
. The structure of
Na
2
PbP
2
O
7
consists of lamina parallel to (0 0 1) plan, constituting
the tridimensional framework. An investigation of ionic conductiv-
ity properties revealed that the number of charge carriers is very
small and the conduction in Na
2
PbP
2
O
7
is low. A comparative study
of electrical properties of ceramic and glassy Na
2
PbP
2
O
7
has shown
∗
Corresponding author.
E-mail address: bassem louati@yahoo.fr (B. Louati).
that no conductivity notable variation is observed as a function of
the crystalline or glassy state of Na
2
PbP
2
O
7
[14].
Single crystals of Ag
2
PbP
2
O
7
are isotype to Na
2
PbP
2
O
7
. The unit
cell parameters are: a = 5.502(6)
´
˚
A, b = 7.008(8)
´
˚
A, c = 10.018(9)
´
˚
A,
˛ = 106.63(6)
◦
, ˇ = 93.89(7)
◦
, = 110.68(6)
◦
and V = 340.8(6) Å
3
.
Ionic conductivity measurements of the crystalline Ag
2
PbP
2
O
7
phase were carried out on powder samples. The Ag
2
PbP
2
O
7
is char-
acterized by a smaller E
activation energy, a smaller Kohlrausch
exponent, ˇ, and a slightly higher value of the glass decoupling
index, log R
(T
g
), resulting in a higher conductivity for the Ag-
material than for the Na-material. The better properties obtained
for Ag
2
PbP
2
O
7
can be attributed to the higher polarizability of Ag
+
ions, more easily deformed (d
10
configuration) to pass through the
bottlenecks and consequently more mobile than the Na
+
ions (rare
gas type configuration) [15].
The aim of this study is to determine the influence of sil-
ver content on the electrical conductivity of the Na
2
PbP
2
O
7
compound. The study focuses on the new mixed compound of
composition (Na
0.8
Ag
0.2
)
2
PbP
2
O
7
. The ac conductivity,
ac
, of the
(Na
0.8
Ag
0.2
)
2
PbP
2
O
7
sample was measured as a function of fre-
quency and temperature with a view to study some electrical
features.
2. Chemical preparation
The (Na
0.8
Ag
0.2
)
2
PbP
2
O
7
compound was synthesized by the
classic ceramic method. The preparation technique which involves
as a first step the mixing powders in an appropriate ratios and
careful grinding using a mortar and pestle. The powders are pres-
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.06.148