Synthesis, characterization and electrical properties of

a lead sodium vanadate apatite

E. Chakroun-Ouadhour, R. Ternane *, D. Ben Hassen-Chehimi, M. Trabelsi-Ayadi

Laboratoire d’Application de la Chimie aux Ressources et Substances Naturelles et a l’Environnement,

Faculte des Sciences de Bizerte, 7021 Zarzouna Bizerte, Tunisia

Received 23 June 2007; received in revised form 10 July 2007; accepted 20 July 2007

Available online 27 July 2007

Abstract

The lacunary lead sodium vanadate apatite Pb8Na2(VO4)6 was synthesized by the solid-state reaction method. The compound

was characterized by X-ray powder diffraction, infrared (IR) absorption spectroscopy and Raman scattering spectroscopy.

By comparing the effect of vanadate and phosphate ions on electrical properties, it was concluded that Pb8Na2(VO4)6 apatite is

better conductor than Pb8Na2(PO4)6 apatite.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds; B. Chemical synthesis; C. Impedance spectroscopy; C. Infrared spectroscopy; C. Raman spectroscopy; D.

Electrical properties; D. Ionic conductivity

1. Introduction

Apatites with general formula M10(XO4)6Y2 where (M = Ca, Sr, Pb . . ., X = P, V, Si . . . and Y = OH, F, Cl, O . . .)crystallize in the hexagonal system with space group P63/m [1] and are prone to extensive ionic substitution [2].

Lead in apatite is of interest from two points of view. First, lead in known as a ‘‘bone seeker’’ in that it accumulates

in bones and teeth, second, it may contribute to deviation from the general formula of apatites. Nevertheless, the only

system, where compounds with the apatite structure could be prepared without anion Y, is the lead system [3]. The

apatites having vacancies in the Y anion sites with general formula Pb8A2(PO4)6 (A: monovalent cation) have been

studied by several authors (A = Na [4–8], Ag [6,7,9], K [4,10]).

Because of the mobility of M and Y ions in such compounds, investigations on electrical measurements were

performed. Several authors have established a correlation between structural properties and ionic conductivity [11–19].

Comparison of electrical properties of fluoroapatites and hydroxyapatites, shows that the electrical behaviour of

lead apatites differs from other apatites. Moreover, fluoroapatites are found to be better ionic conductors than the

hydroxyapatites [16].

In the case of apatites without anion Y, investigations on the electrical properties have been carried out by several

authors [13,19,20].

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Materials Research Bulletin 43 (2008) 2451–2456

* Corresponding author at: LACReSNE-Departement de Chimie, Faculte des Sciences de Bizerte, 7021 Zarzouna Bizerte, Tunisia.

Tel.: +216 98 90 19 16; fax: +216 72 59 05 66.

E-mail address: rternane@yahoo.fr (R. Ternane).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2007.07.030

A lead sodium apatite with vanadium Pb8Na2(VO4)6 was previously synthesized as single crystal and its structure

was solved [21], there is no study reported on the electrical properties.

The present study reports the synthesis, and for the first time the characterization by vibrational spectroscopy and

the ionic conductivity of Pb8Na2(VO4)6.

2. Experimental

The lead sodium apatite was synthesized using stoechiometric amounts of PbCO3, Na2CO3 and V2O5 according to

the following reaction:

8PbCO3þNa2CO3þ 3V2O5 ! Pb8Na2ðVO4Þ6þ 9CO2

The mixture was heated up to 673 K for 1 day and 1023 K for 10 days.

The X-ray diffraction measurements were carried out by a BRUKER D8-advance diffractometer using a

monochromatized Cu Ka radiation (l = 1.5406 A).

The unit cell parameters were determined from XRD data by a least-square refinement.

The infrared spectra were recorded on pellet using 1 mg of apatite powder in 300 mg of powdered spectroscopic

grade KBr, with the FTIR Perkin-Elmer 1000 Spectrometer in the range of 4000–350 cm�1.

The Raman spectra were recorded using a DILOR XY spectrometer equipped with a CCD detector and a spectra

physics Ar laser (exitation at 514.5 nm).

The electrical conductivity was studied using impedance spectroscopy and measurements were performed on an

Hewlett-Packard 4192 A impedance analyzer, the signal frequency range is from 5 Hz to 13 MHz. The sample was

pressed as a pellet and sintered at 1023 K. Electrical measurements were done in the range 298–800 K while the pellet

was kept at steady temperature.

3. Results and discussion

3.1. Characterization

According to the X-ray powder diffraction patterns (Fig. 1), the lead sodium apatite Pb8Na2(VO4)6 posses

hexagonal structure with the P63/m space group. The refined unit cell parameters are a = 10.059(3) A and

c = 7.434(2) A.

E. Chakroun-Ouadhour et al. / Materials Research Bulletin 43 (2008) 2451–24562452

Fig. 1. X-ray diffraction patterns of Pb8Na2(VO4)6.

A comparison of these values with those of Pb8Na2(PO4)6 (a = 9.724(3) A and c = 7.194(6) A) [19] indicates an

increase of the unit cell parameters as is to be expected by the substitution of PO43� ions by VO4

3� ions of bigger size.

In the M10(XO4)6Y2 apatitic lattice, the cations are located on two non-equivalent sites. The site M(I), 4(f) position,

with C3 symmetry is coordinated by nine oxygen, whereas the site M(II), 6(h) position, is seven-fold coordinated (6

oxygen and 1 Y) with Cs symmetry. The triangles delimited by M(II) are equilateral and rotate by 608 from each other

about the c axis at levels z = 1/4 and 3/4, respectively.

Studies on apatites with lack of Y anion, like Pb8A2(XO4)6 (X = P [8,10]; V [21]; A: monovalent cation) have

shown that, in the majority of the cases, the A alkaline ions are mainly localized in the column positions (4f) sites,

while the triangular (6h) sites are occupied by lead cations. The stabilization of the apatite-type structure seems to be

controlled by the presence of a maximum electron density within the tunnels. These negative charges could be brought

either by distinct anions Y� (OH�, F�, Cl�, etc.) or by electron lone pairs (LPs) held by Pb2+ cations or other ions such

as Bi3+ [22]. So, such active electron LPs should be considered as a constituent of the coordination spheres of M(II)

cations, as reported by various authors in different crystalline compounds containing Pb2+, Bi3+ or Tl+ [23,24]. Under

E. Chakroun-Ouadhour et al. / Materials Research Bulletin 43 (2008) 2451–2456 2453

Fig. 2. (a) Infrared spectrum of Pb8Na2(VO4)6 and (b) Raman spectrum of Pb8Na2(VO4)6.

these circumstances, the coordination number of M(II) becomes equal to seven as in the M10(XO4)6Y2 apatite

compounds.

The infrared spectrum of Pb8Na2(VO4)6 is shown in Fig. 2a and the band assignment is listed in Table 1. The two IR

lines observed at 761 and 856 cm�1, correspond to symmetric stretching ns(VO4) and asymmetric stretching nas(VO4)

vibration modes of the VO4 groups, respectively.

The Raman spectrum of this compound is given in Fig. 2b and the band assignments is given in Table 1. The bands

observed at 831 cm�1 and at 731 cm�1 are corresponding to asymmetric stretching nas(VO4) and symmetric stretching

ns(VO4) vibration modes of the VO4 groups, respectively. Three bands are observed in the range 419–362 cm�1

corresponding to asymmetric deformation das(VO4) and the band observed at 314 cm�1 is attributed to the symmetric

deformation ds(VO4) vibration mode of the VO4 groups.

No vibration characteristic of OH groups is observed in 3570 cm�1 range. This confirms that the synthesized apatite

does not contain hydroxyl ions.

3.2. Electrical properties

The measurements were performed in the temperature range 623–729 K. Below 623 K, the conductivity appears

too low to be measured.

The complex impedance diagrams are presented in Fig. 3. The resistance was calculated from the intercept of the

circle with the real axis (Table 2). The temperature dependence of the electrical conductivity follows an Arrhenius-

type law at temperatures between 623 and 729 K (Fig. 4).

Because of the high temperature used to understand the conduction process, the conductivity can be attributed to the

creation of intrinsic defects such as Frenkel or substitution defects.

As demonstrated by Den Hartog in fluorapatite, the conductivity is dominated by cationic mobility and presents an

anisotropy [25].

In the Pb8K2�xNax(PO4)6 system, Laghzizil et al. showed that the disorder defect increases the conductivity and

that only monovalent cations contribute to the conductivity phenomenon which is related to the variation of the apatite

tunnels size [13].

In our study, the activation energy found (Ea = 0.59 eV) is slightly lower than that found in Pb8Na2(PO4)6

(Ea = 0.77 eV) [19].

E. Chakroun-Ouadhour et al. / Materials Research Bulletin 43 (2008) 2451–24562454

Table 1

IR and Raman band assignment for Pb8Na2(VO4)6 (cm�1)

nas(VO4) ns(VO4) das(VO4) ds(VO4)

IR Raman IR Raman IR Raman IR Raman

Pb8Na2(VO4)6 856 831 761 731 – 419, 379, 362 – 314

Fig. 3. Complex impedance diagrams of Pb8Na2(VO4)6 at different temperatures.

It can be deduced that Pb8Na2(VO4)6 with lower activation energy is better conductor than Pb8Na2(PO4)6. This can

be explained by the substitution of phosphate ions by vanadate ones. In fact, P5+ ion is smaller than V5+ with ionic

radius of 0.170 and 0.355 A [26], respectively, and the substitution of PO43� ions by VO4

3� ions leads to the expansion

of the apatite tunnels, which favours the diffusion of mobile ions in the apatitic lattice.

4. Conclusion

This paper presents a study of synthesis, characterization and ionic conductivity of Pb8Na2(VO4)6 apatite. X-ray

diffraction study shows that Pb8Na2(VO4)6 crystallizes in the hexagonal system with P63/m as space group. Infrared

and Raman spectra are reported and band assignments are made. The vanadate apatite Pb8Na2(VO4)6 is found to be

better conductor than the phosphate apatite Pb8Na2(PO4)6.

Acknowledgements

The authors thank Professor H. Boussetta and Dr. A. Madani (Laboratoire de Physique des Materiaux, Faculte des

Sciences de Bizerte) for the conductivity measurements and R. Ternane is grateful to Professors G. Panczer and B.

Champagnon, Director of CECOMO (Centre Commun de Microscopie Optique, Universite Claude-Bernard Lyon 1,

France), for their assistance with the Raman spectra.

E. Chakroun-Ouadhour et al. / Materials Research Bulletin 43 (2008) 2451–2456 2455

Table 2

Resistance and conductivity of Pb8Na2(VO4)6 at different temperatures

Temperature T (K) Resistance R (V) Conductivity s (10�6 S cm�1)

623 86211 3.747

638 82118 3.865

649 66350 5.672

660 56529 5.991

670 51862 6.713

679 44106 8.219

689 39706 8.316

699 33312 9.615

709 28812 11.513

719 23645 13.193

729 20460 15.828

Fig. 4. Arrhenius plot ln(sT) = f(1000/T) of ionic conductivity for Pb8Na2(XO4)6 (X = P, V).

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