Structural, thermal behavior, dielectric and vibrational studies of the new compound, sodium hydrogen arsenate tellurate (Na2H4As2O5.H2TeO4)

Journal of Physics and Chemistry of Solids 75 (2014) 911–920

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Review

Structural, thermal behavior, dielectric and vibrational studies of the new compound, sodium hydrogen arsenate tellurate (Na2H4As2O5.H2TeO4) I. Bechibani a, H. Litaiem a,n, L. Ktari a, N. Zouari b, S. Garcia-Granda b, M. Dammak a a b

Laboratoire de chimie inorganique (LCI), Université de Sfax, Faculté des Sciences de Sfax, BP 1171 Sfax 3000, Tunisia Laboratoire de Chimie Physique et Analytique, Faculté de Chimie, Université d’Oviedo, Oviedo 33006, Spain

art ic l e i nf o Article history: Received 29 December 2013 Received in revised form 11 February 2014 Accepted 17 February 2014 Available online 5 March 2014 Keywords: C. IR C. Raman C. DSC D. Phase transition D. Dielectric properties

Abstract: The structure of Na2H4As2O5.H2TeO4 (NaAsTe) crystallizes in the tetragonal system I4̄ . The unit , Z¼2, and V¼ 241.8(2) 3̊. The main feature of the crystal cell parameters are a ¼b ¼5.576(2) ̊, c¼ 7.773(5) ̊ structure is the coexistence of two independent and different anions, As2O45  and TeO44  , in the unit cell, connected by strong (O–H…O) hydrogen bonds, which make the packing of the crystal. The polyhedra in the structure arranged allows formation of tunnels where hydrogen atoms are placed. The structural cohesion for this material is assured both by the interaction between Na þ and the anionic oxygen atoms and by the presence of strong hydrogen bonds. The NaAsTe material undergoes three endothermic peaks at 417, 421, and 450 K. These transitions, detected by differential scanning calorimetry (DSC) and confirmed by Thermogravimetry, differential thermal analysis (DTA–TG) analyses, are also substantiated by dielectric and conductivity measurements using the impedance spectroscopy techniques. In fact, the peak at 417 K is attributed to ferroelectric– paraelectric phase transition while the second one, at 421 K, is assigned to ionic–protonic conduction. The third anomaly at 450 K can be attributed to the decomposition of the material confirmed by TG analysis. The infrared (IR) and Raman spectra recorded at room temperatures, in the frequency ranges 4000– 400 cm  1 and 200–1500 cm  1, respectively, bear out the presence of anionic groups in the crystal and the presence of strong hydrogen bonds. & 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Microstructural studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Structural analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Calorimetric study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Dielectric studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Electrical proprieties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Vibrational study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1. Interpretation of Raman spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2. Interpretation of IR spectrum: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

n

Corresponding author. Tel.: þ 216 22 371 409; fax: þ 216. 74 274 437. E-mail address: [email protected] (H. Litaiem).

http://dx.doi.org/10.1016/j.jpcs.2014.02.007 0022-3697/& 2014 Elsevier Ltd. All rights reserved.

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1. Introduction

2. Experimental phase

Telluric acid (Te(OH)6 ) has the property of forming addition compounds of considerable importance because Te(OH)6 acts as both acceptor and donor of hydrogen bonds. In fact, the coexistence of different anions in the same unit cell connected by hydrogen bonds and the structural arrangement of all the polyhedra are at the origin of the important physical properties. Among compounds, there are some alkaline sulfate–selenate tellurate compounds, which exhibit a superionic–protonic conduction phase transition at high temperature and are ferroelectric in their low-temperature phases [1–8]. In fact, the family of sulfate tellurate with formula M2SO4.Te(OH)6, where M¼K, Rb, and Cs, present an order–disorder phase transition at 480, 440, and 460 K, respectively. They show a ferroelectric–paraelectric phase transition at 490, 520, and 510 K, respectively [1]. Furthermore, the Na2SeO4  Te(OH)6  H2O (NaSeTe) material shows three-phase transitions observed in the DSC curve at 390, 420, and 430 K. Thus, NaSeTe material presents, on the one hand, a ferroelectric–paraelectric phase transition at 420 K, detected by DSC and DTA and confirmed by the evolution of both the dielectric constant and the dissipation factor versus temperature. On the other hand, we notice a strong jump in the conductivity plot, detected at 430 K, characterizing a superprotonic conduction phase transition. This can be attributed to the H þ mobility due to the breaking of hydrogen bonds. A relaxation study shows that the H þ transfer is probably provided by a hopping mechanism [9]. Hydrogen-bonded crystals have attracted a lot of attention due to their possible use as superionic conductors in technological applications. In order to improve and discover the effect of the hydrogen atom on various properties, we carried out the anionic substitution of SeO4 and SO4 groups by HAsO4 group. In this study, we have paid a great deal of attention to the sodium hydrogen arsenate tellurate as the object of our investigation. The aim of the present work is to study the structural, vibrational, and the dielectric behaviors of Na2H4As2O5.H2TeO4 (NaAsTe) compound. We report here the results obtained by X-ray diffraction, differential thermogravimetric analysis (TG), thermodifference analysis (DTA), differential scanning calorimetry (DSC), dielectric, Raman and infrared (IR) studies.

Crystals of NaAsTe are prepared at room temperature by slow evaporation of an aqueous solution of telluric acid Te(OH)6, sodium carbonate (Na2CO3), and arsenic acid (H3AsO4) with proper molar ratios. The formula for this material was determined by chemical analysis. The structure was solved by using the Crystals program and the graphics were created by the Diamond program [10,11]. Simultaneous TG and DTA analyses were performed using Mettler Toledo model TGA851eLF and Setaram model Setsys Evolution 16 thermobalances. Samples were placed inside uncovered alumina crucibles. The masses of samples used in TG and DTA measurements were 5.334 mg. DSC measurements were carried out by means of a Mettler Toledo DSC model DSC821. Samples in both thermal analyses were heated from 300 to 650 K at a heating rate of 5 K/min. Infrared absorption spectra of suspension of crystalline in KBr have been recorded using JascoFT-IR-420 spectrophotometer in the 4000–400 cm  1 frequency

Table 1 Main crystallographic feature, X-ray diffraction data parameter results of Na2H4As2 O5.H2TeO4 material.

Table 2 Fractional atomic coordinates and equivalent isotropic displacement (Uiso for H atoms) for Na2H4As2O5.H2TeO4 compound.. Atoms

x

y

Z

Uiso/equiv

Te O H Na As O1 O2 O3 H1 H2

0 0.003(2) 0 0.5 0.193(7) 0.154(1) 0.283(2) 0  0.296(3)  0.436(4)

0.5 0.342(2) 0.5 0 0.283(8) 0.212(2) 0.340(2) 0.5 0.197(1) 0.257(2)

0.25 0.032(9) 0.007(2) 0.25 0.021(3) 0.165(1) 0.162(1) 0.092(2) 0.189(5) 0.068(7)

0.0245(4) 0.06(8) 0.0384(7) 0.0245(6) 0.0676(2) 0.0574(8) 0.0676(1) 0.0452(3) 0.0940(5) 0.0100(5)

Table 3 Anisotropic displacement parameters of Na2H4As2O5.H2TeO4. Atoms U11

U22

U33

U23

U13

U12

Te Na As O1 O2 O3 O

0.025(2) 0.033(8) 0.034(1) 0.050(3) 0.063(4) 0.068(1) 0.058(3)

0.0221 0.026(12) 0.041(2) 0.064(4) 0.067(5) 0.043(8) 0.036(3)

0.0004(4) 0  0.0032(7)  0.012(3) 0.007(3) 0  0.008(2)

0.0005(5) 0 0.0021(6) 0.009(3)  0.009(4) 0.0000  0.001(3)

0.004(2) 0 0.0023(2)  0.011(3)  0.0014(4) 0.006(1)  0.002(3)

0.025(2) 0.033(8) 0.068(9) 0.058(3) 0.072(5) 0.063(1) 0.086(5)

Crystal data Formula Crystal system Space group a ¼ b (Å) c (Å) a ¼ β¼ γ V (Å3) Z Formula weight (g mol-1) μ (mm  1) dcal Max crystal dimension (mm3) Experimental details Diffractometer Max Brag angle (1) h ¼ k ¼  8 8, l- ¼  11 7 Parameters refined Observed reflexions F0 44s(F0) Reflexions measured R1 ¼[F2 4 4s(F)2] WR2 (F2)

Na2 H4As2O5.H2TeO4 Tetragonal I4̄ 5.576(2) 7.773(5) 901 241.8(2) 2 415.6 33.684 5.72 0.01  0.02  0.07 Agilent diffraction Xcalibur Gemini RCCD 33.27

O

Na

As

Te

Te Te Te

46 464 1433 0.02 0.06 Fig. 1. The EDS spectrum of the Na2H4As2O5.H2TeO4 at room temperature.

I. Bechibani et al. / Journal of Physics and Chemistry of Solids 75 (2014) 911–920

range. Raman spectra of polycrystalline samples sealed in glass tubes have been recorded on a Labram HR 800 instrument using 632.81 nm radiations from an argon ion laser. The complex permittivity was determined from measurements of electric capacitance in the frequency range 1 kHz–2.9 MHz using HewlettPackard 4192 A LF automatic bridge monitored by an HP Vectra microcomputer. The details of data collection and refinement for the title compound are summarized in Table 1. The final positions and equivalent isotropic thermal parameters for the new compound are shown in Tables 2 and 3.

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3. Results and discussion: 3.1. Microstructural studies The chemical purity of the product was tested by energy dispersive analysis of X-ray (EDAX) measurements (Fig. 1). The EDAX spectrum of NaAsTe reveals the presence of all nonhydrogen atoms: Te, As, Na, and O. Elemental analysis gives these results: for observed atoms, we have Te: 40.01%; As: 16.67%; Na: 11.19%; and O: 23.21%, whereas for the calculated rate, we find Te: 41.81%; As: 17.42%; Na: 11.9%; and O: 24.25%. The scanning electron microscopy (SEM) image is shown in Fig. 2; the crystals are of a parallelepiped shape.

3.2. Structural analysis

Fig. 2. The SEM fracture micrograph of the Na2H4As2O5.H2TeO4.

The X-ray single-crystal analysis of NaAsTe shows that this compound crystallizes in tetragonal system I4̄ with unit cell parameters a¼ b¼5.576(2) ̊ , c ¼7.773(5) ̊ , and V¼241.8(2) Å3. Fig. 3 shows that the structure can be regarded as being built up by As2O45  and TeO44  groups, alternating with Na þ cations, connected by strong O–H…O hydrogen bonds. In the new structure, NaAsTe, the Te atom occupies one special position. Here, it has a tetrahedral coordination formed by oxygen atoms (Fig. 4a). The tetrahedra of TeO4 is formed by four Te–O bonds length, equal to 1.898(7) ̊and O–Te–O angle values ranging from 124.95(5)1 to 141.86(3)1. Fig. 4b shows As2O5 groups. In fact, we observe that As–O distances vary between 1.614(2) and 1.710 (2) ̊ , while the O–As–O angles range between 126.61(3)1 and 130.42(2)1.

Fig. 3. Projection of the Na2H4As2O5.H2TeO4 structure in the (a,c ) plane.

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Fig. 4. (a) The environment around Te atom. (b) H4As2O5 group. (c) Sodium coordination.

The Na atom in the new structure occupies one special position. It is coordinated by 12 oxygen atoms (Fig. 4c), eight of them belonging to As2O5 groups, and the others belong to TeO4 tetrahedra. The Na–O distances range from 2.348(8) to 2.364(7) ̊ , while in the Na2SeO4.Te(OH)6.H2O material, the sodium atom is coordinated by six oxygen atoms, with Na–O distances varying from 2.344(7) to 2.717(7) ̊[9]. The bond lengths, the bond angles, and the sodium coordination of NaAsTe are listed in Table 4. The structure is stable, thanks to strong hydrogen bonds assured with protons belonging to H4As2O5 linked to TeO4 groups. As a consequence, four oxygen atoms from tetrahedral group (TeO4) are linked to both hydroxides O1–H1 and O2–H2. In fact, the O…O bonds vary from 2.334(7) to 2.453(2) ̊and the O–H…O angles range from 116.66(6)1 to 129.43(7)1. The O–H distances range from 0.819(3) to 0.828(5) ̊ . These hydrogen bonds are strong according to the Brown theory [12]. A complete report on the results is given in Table 5. From Fig. 5, we note that the arrangement of the various polyhedra on the (a,b) plane forms tunnels where all the hydrogen atoms are located. These atoms can move through these tunnels resulting in protonic conduction. 3.3. Calorimetric study The Na2H4As2O5.H2TeO4 material presents three phase transitions. In fact, a typical result of the calorimetric study of the NaAsTe compound is presented in Fig. 6. The DSC thermogram reveals three anomalies: an intense peak at T1 ¼ 417 K, a shoulder at T2 ¼ 422 K, and an anomaly at T3 ¼ 450 K. The calculated transition enthalpies, for the first transition at T1 ¼ 417 K and for the third anomaly at T3 ¼ 450 K are ΔH1 ¼ 919.145 Jg  1 and ΔH3 ¼ 230.708 Jg  1, respectively. According to the superposition of TG–DTA curve shown in Fig. 7, we can conclude that the DTA presents two anomalies at 420 K and 448 K. The second anomaly is accompanied by the mass loss observed by TG curve. Accordingly, we can deduce that the decomposition of this material begins at about 445 K.

Table 4 Main interatomic distances (Å) and bond angles (1) in the Na2H4As2O5.H2TeO4 material. Tellurate groups Te–O (Å) Te–O Te–O Te–O Te–O O–Te–O (Å) O–Te–O O–Te–O O–Te–O O–Te–O Arsenic groups As–O (Å) As–O1 As–O1 As–O2 As–O2 As–O3 As–O3 O–As–O (Å) O1–As–O3 O1–As–O3 O3–As–O2 O3–As–O2 Distances Na–O (Å) Na–O1 (  4) Na–O2 (  4) Na–O (  4)

1.898(7) 1.898(7) 1.898(7) 1.898(7) 124.95(5) 124.95(5) 141.86(3) 141.86(3)

1.614(2) 1.614(2) 1.669(3) 1.669(3) 1.710(2) 1.710(2) 130.41(2) 130.41(2) 126.62(3) 126.62(3) 2.354(7) 2.348(8) 2.364(7)

3.4. Dielectric studies The presence of different anions in the structure and the appearance of ferroelectricity in a good number of sulfates,selenates, and tellurates [1–8] render advisable the dielectric examination of the sodium hydrogen arsenate tellurate. The cohesion of

I. Bechibani et al. / Journal of Physics and Chemistry of Solids 75 (2014) 911–920

the NaAsTe structure is assured by both the interaction between Na þ and anionic oxygen atoms and the presence of protons H þ in H4As2O5 and H2TeO4 groups. This favors ionic–protonic conductivity in this material at high temperature. Thus, this property leads us to make electric conductivity measurements of the new compound, NaAsTe. In order to characterize the phase transition at about T¼ 417 K, observed in DSC curve, we have studied the temperature dependence of the permittivity (ε'r) in the 350–500 K temperature range. Fig. 8 shows the evolution of (ε'r) versus temperature at different frequencies for (NaAsTe) material. Two anomalies are observed in this temperature range. The first one detected at T1 ¼417 K can be attributed, by comparison with studied sulfate, selenate, and phosphate tellurate compounds, to a ferroelectric– paraelectric phase transition [1–7]. This hypothesis can be confirmed by the evolution of the loss factor. The second anomaly observed at T2 ¼422 K can be attributed to ionic–protonic phase transition [13–18]. Fig. 8 shows that the value of ε0 r varies significantly with frequency, which characterizes a large dispersion in this frequency range. This behavior confirms the important conductivity in this material. The decrease of ε0 r with increasing frequency is linked to the large conductivity in this material due to charge carriers and to the fast mobility of the proton H þ [19,20]. In fact, when we have a material which presents long-range ion Table 5 Geometrical characterization of hydrogen bonds in the Na2H4As2O5.H2TeO4 compound. O…O (Å) O1…O¼2.334(7) O2…O¼2.453(2)

O-H (Å) O1-H1 ¼0.819(3) O2-H1 ¼0.828(5)

O…H (Å) O…H1 ¼ 1.703 O…H2 ¼ 1.823

O-H…O(Å) O1-H1…O¼ 116.66(6) O2-H2…O¼ 129.43(7)

diffusion, the real part of the permittivity the summation of two contributions:

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ε0 r can be considered as

ε0r ¼ ε0r ðlatt:Þ þ ε0r ðcarr:Þ; where εr0 (latt.) presents the lattice response due to permanent dipole orientations or other motions which do not involve longrange displacement of mobile charge carriers. The second contribution, εr0 (carr.), which presents the conductivity or carrier response associated with long migration, strongly depends on frequency and especially at low frequencies [1,21]. Fig. 9 shows the evolution of the dissipation factor tan δ as a function of temperature. The obtained values of tan δ are relatively higher in agreement with the large contribution of conductivity in this material. In fact, tan δ increases from low temperature, then decreases and presents a minimum in the vicinity of T ¼417 K. The behaviors of the dielectric permittivity εr0 and tan δ are in agreement with ferroelectric–paraelectric phase transition in NaAsTe compound at T¼ 417 K. The NaAsTe structure is built up by As2O45  groups and TeO44  tetrahedra with Na þ cations. This framework can explain that the presence of the ferroelectric–paraelectric phase transition at 417 K is favored by the coexistence of different types of ions. 3.5. Electrical proprieties In order to verify our previous hypothesis about the presence of the ionic–protonic conduction phase transition in this material, we have carried out complex impedance measurements (Z* ¼Z0  iZ″) in the 300–600 K temperature range. Polycrystalline pellets, 13 mm in diameter and 1.05 mm in thickness, were obtained at room temperature by pressing under 4 MP. The pellets were sintered for 4 h in vacuum in order to eliminate the water content in the sample and obtain dense pellets. The complex impedance

Fig. 5. Representation of tunnels in the Na2H4As2O5.H2TeO4 structure.

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I. Bechibani et al. / Journal of Physics and Chemistry of Solids 75 (2014) 911–920

235 61 61KHz Hz Série1

215

450 K

Exo

195

71 Série2 71KHz Hz 81 Série3 81KHz Hz 91 KHz 91 Hz Série4

175

422 K

375

155

400

425

450

475

500

εr′

T (K)

417 K

525

135

Fig. 6. DSC heating curve of Na2H4As2O5.H2TeO4 compound.

115

TG DTA

95

Exo

75

T (K) 55 406

411

416

421

426

Fig. 8. Temperature dependence of ε'r as a function of frequency for Na2H4As2O5. H2TeO4 material.

T(K) 350

400

450

500

550

Fig. 7. TG–DTA heating curve of Na2H4As2O5.H2TeO4 material.

Série1 61Hz 61KHz 61Hz Série1

35

71Hz 71KHz Série2 71Hz 25

81KHz Série3 81Hz 81Hz Série3 91KHz 91Hz Série4

tan δ

plane data show semicircles. Hence, the pellet can be regarded as a parallel combination of resistance and capacitance in the electrical circuit. Some complex impedance diagrams, -Z"(Ω) versus Z'(Ω) Cole–Cole plots, recorded at various temperatures, are given in Fig. 10a. These diagrams show that NaAsTe follows the Cole–Cole law [22]. The difference between the Cole–Cole and the Debye laws is determined by α(π/2) dispersion angle when α ¼0.21. The intercept on the real axis of the zero phase angle extrapolation of the highest-frequency curve determines the bulk ohmic resistance relative to the experimental temperature, which can be used to show the evolution of the conductivity versus the inverse temperature [23]. The curves reflect the temperature dependence of the resistance proving ionic conduction properties. Fig. 10b shows the frequency dependence of the real part of the impedance of NaAsTe at various temperatures. Actually, it is obvious that the magnitude of Z' decreases with the increase in both frequency and temperature, indicating an increase in the conductivity of the material. Moreover, Fig. 10c demonstrates the variation of the imaginary part of impedance with frequency at different temperatures. The magnitude of the imaginary component of the impedance at peak frequency can be considered as a strongly decreasing function of the temperature. Furthermore, the observed peak in these plots corresponds to a relaxation process, and its position corresponds to the condition ωτ ¼1. The thermal evolution of conductivity in Na2H4As2O5.H2TeO4 material is presented in Fig. 11. Arrhenius-type behavior sT¼ s0 exp(-Ea/KT) is observed in three regions of the curve showing the transition of conduction phase transition in this material [8,24].

91Hz Série4

15

5

-5 T (K) T (K) -15 390

410

430

Fig. 9. Thermal evolution of the dissipation factor as function of frequency for Na2H4As2O5.H2TeO4 material.

I. Bechibani et al. / Journal of Physics and Chemistry of Solids 75 (2014) 911–920

100000

383 K 403 K 413 K 418 K 423 K

60000

Z'(Ω)

-Z"(Ω)

80000

2.0x10

5

1.5x10

5

1.0x10

5

917

383 K 403 K 413 K 418 K 423 K 430 K

40000 4

5.0x10 20000

0.0 0 20000

40000

60000 Z'(Ω)

z"(Ω)

0

80000

0

100000

10

101

102

10

3

f(kHz)

7x10

4

6x10

4

5x10

4

4x10

4

3x10

4

2x10

4

383 K 403 K 413 K 418 K 423 K 430 K

1x104 0 0

10

1

2

10

10

3

10

f(kHz) Fig. 10. (a) Complex impedance diagrams – Z'' versus Z' of Na2H4As2O5.H2TeO4 over the temperature range 383–423 K. (b) Frequency dependence of the real parts of impedance at various temperatures. (c) Frequency dependence of the imaginary parts of impedance at various temperatures.

logσT(KΩ−1m-1)

437 K

403 K

1000/T(K-1) Fig. 11. Temperature dependence of log(sT) for Na2H4As2O5.H2TeO4 compound.

Indeed, there is a sharp rise in the conductivity in the second region, whose temperature is between 403 and 437 K. On the one hand, there is an increase in the activation energy from ΔE1 ¼1.02 eV at low temperature to ΔE2 ¼ 1.28 eV at high temperature. On the other hand, the conductivity increases from sT1 ¼7.38 10  5 Ω  1 cm  1 at T1 ¼ 370 K to sT2 ¼ 1.4 10  3 Ω  1 cm  1 at T2 ¼ 437 K. In spite of all the results indicated above, the high-temperature phase transition at about 421 K in the Na2H4As2O5.H2TeO4 compound can be interpreted as an ionic– protonic one, observed previously at the same temperature by DSC and DTA curve. In fact, the proton H þ placed in the tunnels and the Na þ cations are responsible for this conduction. This transition can be due to the breaking of the hydrogen bonds (O…H–O) in this

new compound, which link all the polyhedra and the mobility of the sodium cations. As a result, the proton moves between the potential wells associated with ionic and cationic groups. The difference between ΔE1 and ΔE2 is due to the difficulty of moving the proton in the high-temperature phase caused by the cell deformation introduced by the establishment of the polar phase due to the presence of the ferroelectric–paraelectric phase transition at 417 K. In order to throw some additional lights on the role of the ions in the second transition, dielectric relaxation studies have been consequently undertaken at high temperature, between 429 and 488 K, in the complex modulus M* formalism. For a given temperature and frequency, both the real part M' and the imaginary part M" of the M* complex modulus (M* ¼M' þjM") were calculated from the complex impedance data (Z* ¼ Z'-jZ") using the relations M' ¼ ωC0Z" and M" ¼ ωC0Z'. Fig. 12 shows the variation of real part of electrical modulus as a function of frequency at different temperatures for NaAsTe. It can be seen from the figure that the value of M' is very low (approaching zero) in the low-frequency region for all the temperatures. This indicates that the electrode polarization phenomenon makes negligible contribution to M* and may be ignored when the electric data are analyzed in this form [25,26]. This observation may be related to a lack of restoring forces governing the mobility of charge carriers under the action of an induced electric field. This type of behavior supports the long-range mobility of charge carriers. As the frequency increases, the value of M* increases and ultimately approaches a value of M'α ¼ 1/ ε'α for all temperatures. This supports the conduction phenomena due to short-range mobility of ion carriers [27,28]. The increasing trend in the plot at higher frequencies may be attributed to the bulk effect of the material. The height of the peak decreases with

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0.014

0.010

M'

0.008

Relative intensity

441 K 443 K 445 K 453 K 455 K 463 K

0.012

0.006 0.004 0.002 0.000 -0.002 1

10

100

200

f(kHz) Fig. 12. Frequency dependence of real part of electric modulus for (NaAsTe) at different temperatures.

0.010

0.008 0.007

M"

0.006

600

800

1000

1200

1400

Wave number (cm-1) Fig. 14. Raman spectra at room temperature of Na2H4As2O5.H2TeO4 compound.

Relative intensity

441 K 443 K 445 K 453 K 455 K 463 K

0.009

400

0.005 0.004 0.003 0.002 0.001 0.000 -0.001 1

10

100

f(kHz) Fig. 13. Frequency dependence of imaginary part of electric modulus for (NaAsTe) at different temperatures.

4000

3500

3000

2500

2000

1500

1000

500

-1

Wave number (cm ) Fig. 15. IR spectra at room temperature of Na2H4As2O5.H2TeO4 compound.

the rise in temperature, suggesting a plurality relaxation mechanism [29]. Fig. 13 shows the variation of M" as a function of frequency at different temperatures for NaAsTe. It is evident from the figure at lower frequencies that M" exhibits a low value, whereas at higher frequencies well-defined peaks are observed. The heights of the peaks are found to decrease with increasing temperature. The broad nature of the peaks can be interpreted as being the consequence of distribution of relaxation time. Moreover, such peaks are broader than the Debye peak, which is usually attributed to the represented ionic conductor. The peak frequency of M" shifts toward the high-frequency region as the temperature increases, which implies the conductivity relaxation suggesting that the dielectric relaxation is activated thermally, in which a hopping process of charge carriers is predominant. By comparison with other alkaline sulfate, selenate tellurate material studied, we can confirm that the protonic conduction in the NaAsTe material can be due to a hopping mechanism. As a consequence, the H þ ions in the NaAsTe material move from one potential minimum to the other and the basic transport mechanism is thermally activated, hopping across an energy barrier. 3.6. Vibrational study In the present investigation, IR and Raman spectroscopic studies of Na2H4As2O5.H2TeO4 have been performed and analyzed in order to confirm the presence and the independence of the two

anions (As2O45  and TeO44  ). These two analyses give more importance to strong hydrogen bonds in the new crystal lattice. Raman and IR spectra of Na2H4As2O5.H2TeO4 at room temperature are shown in Figs. 14 and 15, respectively. The observed Raman and IR bands are given in Table 6.

3.6.1. Interpretation of Raman spectrum According to the literature of the addition compounds based on tellurate, arsenate, and materials which are stable, thanks to the hydrogen bonds, we could interpret the different peaks observed in Fig. 14. In fact, the two bands at 244 and 295 cm  1 correspond to the lattice mode and the ν(O–H…O) [30]. While the peaks at 372 and 415 cm  1 are probably attributed to the mode δ(AsO2) [31,32]. The vibration modes of As–O–As are detected at 623, 650, and 664 cm  1 [33–35]. In addition, the band observed at 718 cm  1 can be attributed to stretching vibration mode of AsO2 [36]. The δ(As–O…H) bending vibration mode is assigned to the peaks at 1237 and 1327 cm  1 [37,38]. The metatellurate anion (TeO24  ) should have Td symmetry and, therefore, four internal modes, namely A1 (ν1), E (ν2) and 2F2 (ν3 and ν4). The flattened tetrahedrons form a square outline and help produce (in many of these minerals) tetragonal (fourfold) C4v symmetry, which is an uncommon symmetry in minerals. In fact, the peak which appears at 327 cm  1 can be attributed to vibration ν4(TeO4) [33,39], while the frequencies at 423 and 429 cm  1 are

I. Bechibani et al. / Journal of Physics and Chemistry of Solids 75 (2014) 911–920

Table 6 Infrared and Raman frequencies for Na2H4As2O5.H2TeO4 material. IR (cm  1)

Raman (cm  1)

– 244 295

)

9 > 321 = 32 7 > 37 2 ;

_

– 427

415 423 429 –

560 9 620 > = > ; 667

623 650

)

Assignment

Lattice mode and ν(O…H-O)

ν4(TeO4) and ν(AsO2)

δ(AsO2) ν2(TeO4) δ(As2O)

9 > = > ;

ν(As–O–As) and ν3(TeO4)

664

9 720 > = 750 > 790 ;

718

δ(AsO2) and ν1(TeO4)

860

835

δ(As2O) and ν1(TeO4)

9 1236 =

9 1237 = ; 1327

; 1440 9 1592 > > > > > 1669 > > = 1811 > > > > > > > ; 2285 3578 3649

–

919

4. Conclusion The sodium hydrogen arsenate tellurate crystallizes in the I4̄ space group. The structure can be regarded as an arrangement of H4As2O5 and H2TeO4 groups alternating with Na þ cations. The cohesion of this structure is assured by both the interaction between Na þ and the anionic oxygen atoms and by the presence of strong hydrogen bonds. The arrangement of different anionic polyhedra in this structure allows formation of tunnels where some hydrogen atoms are placed. The most interesting feature in the Na2H4As2O5.H2TeO4 material is the presence of three- phase transitions detected by the DSC curve at 417, 422, and 450 K and confirmed by DTA-TG analysis. In fact, the title compound presents a ferroelectric–paraelectric phase transition at 417 K, confirmed by the evolution of both the dielectric constant and the dissipation factor versus temperature. On the other hand, the strong jump in the conductivity plot detected at 422 K, confirms the presence of a protonic–ionic conduction phase transition at this temperature. The decomposition of this material starts at about T ¼445 K, as indicated in ATDTG themograms.

Acknowledgment

δ(As–OH)

This work is supported by the Minister of Higher Education and Research of Tunisia. All the authors express their thanks to Professor H. Khemakhem for his help in the Raman spectroscopy measurements. As–OH free and the strong hydrogen bond.

References

) ν(OH)

associated with the vibration ν2(TeO4) [40,41]. In addition, the peaks appearing at 623, 650, and 664 cm  1 are related to the vibration ν3(TeO4) [33,35,42]. while the vibration ν1(TeO4) appears around 835 cm  1 [36]. We can remark that the peak intensity ν1(TeO4) is lower than the peak intensity ν3(TeO4). This is in accordance with compounds already studied in the literature, like Bi5Fe3O9 (TeO3) (TeO4)2.H2O and Cu3TeO4 (OH)4 [42,43].

3.6.2. Interpretation of IR spectrum: In order to confirm the results shown by the Raman spectra, and to gain more information on the groups present in this new structure, we have undertaken the IR study at room temperature in the 4000–400 cm  1 frequency range. The IR spectrum of this compound shows only the internal vibration. In fact, the shoulder at 560 cm  1 corresponds to the mode δ(AsO2) [44]. But the shoulder at 620 cm  1 and the band detected at 667 cm  1 are related to the As–O–As vibration [33,34]. The peak around 860 cm  1 can be assigned to the mode δ(As2O) [36]. The δ(As– O…H) bending vibration mode is assigned to the peaks at 1236 and 1440 cm  1 [37,38]. The band which appears at 427 cm  1 is associated with the vibration ν2(TeO4). But the shoulder at 620 cm  1 and the band detected at 667 cm  1 are related to the vibration ν3(TeO4) [33,42]. The peak at 750 cm  1 and the band around 860 cm  1 can be attributed to the vibration ν1(TeO4) [40,41]. While the bands which appear at 1592 and 1661, 1881, 2285 cm-1 can be attributed to the presence of As-O…H free and the presence of strong hydrogen bonds, respectively. The two bands that appear at 3578 and 3649 cm  1 may be associated with vibration ν(OH) [36].

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