Synthesis, Crystal Structure and Electrical Properties of a New Mixed Compound (Na0.71Ag0.29)2CoP2O7

doi:10.4236/csta.2012.13013

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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

Email: *[email protected]

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.

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.

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

Na2

Ag 3

Ag 2

O8i ii

O1 3

O7v

Ag 1

Na1

Co1

O12iii

Ag 5

O10i

O1 0

Na5

Co2

O11iv

O9vi

O1 4

O5ii

O1 1

O1i ii

O6 P3

O6iv

O4i v

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

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.

0 20000 40000 60000 80000 100000 120000

Re (Z) (Ω.cm)

-lm (Z) (Ω.cm)

70000

60000

50000

40000

30000

20000

10000

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.

a0.71Ag0.29)2CoP2O7

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)

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

CoP2O7 sample.

R. MARZOUKI ET AL.

(a) (b)

Figure 9. Dimen

d the unit cell parameters are a = 6.4170(3) Å,

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

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