Crystal Structure Theory and Applications, 2012, 1, 68-73
http://dx.doi.org/10.4236/csta.2012.13013 Published Online December 2012 (http://www.SciRP.org/journal/csta)
Synthesis, Crystal Structure and Electrical Properties of
a New Mixed Compound (Na0.71Ag0.29)2CoP2O7
Riadh Marzouki1, Abderrahmen Guesmi1,2, Mohamed Faouzi Zid1*, Ahmed Driss1
1Département de Chimie, Faculté des Sciences, Université Tunis El Manar, Tunis, Tunisie
2Institut Préparatoire aux Etudes d’Ingénieur, Université Tunis El Manar, Tunis, Tunisie
Received October 9, 2012; revised November 15, 2012; accepted November 24, 2012
ABSTRACT
A new cobalt diphosphate (Na0.71Ag0.29)2CoP2O7, is synthesized by solid state reaction method and characterized by
single-crystal X-ray diffraction. The title material crystallizes in the triclinic space group P-1 with a = 6.4170(3) Å, b =
9.4510(2) Å, c = 10.9350(3) Å,  = 115.240(2)˚,  = 80.190(3)˚ and  = 106.810(2)˚. The structure presents a
centro-symmetrical clusters Co4P4O28 consists of two Co2O11 units and two P2O7 pyrophosphate groups. The junction
between clusters is assured by two P2O7 groups to form a three-dimensional anionic framework having different inter-
connecting tunnels running along [100] and [010]. The former contains the Na+ and Ag+ cations. The conductivity
measurements of (Na0.71Ag0.29)2CoP2O7 are studied over a temperature interval from 783 to 903 K using the frequency
response analyzer with 0.5 V amplitude signal over the range of 13 MHz - 5 Hz.
Keywords: Diphosphate; X-Ray Diffraction; Anionic Framework; Tunnels; Conductivity
1. Introduction
Phosphate materials have vast applications in several
domains as electric, pyroelectric, ferroelectric, magnetic,
catalytic processes, state laser materials, etc. [1-7]. Dur-
ing the last years, there have been many studies of the
compounds with general formula A2BP2O7 (A = mono-
valent cation, B = divalent ion) concerned the structural
features and the electrical properties [8-12]. With regard
to cobalt phases members, electrical studies were per-
formed only for the tetragonal formula Na2CoP2O7 (bidi-
mensional) [8]. It shows that the latter is an ionic con-
ductor material. We have now prepared a new mixed di-
phosphate of the triclinic form. The synthesis, the struc-
tural study and the electrical properties of
(Na0.71Ag0.29)2CoP2O7 (tridimensional) material are dis-
cussed here.
2. Experimental
2.1. Synthesis of the Title Compound
A mixture of high-purity reagents (NaNO3/AgN O3,
Co(NO3)2·6H2O and NH4H2PO4) as polycrystalline form,
with a Na:Ag:Co:P molar ratio of 1:1:1:2, is dissolved in
deionised water to give a pink solution. After evaporation
to dryness at 70˚C in the oven, the residue, placed in
porcelain crucible, is slowly annealed in air to 400˚C for
24 h, in order to eliminate volatile products. In a second
step, it was progressively heated at 620˚C for 5 days. The
sample was slowly cooled at 5˚C/24 h to 580˚C and fi-
nally quenched to room temperature. Purple single crys-
tals of the title compound are extracted from the flux
matrix with boiling water. A qualitative EDX (energy-
dispersive X-ray spectroscopy) analysis (model: Philips
XL 30) detected the presence of Na, Ag, Co, P and oxy-
gen elements. A polycrystalline powder of (Na0.71Ag0.29)2
CoP2O7 was obtained by treating a stoichiometric mix-
ture of the above reagents. The powder X-ray diffraction
pattern was in agreement with single-crystal structure.
2.2. Materials and Physical Measurements
Impedance spectroscopy measurements were carried out
in a Hewlet-Packar 4192-A automatic bridge monitored
by a HP microcomputer. Impedance spectra were re-
corded in the 13 MHz - 5 Hz frequency range with 0.5 V
alternative signal. Pellet was prepared by uniaxial shap-
ing followed by isostatic pressing at 2.5 kbar and sinter-
ing at 540˚C for 2 h in air with 5 K·min–1 heating and
cooling rates. The thickness and surface of pellet were
about 0.356 cm and 0.454 cm2 having a geometric factor
of e/S = 0.78 cm–1. Platinum electrodes were painted in
the two faces of the pellet with a platinum paste to ensure
good electric contacts and then painted pellet was carried
out at steady-state temperatures in still air.
*Corresponding author.
C
opyright © 2012 SciRes. CSTA

R. MARZOUKI ET AL. 69
2.3. Crystal Structure Determination
A suitable single crystal with dimensions 0.24 × 0.21 ×
0.16 mm3 was chosen for the structure determination.
The data were collected on an Enraf-Nonius CAD-4 dif-
fractometer using the MoK

(λ = 0.71069 Å) radiation at
room temperature. The structure was determinate by di-
rect methods using SHELXS-97 program [13]. In the clo-
sest solution proposed by program, only some atoms of
cobalt and phosphor were located. Using SHELXL-97
program [14], refinements followed by Fourier differ-
ences are necessary to find the positions of others atoms
remaining in the lattice to an R factor of 2.55% for all re-
flections. The structure graphics were drawn with dia-
mond 2.1 supplied by Crystal Impact [15]. A summary of
crystallographic data, recording conditions and structure
refinement results of the title compound is given in Table 1.
The atomic coordinates and isotropic thermal factors
are presented in Table 2. Table 3 contains the main in-
teratomic distances in coordination polyhedra of the stu-
died structure.
3. Results and Discussion
The title compound is a new member of isostructural
phases family including Na7Mg4,5 (P2O7)4 2) [16],
Na2CoP2O7 3) [17], Na3.12 Fe2.44 (P2O7)2 4) [18], Na3.64
Mg2.18(P2O7)2 5) and Na3.64Ni2.18(P2O7) 6) [19]. This fam-
ily of phases crystallizes in a centrosymmetric lattice, in
Table 1. Crystal data refinement results of
(Na0.71Ag0.29)2CoP2O7 compound.
Crystal data
Crystal shape: Prism Color: Purple
Crystal system: Triclinic Space group: P-1
Cell parameters:
a = 6.417(3) Å α = 115.24(2)˚
b = 9.451(2) Å β = 80.19(3)˚
c = 10.935(3) Å γ = 106.81(2)˚
V = 573.4(3) Å3
Z = 2
 = 3.801 g·cm–3 T = 298 K
Data collection
4980 measured reflections 2166 reflections with I > 2σ(I)
2491 independent reflections Rint = 0.02
H = −8→8 Tmin = 0.554; Tmax = 0.402
K = −12→12 θmax = 26.97˚; θmin = 2.06˚
L = −13→13 Decay = 1%
Refinement
R [I > 2σ(I)] = 0.0255 wR2 (F2) = 0.0618
S = 1.07 Extinction coefficient: 0.0031 (6)
Δρmax = 0.71 e·Å−3 Δρmin = −0.56 e·Å−3
258 parameters 2491 reflections
Table 2. Atomic coordinates and isotropic thermal factors
of (Na0.71Ag0.29)2CoP2O7.
Atomsx y z Uiso* Occupancy
Co1 0.3545(7)0.26562(5)0.76417(4) 0.00967(2)1
Co2 0.2783(7)0.61021(5)0.71736(4) 0.01031(2)1
P1 0.4257(3)0.65654(1)0.95528(8) 0.00855(8)1
P2 0.2858(4)0.27129(1)0.46202(8) 0.00982(8)1
P3 0.1219(3)0.63778(1)0.18316(9) 0.00952(8)1
P4 −0.0770(4)0.10713(1)0.28674(9) 0.01362(9)1
O1 0.6880(4)0.2606(3)0.7245(2) 0.0182(5) 1
O2 0.2116(5)0.4255(3)0.5341(3) 0.0215(6) 1
O3 0.4444(4)0.5208(3)0.8156(2) 0.0112(5) 1
O4 0.0304(4)0.2645(3)0.8112(3) 0.0206(6) 1
O5 0.2278(4)0.5753(3)0.0322(2) 0.0140(5) 1
O6 −0.0161(4)0.5167(3)0.7972(3) 0.0195(6) 1
O7 0.9557(5)−0.0518(4)0.8485(3) 0.0369(8) 1
O8 0.5668(4)0.7289(3)0.6619(3) 0.0221(6) 1
O9 0.2841(4)0.0145(3)0.6998(3) 0.0153(5) 1
O10 0.3788(4)0.2781(3)0.9609(2) 0.0162(5) 1
O11 0.1141(5)0.7259(3)0.6656(4) 0.0310(7) 1
O12 0.9249(4)0.8744(3)0.5989(3) 0.0145(5) 1
O13 0.3769(4)0.2275(3)0.5562(2) 0.0170(5) 1
O14 0.3543(4)0.7872(3)0.9368(3) 0.0164(5) 1
Na1 0.024(2)0.2050(4)0.0244(2) 0.019(3) 0.899(2)
Ag1 0.027(4)0.212(3)0.016(2) 0.017(3) 0.101(2)
Na2 0.592(3)0.0065(6)0.8113(5) 0.0229(7) 0.340(2)
Ag2 0.5754(4) −0.0075(2) 0.8338(3) 0.0229(7) 0.503(9)
Ag3 0.6006(2) −0.0203(7) 0.8560(7) 0.0229(7) 0.157(8)
Ag4 0.7170(2)0.0533(1)0.54694(9) 0.0492(5) 0.313(2)
Na3 0.7170(2)0.0533(1)0.54694(9) 0.0492(5) 0.424(7)
Na4 1/2 0 1/2 0.043(2) 0.526(2)
Na5 0.2060(2)0.5653(9)0.4081(7) 0.0278(1) 0.912(2)
Ag5 0.2430(3)0.5330(2)0.3776(7) 0.0390(3) 0.088(2)
*Uéq = (1/3) ∑i∑j Uijai*aj*ai·aj.
Table 3. Main interatomic distances (Å) in (Na0.71Ag0.29)2
CoP2O7 compound.
Octahedron Co(1)O6 Octahedron Co(2)O6
Co1—O4 2.053 (3) Co2—O11 2.006 (3)
Co1—O9 2.096 (2) Co2—O8 2.012 (3)
Co1—O1 2.120 (3) Co2—O2 2.031 (3)
Co1—O13 2.132 (3) Co2—O6 2.060 (3)
Co1—O10 2.134 (3) Co2—O3 2.165 (2)
Co1—O3 2.149 (2) Co2—O14 2.302 (3)
Tetrahedron P(1)O4 Tetrahedron P(2)O4
P1—O10i 1.502 (3) P2—O2 1.505 (3)
P1—O14 1.532 (2) P2—O13 1.510 (2)
P1—O3 1.532 (2) P2—O8iii 1.514 (3)
P1—O5ii 1.592 (3) P2—O12iii 1.617 (3)
Tetrahedron P(3)O4 Tetrahedron P(4)O4
P3—O4iv 1.506 (2) P4—O7v 1.500 (3)
P3—O6iv 1.511 (3) P4—O11iv 1.514 (3)
P3—O1iii 1.531 (3) P4—O9vi 1.517 (3)
P3—O5 1.598 (3) P4—O12iii 1.637 (3)
Symmetry codes: (i): −x + 1, −y + 1, −z + 2; (ii): x, y, z + 1; (iii): −x + 1, −y +
1, −z + 1; (iv): −x, −y + 1, −z + 1; (v): −x + 1, −y, −z + 1; (vi): −x, −y, −z + 1.

R. MARZOUKI ET AL.
70
the space group P-1. The 3 structure is to claim to be not
centrosymmetric (P1 GE). For Na2CoP2O7 material, we
note the stoichiometry of the chemical composition. The
report of the formula as determined from 3) Na/Co = 2,
which agrees with the results found in the structural
study of the title compound (Na, Ag)/Co = 2.
The asymmetric unit in (Na0.71Ag0.29)2CoP2O7 com-
pound is shown in Figure 1. The structure is composed
of two octahedrons sharing corner and forming Co2O11
group. The latter is linked on one side by edge with P2O7
group. On the other side, a second diphosphate is con-
nected sharing corners with Co1O6 and Co2O6. The
compensation of charge in the asymmetric unit is ensured
by Ag+ and Na+ cations.
In the anionic framework, the Co2O11 octahedral
groups are arranged in the (1 –1 0) plane (Figure 2). The
connection between two Co2O11 units and two diphos-
phate groups is assured by mixed bridges Co1–O–P and
sharing edges with Co1O6 octahedra, to form the
Co4P4O28 cluster (Figure 3).
Along the three directions of the cell, the junction be-
tween these clusters is provided by two diphosphates
groups sharing a corner with the CoO6 octahedra thus
forming a three-dimensional framework (Figure 4). How-
ever, the structure belongs to the dichromate family [20]
which the conformation of this group is eclipsed.
The three-dimensional network shows the existence of
two types of tunnels along [100] with hexagonal and de-
cagonal sections (Figure 5(a)). A projection of the ani-
Na/Ag
Na4
O1
Na2
Ag 3
Ag 2
O8i ii
O1 3
O9
P2
O7v
O8
Ag 1
Na1
Co1
O3
O12iii
Ag 5
O10i
O1 0
O2
Na5
P4
P1
Co2
O11iv
O9vi
O1 4
O4
O5ii
O1 1
O1i ii
O6 P3
O6iv
O4i v
a
b
c
Figure 1. Asymmetric unit of (Na0.71Ag0.29)2CoP2O7 com-
pound.
Figure 2. Octahedral representation of the structure show-
ing the arrangement of Co2O11 groups in the (1 –1 0) plane.
onic framework in the b direction is given in Figure
5(b).
Figure 3. Projection of Co4P4O28 cluster of (Na0.71Ag0.29)2
CoP2O7 viewed near the [100] direction.
Figure 4. Junction between clusters ensured by the diphos-
phate groups in bc plane of (Na0.71Ag0.29)2CoP2O7 com-
pound.
(a)
(b)
Figure 5(a). Projection of (Na0.71Ag0.29)2CoP2O7 structure
along [100] direction showing tunnels where monovalent
cations are located; (b): Projection of (Na0.71Ag0.29)2CoP2O7
structure along [010] direction showing the windows and
the channels.

R. MARZOUKI ET AL. 71
It shows the presence of channels and quadrilateral
windows along this direction. The monovalent cations
are located in these tunnels.
The electrical properties of the title compound are in-
vestigated using complex impedance spectroscopy (CIS).
The electrical data exploitation was realized in the ther-
mal range 783 - 903 K. The Nyquist plots at different
temperature for (Na0.71Ag0.29)2CoP2O7 material are shown
in Figure 6. We have used the Zview software [21] to fit
these curves. The bulk ohmic resistance relative to each
experimental temperature is deduced from complex im-
pedance diagrams. It is the intercept Z0 on the real axis
of the zero phase angle extrapolation of the highest fre-
quency curve. The resistivity parameters R for this com-
pound vary with temperature according to Arrhenius-
type laws. The (Na0.71Ag0.29)2CoP2O7 impedance dia-
grams show only one typical semicircle arc with a spike
at lower frequencies. The best fit is obtained when we
used an equivalent circuit composed of a resistor, R con-
nected in parallel with a constant phase element, CPE
(Figure 7) [22]. No additional blocking effect could be
evidenced at lower frequencies (f ≤ 20 Hz). Values of
electric parameters calculated for (Na0.71Ag0.29)2CoP2O7
compound, at different temperatures, after fitting are il-
lustrated in Table 4.
A linear plots of log (
 T(S.K·cm–1)) vs. 103/T (K–1) is
represented in Figure 8. The conductivity value
 at 683
K is 2.61  10–7 S·cm–1 and the activation energy de-
duced from the slope is Ea = 1.368 eV (Figure 8). Com-
pared to the activation energies observed in Na2CoP2O7
material (Ea = 0.63 eV) [8], in NaAgZnP2O7 (Ea = 0.76
eV) [23], in Na2PbP2O7 (Ea = 0.90 eV) [24] and
853
K
883 K
893K
903 K
90000
80000
70000
60000
50000
40000
30000
20000
10000
0 0 50000 100000 150000
Re (Z) (Ω.cm)
-lm (Z) (Ω.cm)
Figure 6. Impedance spectra recorded on (Na0.71Ag0.29)2
CoP2O7 sample over the temperature ranges 853 - 903 K.
R1
0 20000 40000 60000 80000 100000 120000
Re (Z) (Ω.cm)
-lm (Z) (Ω.cm)
70000
60000
50000
40000
30000
20000
10000
0
CPE1
Figure 7. Impedance spectra recorded on (Na0.71Ag0.29)2
Ag2PbP2O7 (Ea = 0.78 eV) [25], (Na0.71Ag0.29)2CoP2O
CoP2O7 sample over the temperature ranges 853 - 903 K.
7
a0.71Ag0.29)2CoP2O7
co
is work we have synthesized a new
tion. This mate-
able 4. Electrical values of the equivalent circuit parame-
T (K) R (×105 Ω.cm) C (×10–12 F) P  (×105 S·cm–1)
exhibit a low electric conductivity.
Data from the structural study of (N
mpound show that the monovalent cations are located
in 3 types of tunnels whose section dimensions are illus-
trated in Figure 9. In the [100] direction, the Na+ and
(Na/Ag)+ ions are located in a tunnel of hexagonal sec-
tion with maximum section equal to 4.671 (6) Å (Figure
9(a)). The other tunnel contains the Na+ and Ag+ ions.
The smaller sections of these tunnels in this direction are
3.699 (3) Å and 3.460 (5) Å which are inferior to 2 (ro2–
+ rNa+) = 5.18 Å and 2 (ro2– + rAg+) = 5.40 Å according to
Shannon [26]. Furthermore, in the b direction, the tunnel
of hexagonal section (Figure 9(b)) contains a bottleneck
of small width equal to 2.917 (3) Ǻ. It is smaller also
than twice the sum of ray ro2– = 1.42 Å and rNa+ = 1.18 Å
(5.18 Ǻ) for Na+ and 2 (ro2– + rAg+) = 5.40 Å for Ag+ ac-
cording Shannon [26]. These geometric factors are caus-
ing a low mobility of the cations (Ea >1 eV).
4. Conclusion
In the summary, in th
diphosphate compound of composition
(Na0.71Ag0.29)2CoP2O7 by solid state reac
rial was characterized by X-ray diffraction. The sample cry-
stallized in triclinic symmetry with P-1 space group (Z = 2)
T
ters calculated for (Na0.71Ag0.29)2CoP2O7 sample at different
temperatures.
783 6.51 8.0 0.9 0.12
813 3.09 8.7 0.9 0.25
853 1.08 10.9 0.9 0.73
883 0.77 11.8 0.9 1.02
893 0.59 8.4 0.9 1.33
903 0.50 9.9 0.9 1.57
Figure 8. Conductivity Arrhenius plots of (Na0.71Ag 29)2
0.
CoP2O7 sample.

R. MARZOUKI ET AL.
72
(a) (b)
Figure 9. Dimen
d the unit cell parameters are a = 6.4170(3) Å,
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