Structural, dielectric and vibrational studies in the dipotassium sulfate selenate tellurate mixed solid solution

Journal of Alloys and Compounds 428 (2007) 8–16

Structural, dielectric and vibrational studies in the dipotassium sulfate selenate tellurate mixed solid solution M. Dammak ∗ , A. Hadrich, T. Mhiri Laboratoire de L’Etat Solide, Facult´e des Sciences de Sfax, BP 802, 3018 Sfax, Tunisia Received 8 November 2005; received in revised form 15 March 2006; accepted 16 March 2006 Available online 27 April 2006

Abstract Synthesis, crystal structure, DSC characterization, dielectric and Raman measurements are given for a new mixed solution ¯ with the folK2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 (KSSeTe). X-ray studies showed that the title compound crystallizes in the triclinic system (P 1) ˚ 3 and ˚ b = 6.5680(3) A, ˚ c = 13.1550(6) A, ˚ α = 102.30(2)◦ , β = 90.07(2)◦ , γ = 116.98(2)◦ , Z = 2, V = 474.60(4) A lowing parameters: a = 6.2560(3) A, −3 + ρcal = 2.86 g cm . The structure can be regarded as being built of isolated TeO6 octahedra, SO4 and SeO4 tetrahedra and K cations. The main feature of this structure is the coexistence of three different anions (TeO6 6− , SO4 2− and SeO4 2− groups) in the unit cell, connected by O H· · ·O hydrogen bonds. Crystals of K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 undergo three endothermic peaks at 425 K, 465 K and 480 K. These transitions detected by DSC and analyzed by dielectric measurements using the impedance and modulus spectroscopy techniques. IR and Raman scattering measurements on K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 material taken between 300 K and 620 K are reported in this paper. The spectra indicate clearly these phase transitions. © 2006 Elsevier B.V. All rights reserved. Keywords: Structure; Conductivity; DSC; Raman; Sulphate; Selenate tellurate

1. Introduction The compounds of general formula M2 XO4 ·Te(OH)6 (M = Na+ , K+ , NH4 + , Rb+ , Cs+ and X = S, Se, P) undergo structural phase transitions and interesting physical properties such as superprotonic conduction and ferroelectricity [1–4]. The structures of some sulfate and selenate tellurate have been investigated using X-ray diffraction method. In fact, the potassium selenate tellurate K2 SeO4 ·Te(OH)6 (KSeTe) crystallizes in the monoclinic system Cc [1]. The dielectric studies show that the material exhibit a ferroelectric–paraelectric phase transition at 433 K and a superionic protonic conduction one at 480 K [2]. On the other hand, the potassium sulfate tellurate K2 SO4 ·Te(OH)6 (KSTe) crystallizes in the centrosymmetric triclinic space group and presents a ferroelectric–paraelectric phase transition at 460 K and a ionic protonic conduction phase at 490 K [3,4].

∗

Corresponding author. Tel.: +216 98 63 62 72; fax: +216 74 27 44 37. E-mail address: [email protected] (M. Dammak).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.045

In the present work, the X-ray, calorimetric, dielectric and vibrational studies of mixed crystal K2 (SO4 )0.9 (SeO4 )0.1 · Te(OH)6 (KSSeTe) were carried out in order to determine the effect of the partial substitution of sulfate by selenate anion. 2. Experimental Transparent, colorless single crystals of the title composition were grown from aqueous solution of a mixture of H6 TeO6 , K2 SeO4 and K2 SO4 at room temperature. H6 TeO6 + (1 − x)K2 SeO4 + xK2 SO4 → K2 (SeO4 )1−x (SO4 )x ·Te(OH)6 The formula is determined by refinement of the crystal structure and confirmed by chemical analysis. The crystal used for the structure determination was a small almost parallelipipedic (0.22 mm × 0.3 mm × 0.4 mm). Reflections (2152) were recorded, at room temperature, on a Enraf Nonius Kappa CCD diffractometer using the Mo K␣ radiation [5]. Equivalent reflections averaged as I/σ(I) > 4 to give 2057 unique reflections (Rint = 0.032). The structural determination shows that ¯ The average density values, measured at room the proper space group is P 1. temperature, with CCl4 as pycnometric liquid, are in agreement with the calculated density. Formula unit in the cell of crystal is deduced from these values.

M. Dammak et al. / Journal of Alloys and Compounds 428 (2007) 8–16 Table 1 Main Crystallographic data for K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 Formula Formula weight (g mol−1 ) Crystal system ˚ a (A) ˚ b (A) ˚ c (A) α (◦ ) β (◦ ) γ (◦ ) ˚ 3) V (A Z Space group θ min /θ max (◦ ) T (K) Diffractometer ˚ λ (Mo K␣) (A) −8 ≤ h ≤ 7 −8 ≤ k ≤ 8 −16 ≤ l ≤ 17 ρcal (g cm−3 ) μ (cm−1 ) Total reflections Reflection with I > 4σ(I) ˚ −3 ) Min, max, ρ (e A R (F)a (%) wR (F)a (%) W = 1/[σ 2 (F0 )2 + (0.029 × P)2 + 0.34 × P] with P = (F02 + 2Fc2 )/3 R values are defined as WR2 = (   R1 = ||Fo | − |Fc ||/ |Fo |. a

K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 408.6 Triclinic 6.2560(3) 6.6580(3) 13.1550(6) 102.30(2) 90.070(2) 116.984(2) 474.60(4) 2 P 1¯ 3.1/27.65 293(2) Enraf-Nonius Kappa CCD 0.71073

2.86 81.04 2152 2057 −1.11; 0.85 2.4 6.2



Table 2 Fractional atomic coordinates and temperature factors for K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 Atom

X

Y

Z

˚ 2) Ueq (A

Te1 Te2 S/Se K1 K2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 H1 H2 H3 H4 H5 H6

0 0.5 0.75128(9) 0.57840(11) 0.1932(1) 0.2906(3) −0.0596(3) 0.1769(4) 0.3949(3) 0.2324(3) 0.3088(3) 0.8953(3) 0.9146(4) 0.6352(4) 0.5595(4) 0.3809(73) −0.0691(73) 0.1387(74) 0.3060(81) 0.3105(75) 0.1870(52)

0 0 0.4649(1) 0.2333(1) 0.2585(1) 0.2812(3) −0.0369(3) −0.1708(3) −0.3129(3) 0.0045(3) −0.1108(3) 0.4711(3) 0.5491(4) 0.6148(4) 0.2267(3) 0.2636(67) −0.1593(73) −0.2669(71) −0.3412(75) 0.1375(42) −0.2455(38)

0 0.5 0.24901(4) −0.14885(5) 0.35057(5) 0.0546(2) 0.1402(1) −0.0065(2) 0.5110(1) 0.5677(2) 0.3668(1) 0.3407(1) 0.1692(2) 0.2813(2) 0.2056(2) 0.1029(33) 0.1441(30) −0.0624(31) 0.5549(32) 0.6157(26) 0.3727(29)

0.01073(9) 0.01036(9) 0.0115(2) 0.0222(1) 0.0244(1) 0.0243(4) 0.0201(4) 0.0208(4) 0.0182(4) 0.0216(4) 0.0194(4) 0.0216(4) 0.0259(4) 0.0260(4) 0.0264(4) 0.040(11) 0.044(12) 0.041(11) 0.048(12) 0.063(14) 0.039(11)

Ueq =

1 3

 i

2

2

[w(Fo2 − Fc2 ) ]/[w(Fo2 ) ])1/2 and

All subsequent computations were carried out using the computer program SHELX [6,7]. The structure was solved by conventional Patterson and difference-Fourier techniques and refined by the full matrix least squares procedure. The chemical crystal data, the parameters used for X-ray diffraction data collection and the strategy used for the crystal structure determinations and their results are listed in Table 1. The final positional and Ueq parameters are given in Tables 2 and 3. Hydrogen atoms were not determined or calculated but were refined isotropically (SHELXL 97 [7]).

9

Uij aj∗ ai∗ ai aj .

j

SETERAM DSC 92 thermoanalyzer is used to perform thermal treatment on sample of KSSeTe. The sample was heated, in air, at heating rate of 3 K mn−1 from room temperature to 600 K. An empty crucide was used as reference. The impedance spectroscopy measurements were performed on a ceramic disc of about 13 mm in diameter and about 1 mm in thickness. These pellets, dense and translucent, were obtained by sintering, at room temperature, for a few hours under 200 MPa stress. The complex permittivity was determined from measurements of electric capacitance in the frequency range 100 Hz–13 MHz, using Hewlett-Packard 4192 A LF automatic bridge monitored by a HP vectra micro computer. Temperature was measured with a Chromel Alumel thermocouple close to the sample. Raman spectra of crystals in sealed glass cell (2 mm in diameter), were obtained between 300 K and 620 K on a multichannel X–Y Dilor spectrometer equipped with a CCD detector cooled with liquid nitrogen. The 514.5 nm

Table 3 Anisotropic displacement parameters of K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 Atom

U11

U22

U33

U23

U13

U12

Te1 Te2 S/Se K1 K2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10

0.0083(1) 0.0084(1) 0.0101(3) 0.0171(3) 0.0166(3) 0.0156(9) 0.0261(10) 0.0246(10) 0.0200(9) 0.0131(8) 0.0170(9) 0.0162(9) 0.0227(10) 0.0307(9) 0.0220(8)

0.0105(1) 0.0089(1) 0.0105(3) 0.0203(3) 0.0236(3) 0.0143(9) 0.0235(9) 0.0252(10) 0.0123(8) 0.0169(9) 0.0189(9) 0.0239(9) 0.0322(11) 0.0292(9) 0.0133(9)

0.0119(2) 0.0126(2) 0.0140(4) 0.0308(3) 0.0342(4) 0.0335(11) 0.0153(9) 0.0187(10) 0.0216(10) 0.0300(11) 0.0149(9) 0.0226(10) 0.0248(11) 0.0251(9) 0.0340(9)

0.0034(1) 0.0030(9) 0.0040(2) 0.0087(2) 0.0079(3) 0.0067(8) 0.0089(7) 0.0010(8) 0.066(7) 0.0006(8) 0.0047(7) 0.0112(8) 0.0160(9) 0.0005(8) 0.0028(8)

0.00176(9) 0.00211(9) 0.0026(2) 0.0012(3) 0.0050(3) −0.0083(8) 0.0074(8) 0.0009(8) 0.0070(8) 0.0077(8) −0.0037(7) −0.0006(7) 0.0116(8) 0.0021(9) −0.0067(9)

0.0028(1) 0.0028(1) 0.0045(2) 0.0087(2) 0.0096(2) −0.0015(8) 0.0136(8) 0.0185(8) 0.0059(7) 0.0048(7) 0.0018(8) 0.0053(8) 0.0105(9) 0.0221(9) 0.0010(8)

The anisotropic displacement exponent takes the form: (−2π2

 i

j

Uij hi hj ai aj∗ )x.

10

M. Dammak et al. / Journal of Alloys and Compounds 428 (2007) 8–16

radiation of a spectra-physics 2000 argon ion laser was used for excitation with a power about 200 mW. A furnace built in the laboratory was used for hightemperature experiments. The resolution was between 0.5 cm−1 and 2 cm−1 and the accuracy of the sample temperature was about 5 K. The increment of temperature was in the range 20–30 K between each spectrum. Infrared absorption spectra of suspension of crystalline powders in KBr have been recorded using Jasco-FT-IR-420 spectrophotometer in the (200–4000 cm−1 ) frequency range.

3. Results and discussion 3.1. Structure description The KSSeTe material crystallizes in the triclinic system ˚ b = 6.5680(3) A, ˚ with unit cell parameters: a = 6.2560(3) A, ˚ α = 102.30(2)◦ , β = 90.07(2)◦ , γ = 116.98(2)◦ . c = 13.1550(6) A, A projection of the KSSeTe structure on the ac plane is depicted in Fig. 1. The main feature of this structure is the coexistence of three different anions (TeO6 6− , SeO4 2− and SO4 2− ) in the same crystal. This structure type can be the origin of the ferroelectric polar phases in this material, so what dielectric measurements were performed to justify this statement. The structure is being built by planes of pure TeO6 octahedra parallel to the (1 0 0) plane at x/a = 0 and x/a = 1/2 and to the (0 0 1) one at x/c = 0 and x/c = 1/2 alterning with planes of pure S/SeO4 tetrahedra at x/a = 1/4, x/a = 3/4, x/c = 1/4 and x/c = 3/4. The K+ cations are intercalated between these kinds of polyhedra. As in the other alkali sulfates and selenates tellurates, the Te atom has an octahedral environment constituted by six oxygen atoms. The Te atom in TeO6 octahedra in KSSeTe, occupies two special positions. In consequence, the structure shows two kinds of octahedra Te1 O6 and Te2 O6 , with Te O values between ˚ and 1.946(7) A. ˚ The O Te O angles varying from 1.867(7) A ◦ ◦ 88.83(8) to 91.17(8) . These values are different to those observed in the based compounds. Indeed, in the KSTe structure the Te O distances vary ˚ to 1.938 A ˚ with O Te O angles between 89.3◦ and from 1.914 A

Fig. 1. Projection of K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 crystal structure at 293 K on the ac plane.

90.5◦ [3]. Whereas in the KSeTe structure, the Te O distances ˚ to 1.946 A ˚ and the O Te O angle values vary from 1.867 A ◦ ◦ are between 88 and 92.8 [1]. The light distortion in octahedrons, between KSTe and KSSeTe structures, can be the result of the partial anionic substitution of sulphur by the selenium atom having different radiuses. This phenomenon is confirmed by the increasing of the distortion in the Te(OH)6 groups in the KSeTe structure. In the KSSeTe structure, the sulphur and the selenium atoms occupies the same site of a statistical manner in proportions respective 9/10 and 1/10. By comparison of the tetrahedral positions in KSSeTe and KSTe structure, we notice that the positions of tetrahedral groups, in our structure, are displaced of 1/4 according to the b axis. This fact can be a consequence of octahedron distortions in this structure. In the tetrahedral ˚ S/SeO4 groups the distances S/Se O are between 1.474(2) A ˚ and 1.488(2) A. In comparison with the sulfate tellurate studies, the S/Se O distances and O S/Se O angles show an intermediate state. Indeed, in the pure compound K2 SO4 ·Te(OH)6 , S O ˚ to 1.503 A ˚ and the Se O disdistances spread from 1.453 A ˚ ˚ tances are between 1.632 A and 1.641 A in K2 SeO4 ·Te(OH)6 compound [1,3]. This behavior confirms the effect of the selenium cation insertion in the tetrahedra disorder. The K+ cations, distributed on two sites, are located between tetrahedral and octahedral planes. The K O distances are given in Table 4. In the KSTe structure the environnement of the K atom is octahedral and the potassium atom is coordinated by nine oxygen atoms in the corresponding KSeTe compound [1,3], whereas in the mixed KSSeTe structure, the cation K+ is coordinated by eight oxygen atoms. In fact, the K(2) atom is coordinated by three oxygen atoms belonging to S/SeO4 group, three atoms to Te1 O6 octahedra, one atom belonging to another Te1 O6 octahedra and one oxygen atom to Te2 O6 . The K(1) cation is tied to three oxygen atoms of S/SeO4 tetrahedra, three oxygen belonging to Te2 O6 octahedron, one oxygen to another Te2 O6 groups and one oxygen atom of Te1 O6 . The K O distances vary from ˚ to 3.269 A ˚ for the first potassium atom K(1), whereas 2.707 A ˚ and 3.036 A ˚ for the second these distances are between 2.808 A ˚ and 3.391 A ˚ potassium atom. These values are between 2.755 A ˚ in the in the KSeTe structure and vary from 2.713 to 2.987 A KSTe one [1,3]. In an attempt to locate the hydrogen atom of the OH groups belonging to the Te(OH)6 octahedra we have used the results of Brown and Shannon and Novak concerning the bond strength [8,9]. In the KSSeTe structure, the S/SeO4 tetrahedrons are connected with tellurate octahedrons by O H· · ·O hydrogen bonds assured by protons belonging to hydroxide groups. In fact, in the tetrahedral groups, one oxygen atom O8 is tied to two hydrogen atoms and each other oxygen atom (O7 , O9 and O10 ) every one is linked to one hydrogen atom (Fig. 2). In consequence, the structure shows the presence of five hydrogen bonds, with O· · ·O ˚ and 2.794 A. ˚ The O· · ·H distances distances between 2.652 A ˚ to 1.99 A ˚ and in the network hydrogen bonds vary from 1.79 A the obtained O H· · ·O angle values varying from 160.50(4)◦ to 178.10(3)◦ . These values are different to those observed in both KSTe and KSeTe structures (Table 5).

M. Dammak et al. / Journal of Alloys and Compounds 428 (2007) 8–16

11

Fig. 2. Projection of the K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 structure showing the hydrogen bonds.

These O· · ·H and O H· · ·O values explain the origin of a superprotonic conduction phase transition at high temperatures characterized by the breaking of the hydrogen bonds which link tetrahedral and octahedral groups. On the other hand, the presence of two different and independent anions S/SeO4 2− and TeO6 6− in the same crystal can favour interesting physical properties such us ferroelectricity due to the presence of different charge carriers. This interpretation was confirmed by conductivity and dielectric measurements. 3.2. Calorimetric studies A typical result of the calorimetric study is presented in Fig. 3. The DSC curve shows three endothermic peaks at 425 K, 465 K and 480 K. The first, at 425 K, present a very weak enthalpy value of about of some joules by gram. The second pick toward 465 K is little energizing, H = 27 J g−1 and could be due to the ferropara´electrique phase transition. The third anomaly observed toward 480 K represents the contribution of several phenomena as the transition of ionic protonic conduction and the begin-

Fig. 3. Differential scanning calorimetry of K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 .

ning of the decomposition of the material. The enthalpy of this anomaly is 351.04 J g−1 . 3.3. Dielectric studies Polycrystalline pellets were obtained at room temperature under 200 MPa pressure. The pellets were sintered at 400 K for 12 h in vacuum. This processing was applied to eliminate, as much as possible, the water content in the sample and to obtain dense pellets [10]. Some complex impedance diagrams showing Z versus Z , i.e. Cole–Cole plots [11], recorded at different temperatures are presented in Fig. 4. The resistance was determined by extrapolation of the circular arc centred under the Z axis to zero frequency [12]. These curves show the temperature dependence of the resistance proving the ionic conduction properties of the new mixed solution K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 . The temperature dependence of the conductivity is presented in

Fig. 4. Complex impedance curves of K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 at various temperatures.

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M. Dammak et al. / Journal of Alloys and Compounds 428 (2007) 8–16

Table 4 ˚ and angles (◦ ) of K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 Atomic distances (A) K1

K2

(a) Potassium coordination O10 (a) 2.707(2) 2.760(2) O3 (a) 2.794(2) O9 (b) O9 (c) 2.869(2) O2 (d) 2.911(2) O1 2.927(2) 2.988(2) O6 (a) O1 3.269(2)

O5 (e) O7 (f) O4 (g) O6 O2 O4 (h) O9 O10

2.808(2) 2.882(2) 2.896(2) 2.906(2) 2.985(2) 2.992(2) 3.012(2) 3.036(2)

K1

K2

(b) Sulfate/selenate groups S(Se) O10 = 1.474(2) S(Se) O9 = 1.477(2) S(Se) O8 = 1.480(2)

O10 S/Se O9 = 107.90(1) O10 S/Se O8 = 110.34(1) O9 S/Se O8 = 109.70(1)

S(Se) O7 = 1.488(2)

O1 S/Se O7 = 110.20(1) O9 S/Se O7 = 109.90(1) O8 S/Se O7 = 108.70(1)

K1

K2

(c) Tellurate groups Te1 O1 = 1.897(7) Te1 O1 (d) = 1.938(7) Te1 O2 = 1.946(7) Te1 O2 (d) = 1.946(7) Te1 O3 (d) = 1.867(7)

O3 (d) Te1 O3 = 180 O3 Te1 O1 = 90.38(9) O3 Te1 O1 (d) = 89.62(9) O1 Te1 O1 (d) = 180 O3 (d) Te1 O2 (d) = 88.83(8) O3 Te1 O2 (d) = 91.17(8) O1 Te1 O2 (d) = 90.22(9) O1 (d) Te1 O2 (d) = 89.78(9) O2 (d) Te1 O2 = 180

Te1 O3 = 1.898(7)

Te2 Te2 Te2 Te2 Te2

O4 (h) = 1.897(7) O4 = 1.938(7) O5 (h) = 1.867(7) O5 = 1.898(7) O6 = 1.946(7)

O5 Te2 O5 (h) = 180 O5 Te2 O4 = 90.23(8) O5 (h) Te2 O4 = 89.77(8) O4 Te2 O4 (h) = 180 O5 Te2 O6 (h) = 90.36(9)

Te2 O6 (h) = 1.946(7)

O5 (h) Te2 O6 (h) = 89.64(9) O4 Te2 O6 (h) = 88.62(8) O4 (h) Te2 O6 (h) = 90.38(8) O6 (h) Te2 O6 = 180

Fig. 5 in a log(σT) versus 1000/T. The experimental points are located on three sides of a line above and below approximately 480 K. The linearity shown in the log(σT) versus T plots indicates clearly an Arrhenius type law (σT = σ 0 exp(−Ea/kT)) Table 5 Distances and hydrogen bond angles in K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 O· · ·O

O· · ·H

O7 · · ·O4 = 2.710(3) O10 · · ·O1 = 2.784(3) O8 · · ·O3 (a) = 2.701(3) O8 · · ·O2 (i) = 2.794(3) O9 · · ·O5 (j) = 2.652(3) (h)

O· · ·H O

O7 · · ·H4 = 1.93(4) O10 · · ·H1 = 1.87(4) O8 · · ·H3 (a) = 1.98(4) O8 · · ·H2 (i) = 1.99(4) O9 · · ·H5 (j) = 1.79(2) (a)

O7 · · ·H4 (h) O4 (h) = 166.50(4) O10 · · ·H1 O1 = 178.10(3) O8 · · ·H3 (a) O3 (a) = 160.80(3) O8 · · ·H2 (i) O2 (i) = 174.30(3) O9 · · ·H5 (j) O5 (j) = 160.50(4)

Symmetry code: (a) −x + 1, −y, −z; (b) −x + 1, −y + 1, −z; (c) −x + 2, −y + 1, −z; (d) −x, −y, −z; (e) −x, −y, −z + 1; (f) x − 1, y, z; (g) x, y + 1, z; (h) −x + 1, −y, −z + 1; (i) x + 1, y + 1, z; (j) −x + 1, −y + 1, −z + 1.

Fig. 5. Temperature dependence of log σT for K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 .

characterizes the law and the high temperature domain with a sudden, sharp increase of the conductivity up to 480 K. In fact, in the first region the activation energy is E1 = 0.042 eV. Starting at temperature 480 K, a break in the curve is observed at high temperature on increasing activation energy E2 = 0.24 eV. This fact is due to the difficulty of the proton displacement caused by the cell deformation introduced by the establishment of polar phase due to the presence of the ferroelectric–paraelectric phase transition at 465 K. On the other hand, the conductivity increases sharply from σ = 1.03 × 10−5 −1 m−1 at 465 K to σ = 5 × 10−3 −1 m−1 at 503 K. In spite of all results indicated above, the high temperature phase transition in the KSSeTe compound can be interpreted as an ionic–protonic one observed at 480 K in the differential scanning calorimetric trace. The K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 structure is stabilized by O H· · ·O hydrogen bonds. In consequence, the transition at 480 K can be characterized by the breaking of these hydrogen bonds and the proton moves between the potential wells. The conductivity plot, above 480 K, exhibits two parts with one anomaly at 503 K. This phenomenon is due to the superposition of many mechanisms such as the breaking of hydrogen bonds and the contribution of the K+ cation in the ionic conductivity at high temperatures. This fact was confirmed by the modulus study. In consequence, in the temperature range studied, the conductivity in the mixed compound was assured by K+ and H+ ions. Fig. 6 illustrates the temperature dependence of the permittivity εr in the range 400–600 K for KSSeTe salt. These curves show one anomaly at 465 K. The most intense peak at 465 K can be the summation of two peaks which characterize the ferroelectric–paraelectric and the ionic–protonic conduction phase transitions, respectively, since the temperatures of the two phase transitions are very near. The evolution of εr for various frequencies shows that there is a significant variation with the frequency in this material due to the fact that the material presents a long-range ion diffusion. In consequence, two polar-

M. Dammak et al. / Journal of Alloys and Compounds 428 (2007) 8–16

Fig. 6. Temperature dependence of εr as a function of frequency for K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 .

ization mechanisms are possible and the real part of dielectric constant can be presented as: εr = εr(latt.) + εr(carr.) where εr(latt.) presents the lattice response due to permanent dipole orientation or other motions which do not involve long-range displacement of mobile charge carriers. In this contribution we observe the changes caused by the ferroelectric–paraelectric transition. εr(carr.) presents the conductivity relaxation, or carrier response, associated with long-range migration. The second contribution is very linked to the frequency and especially to the low frequency. This part of the permittivity characterizes the conductivity mechanisms. Fig. 7 shows the dissipation factor (tan δ) evolution as a function of temperature. The values of the dissipation factor are relatively important in agreement with the important contribution of the conductivity in this material which makes this compound a ferroelectric with diffuse character [13]. On the other hand, tan δ increases from low temperature, presents a maximum then decreases and presents a minimum in the vicinity of Tc . This behaviour corroborates the presence of a ferroelectric–paraelectric phase transition at Tc = 465 K [13,14]. The values of ferroelectric–paraelectric temperature phase transition does not change with increasing frequency, this suggests that this compound does not present a dipolar-type relaxation in

13

Fig. 7. Thermal evolution of the dissipation factor as a function of frequency for K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 .

this frequency range. This phase transition is detected at 460 K in the K2 SO4 ·Te(OH)6 compound. In consequence, the anionic substitution of sulphate group by the selenate one increases the ferroelectric–paraelectric phase transition temperature and decreases the protonic conduction phase transition one. In order to throw some additional light on the role of K+ and H+ ions, and to identify the entities contributed to the conductivity phenomenon, dielectric relaxation studies have been consequently undertaken at high temperature, in the complex modulus M* formalism. For a given temperature and frequency, the real part M and the imaginary part M of the M* complex modulus (M* = M + jM ) were calculated from the complex impedance data (Z* = Z − jZ ) using the relations M = ωC0 Z and M = ωC0 Z . The plots of log M and the nor malized M  /Mmax imaginary part of the complex modulus of KSSeTe versus log f are given in Figs. 8 and 9 at various temperatures. Whatever the temperature, M reaches a constant value  = 1/ε ) at high frequencies and at low frequencies it (M∞ ∞ approaches zero, which indicates that the electrode polarisation phenomenon makes a negligible contribution to M* and may be ignored when the electric data are analysed in this form [15]. Differently to the both potassium sulfate and selenate com pounds the M  /Mmax spectra relative to a given temperature shows two asymmetrical peaks. For the first one, the modulus peak maximum shifts to higher frequencies as temperature increases. The region of the left of the first peak maximum is where the H+ protons are mobile over long distances, whereas

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M. Dammak et al. / Journal of Alloys and Compounds 428 (2007) 8–16

Fig. 8. Plots of log M vs. log f for K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 at various temperatures.

the region of the right is where the ions are spatially confined to their potential wells. The frequency range where the peak occurs indicative of the transition from short-range to long-range mobility at decreasing frequency and is defined by the condition ωτ σ = 1, where τ σ is the most probable constitution proton relaxation time [16]. This phenomenon confirms that the proton transport in K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 is probably due to a hopping mechanism [17]. The presence of the second peak  in the M  /Mmax spectra shows that the potassium cation K+ contribute considerably in the ionic conductivity in the mixed solid solution KSSeTe. This phenomenon is not observed in the KSTe and KSeTe material. In consequence, the partial anionic substitution is the origin of the increasing of the cation disorder at high temperature. 3.4. Vibrational studies In order to gain more information on the crystal dynamics, on the degree of disorder in the different phases and the mechanisms involved in the transitions, we have undertaken a infrared study

 ) vs. log(f) for K (SO ) Fig. 9. Plots of normalized modulus (M  /Mmax 2 4 0.9 (SeO4 )0.1 ·Te(OH)6 .

Fig. 10. Raman spectra of crystalline K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 between 300 K and 520 K in the range 10–1200 cm−1 (λ0 = 514.5 nm).

at room temperature (Fig. 11) and a Raman study between 300 K and 620 K (Fig. 10) in the range 10–1200 cm−1 . Frequencies and assignments of the IR and Raman peaks in the different phases are given in Table 6. The observed frequencies are interpreted on the basis of the characteristic frequencies of the Te(OH)6 , SeO4 and SO4 groups [18–20]. The stretching and bending vibrations for compounds containing the TeO6 group normally occur in the range of 550–750 cm−1 and 350–450 cm−1 , respectively [21]. At room temperature, the Raman spectrum of mixed compound K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 is very resolved which confirm the presence of the neat phase. The intense peaks around 650 cm−1 in the Raman spectra and at 644 cm−1 in the IR spectra are assigned to ν1 (TeO6 ) whereas the ν2 (TeO6 ) at 624 cm−1 . The narrow and intense band at 975 cm−1 in both IR and Raman spectra is attributed to ν1 (SO4 ). The weak lines at 1075 cm−1 in Raman and at 1060 cm−1 in IR are assigned to ν3 (SO4 ). On the other hand, the vibration ν3 (SeO4 ) appears at about 880 cm−1 in both IR and Raman. The band at 842 cm−1 is attributed to ν1 (SeO4 ) whereas the short lines at 317 cm−1 and 360 cm−1 are assigned to ν2 (SeO4 ) and ν4 (SeO4 ), respectively [22]. The vibration ν2 (TeO6 ) appears at 220 cm−1 and 230 cm−1 in IR and Raman, respectively [18–20]. From the thermal evolution of Raman spectra for the mixed K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 solution, we can deduce that when temperature increases some bands present a decrease in intensity and an increase of width, which is in agreement of the establishment of a disorder with the paraelectric phase. This fact confirms our last interpretation that the superprotonic phase transition observed in this salt is due to the breaking of

M. Dammak et al. / Journal of Alloys and Compounds 428 (2007) 8–16

15

Fig. 11. IR spectra at room temperature of K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 .

the hydrogen bond which can favour the disorder phase. So, two phase transitions at 465 K and 480 K have been evidenced in this compound by Raman spectroscopy. All of these phase transitions are reversible and hysteresis was not observed. Furthermore, the spectra never show the presence of both phases. The first phase transition presents a second character and is of ferroelectric–paraelectric type given by dielectric measurements. The second phase transition shows the proton conduction transition given by electrical and DSC results. These two phenomenons are illustrated by the Raman spectra of this compound at 467 K and 483 K, respectively. At high tem-

perature, the transformation of this compound was evidenced (503 K). The line at 650 cm−1 assigned to ν1 (TeO6 ) stretching shifts by about 10 cm−1 , decreases in intensity and broadens through temperature increases, whereas the bands relative to ν2 (SO4 ) at 446 cm−1 , 460 cm−1 and 480 cm−1 increase in intensity on raising the temperature, they become well defined at 473 K then decrease strongly in intensity showing the decomposition of our compound. The ν5 (TeO6 ) bands which appear at 323 cm−1 , 351 cm−1 and 364 cm−1 decrease in intensity and broaden as the temperature is increased. They disappear at the second phase

Table 6 Observed Raman frequencies (cm−1 ) and band assignments for K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 at different temperatures IR

Raman

Assignment

297 K

297 K

443 K

453 K

467 K

483 K

503 K

3428 vw

– – –

– – –

– – –

– 1120 sh 1055 vw

– 1120 sh 1055 vw

– 1120 sh 1045 vw

νOH

1060 w 975 s 873 w 852 m

1075 vw 975 s 883 w 842 w

1074 vw 978 s 880 w 830 w

1075 vw 975 vs 880 w 830 w

1070vw 975 vs – 830 vw

– 960 vs – –

– 960 s – –

ν3 (SO4 ) ν1 (SO4 ) ν3 (SeO4 ) ν1 (SeO4 )

644 w

650 vs 624 sh

652 vs 620 sh

650 vs 625 sh

670 m 600 sh

660 m 600 sh

660 w 580 sh

ν1 (TeO6 ) ν2 (TeO6 )

475 sh

–

490 w

480 m

470 m

470 w

ν3 (TeO6 ) ν2 (SO4 )

433 sh 410 sh

450 sh 360 sh

450 vw 350 sh

450 sh 345 sh

450 sh 330 vw

440 sh –

430 sh –

ν5 (TeO6 ) ν4 (SeO4 )

330 sh

342 sh 317 sh

– 317 sh

– 315 sh

– –

– –

– –

ν4 (TeO6 ) ν2 (SeO4 )

220 sh

– 57 sh

– 55 sh

230 vw 54 sh

230 m 52 vw

230 m –

220 vw –

ν6 (TeO6 ) T(K+ )

597 m

Relative intensities: vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder.

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M. Dammak et al. / Journal of Alloys and Compounds 428 (2007) 8–16

transition near 483 K. The narrow peak detected at 982 cm−1 relative to ν1 (SO4 ) remains constant in intensity and in wavelength number but it decreases in intensity and broadens near the second transition then disappear at high temperature. Whereas, the weak peak at 842 cm−1 attributed to ν1 (SeO4 ) decreases in intensity and broadens near the first transition and disappears at the second phase transition. On the other hand, the band at 360 cm−1 attributed to ν4 (SeO4 ) disappears at the first transition. These modes can be considered as a soft mode accompanying the disordered paraelectric phase at high temperature of our material. 4. Conclusion The new mixed composition K2 (SO4 )0.9 (SeO4 )0.1 ·Te(OH)6 (KSSeTe) was characterized at room temperature by X-ray, dielectric and vibrational investigations. It is found that crystals of KSSeTe crystallize in the triclinic system P 1¯ with two formula units in the cell. The structure is built by anionic TeO6 6− , SeO4 2− and SO4 2− groups linked by O H· · ·O hydrogen bonds and K+ cation. The main feature of the type of the mixed solution M2 (SO4 )1−x (SeO4 )x ·Te(OH)6 and the alkaline sulfate tellurate structure is the presence of different and independent anions SO4 2− and/or SeO4 2− and TeO6 6− in the same unit cell which can be in the origin of important physical properties, as the ferroelectricity and the superprotonic conduction. The measurements of some electrical properties combined with DSC, Raman spectroscopy and X-ray examination indicate the existence of three phase transitions at 425 K which can favour the ferroelectric phase, and at 465 K and 480 K attributed to the ferroelectricity and the superprotonic conduction, respectively. The ionic conductivity is attributed to H+ and K+ mobility due to the breaking of hydrogen bonds. A relaxation study shows the H+ transfer is probably provided by a hopping mechanism.

Acknowledgement We wish to express our thanks to Dr. N. Zouari for his dielectric measurements and fruitful discussions and interpretations. References [1] M. Dammak, H. Khemakhem, T. Mhiri, A.W. Kolsi, Solid State Chem. 145 (1999) 612. [2] M. Dammak, H. Khemakhem, N. Zouari, T. Mhiri, A.W. Kolsi, Solid State Ionics 127 (2000) 125. [3] R. Zilber, A. Durif, M.T. Averbuch-Pouchot, Acta Crystallogr. B 36 (1980) 2743. [4] C. Boudaya, N. Chabchoub, H. Khemakhem, R. Von der M¨uhll, J. Alloys Compd. 352 (2003) 304. [5] Nonius, Kappa CCD Server Software, Nonius B.V. Delft, The Netherlands, 1997. [6] G.M. Sheldrick, SHELXS’93, Program For The Solution Of Crystal Structures, University of G¨ottingen, Germany, 1993. [7] G.M. Sheldrick, SHELXL’97, Program For Crystal Structure Determination, University of G¨ottingen, Germany, 1997. [8] I.D. Brown, R.D. Shannon, Acta Crystallogr. A 28 (1973) 266. [9] A. Novak, Hydrogen Bonding in Solids, Springre-Verlag, Berhin, Heldelberg, New York, 1974, p. 177. [10] H. Khemakhem, R. Von der M¨uhll, A. Daoud, J. Ravez, Phys. Stat. Sol. (a) 160 (1997) 243. [11] K.S. Cole, R.H. Cole, J. Chem. Phys. 43 (1941) 341. [12] J.F. Bauerle, J. Phys. Chem. 30 (1969) 2657. [13] H. Khemakhem, J. Ravez, A. Daoud, Ferroelectrics 188 (1996) 41. [14] H. Khemakhem, J. Ravez, A. Daoud, Phys. Stat. Sol. (a) 161 (1997) 557. [15] F.S. Howell, R.A. Bose, P.B. Macedo, C.T. Moynihan, J. Phys. Chem. 78 (1974) 639. [16] H.K. Palet, S.W. Martin, Phys. Rev. B45 (1992) 10292. [17] B.V.R. Chowdari, R. Gopalakrishnan, Solid State Ionics 23 (1987) 225. [18] M. Dammak, H. Khemakhem, T. Mhiri, A.W. Kolsi, A. Daoud, J. Alloys Compd. 280 (1998) 107. [19] K. Viswanthan, V.U. Nayar, G. Aruldhas, Infrared Phys. 26 (1986) 89. [20] G. Sekar, V. Ramakrishnan, G. Arulghas, Infrared Phys. 27 (1987) 253. [21] R. Allmann, W. Hasse, Inorg. Chem. 15 (1976) 804. [22] M. Gargouri, T. Mhiri, A. Daoud, J.M. Reau, Solid State Ionics 125 (1999) 193.